The Honolulu Volcanics are a group of
volcanoes
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates are ...
which form a
volcanic field
A volcanic field is an area of Earth's crust that is prone to localized volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters of up to 100 volcanoes ...
on the island of
Oʻahu
Oahu () ( Hawaiian: ''Oʻahu'' ()), also known as "The Gathering Place", is the third-largest of the Hawaiian Islands. It is home to roughly one million people—over two-thirds of the population of the U.S. state of Hawaii. The island of O’ ...
,
Hawaiʻi
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
, more specifically in that island's southeastern sector and in the
city
A city is a human settlement of notable size.Goodall, B. (1987) ''The Penguin Dictionary of Human Geography''. London: Penguin.Kuper, A. and Kuper, J., eds (1996) ''The Social Science Encyclopedia''. 2nd edition. London: Routledge. It can be def ...
of
Honolulu
Honolulu (; ) is the capital and largest city of the U.S. state of Hawaii, which is in the Pacific Ocean. It is an unincorporated county seat of the consolidated City and County of Honolulu, situated along the southeast coast of the island ...
from
Pearl Harbor
Pearl Harbor is an American lagoon harbor on the island of Oahu, Hawaii, west of Honolulu. It was often visited by the Naval fleet of the United States, before it was acquired from the Hawaiian Kingdom by the U.S. with the signing of the Re ...
to the
Mokapu Peninsula. It is part of the
rejuvenated stage of Hawaiian volcanic activity, which occurred after the main stage of volcanic activity that on Oʻahu built the
Koʻolau volcano. These volcanoes formed through dominantly
explosive eruption
In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such ...
s and gave rise to
cinder cone
A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...
s,
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s,
tuff cone
Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and s ...
s and
volcanic island
Geologically, a high island or volcanic island is an island of volcanic origin. The term can be used to distinguish such islands from low islands, which are formed from sedimentation or the uplifting of coral reefs (which have often formed ...
s. Among these are well known landmarks such as
Diamond Head and
Punchbowl Crater
Punchbowl Crater is an extinct volcanic tuff cone located in Honolulu, Hawaii. It is the location of the National Memorial Cemetery of the Pacific.
The crater was formed some 75,000 to 100,000 years ago during the secondary activity of the Hon ...
.
Volcanic activity began less than one million years ago and occurred at between 40 and 30 separate volcanic vents, some of which are submarine. Sea level varied during the activity of the volcanic field, and some volcanic eruptions have been dated through correlation with individual sea level fluctuations. The field erupted various kinds of lavas of mostly
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
ic type with a high content of
xenolith
A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in igne ...
s. During eruptions, ascending magma often underwent interactions with water and thus caused
steam explosion
A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, occurring when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten m ...
s and the formation of particular volcanic structures such as tuff cones. The last eruption took place 35,000 or 76,000 years ago and future hazardous eruptions are possible.
Geography and geomorphology
The Honolulu Volcanics are a series of volcanoes in the southeastern sector of
Oʻahu
Oahu () ( Hawaiian: ''Oʻahu'' ()), also known as "The Gathering Place", is the third-largest of the Hawaiian Islands. It is home to roughly one million people—over two-thirds of the population of the U.S. state of Hawaii. The island of O’ ...
and includes
dikes
Dyke (UK) or dike (US) may refer to:
General uses
* Dyke (slang), a slang word meaning "lesbian"
* Dike (geology), a subvertical sheet-like intrusion of magma or sediment
* Dike (mythology), ''Dikē'', the Greek goddess of moral justice
* Dikes, ...
,
lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s,
spatter cone
Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and s ...
s,
tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they rem ...
deposits,
tuff cone
Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and s ...
s, and
mesa
A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. Mesas characteristically consist of flat-lying soft sedimentary rocks capped by ...
s where the surrounding terrain has been eroded away. Vents span the area southeast of a line between
Mokapu Peninsula and
Pearl Harbor
Pearl Harbor is an American lagoon harbor on the island of Oahu, Hawaii, west of Honolulu. It was often visited by the Naval fleet of the United States, before it was acquired from the Hawaiian Kingdom by the U.S. with the signing of the Re ...
, and extend from the ridges of
Koʻolau volcano to below sea level and to the coast plain of southern Oʻahu.
The system takes its name from
Honolulu
Honolulu (; ) is the capital and largest city of the U.S. state of Hawaii, which is in the Pacific Ocean. It is an unincorporated county seat of the consolidated City and County of Honolulu, situated along the southeast coast of the island ...
, the capital of
Hawaiʻi
Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...
, as craters are scattered in and around the city. The volcanic system includes well-known landmarks of Honolulu such as
Diamond Head,
Koko Head
Koko Head is the headland that defines the eastern side of Maunalua Bay along the southeastern side of the Island of Oahu in Hawaii. On its western slope is the community of Portlock, a part of Hawaii Kai. Koko Head (at 642 ft or 196 m) is a ...
,
Punchbowl Crater
Punchbowl Crater is an extinct volcanic tuff cone located in Honolulu, Hawaii. It is the location of the National Memorial Cemetery of the Pacific.
The crater was formed some 75,000 to 100,000 years ago during the secondary activity of the Hon ...
,
Rabbit Island,
Tantalus
Tantalus ( grc, Τάνταλος ) was a Greek mythological figure, most famous for his punishment in Tartarus: he was made to stand in a pool of water beneath a fruit tree with low branches, with the fruit ever eluding his grasp, and the wate ...
,
Hanauma Bay
Hanauma (; )
is a marine embayment formed within a tuff ring and located along the southeast coast of the Island of Oahu in the Hawaii Kai neighborhood of East Honolulu, in the Hawaiian Islands.
Hanauma is one of the most popular tourist destina ...
(notable as a
snorkeling
Snorkeling ( British and Commonwealth English spelling: snorkelling) is the practice of swimming on or through a body of water while equipped with a diving mask, a shaped breathing tube called a snorkel, and usually swimfins. In cooler waters, a ...
site) and the Mokapu Peninsula,
which is the location of
Marine Corps Base Hawaiʻi. The
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
military has made use of some of the volcanic islands that were formed by the Honolulu Volcanics. The Koko area is designated as the Koko Head Regional Park
and Hanauma Bay is also a state park. Parts of this system are among the best known volcanic vents of Hawaiʻi.
About 30–40 vents have been identified. Most
cinder cone
A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...
s on Oʻahu are quite large, over high and up to wide. Some of the lava flows filled deep valleys cut into the older Koʻolau volcano and displaced streams that previously ran through these valleys; for example, water passing over a lava flow in Kamanaiki Valley forms a
waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in several wa ...
. Together with
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
s coming down from the mountains and
coral reef
A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.
Co ...
growth, the deposits of the Honolulu Volcanics have formed the coastal plain on which the city of Honolulu and military installations are built.
The vents of the Honolulu Volcanics follow northeastward-trending alignments that are at right angles to the
rift zone
A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed t ...
of the Koʻolau volcano. From northwest to southeast these are the Haʻikū Rift, the Tantalus Rift, the Kaimukī/Kaʻau Rift and the Koko Head/Koko Rift, but each rift has had eruptions at different times and with different compositions. It is not clear whether these alignments are in any way related to the structure of the previous Koʻolau volcano, instead of being controlled by the
crust of the
Pacific Ocean
The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, but the trends along the Koko and Tantalus Rifts are parallel to that of the
flexural arch of
Hawaiʻi Island
Hawaii ( ; haw, Hawaii ) is the List of islands of the United States by area, largest island in the United States, located in the U.S. state, state of Hawaii. It is the southeasternmost of the Hawaiian Islands, a chain of High island, volcanic ...
. There is also a hypothetical "Diamond Head
fault" that may be associated with
earthquake
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
s on Oahu that occurred in 1948, 1951 and 1961–1981, but it is not parallel to these alignments and its very existence is questionable.
Submarine vents are also known, including a high solitary cone with two ridges off northeast Oʻahu, which is covered by
pillow lava
Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...
s and
volcaniclastic
Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...
sediments. At least five cones are found off the southwestern extension of the Koko Rift where they are situated on a southwestward extending ridge. Another set of submarine vents is found south of Diamond Head. It was once proposed that some seamounts (underwater mountains) off northeastern Oʻahu such as
Tuscaloosa Seamount are related to the volcanic series;
today however they are considered to be fragments of the giant
Nuʻuanu Slide off northeastern Oʻahu.
File:Honolulu County, HI, USA - panoramio (7).jpg, Hanauma Bay in the foreground, Koko is in the middle
File:Oahu-DiamondHead-Punchbowl-Honolulu.jpg, Diamond Head to the left, Punchbowl in the middle
Description of individual volcanoes
Most of the present-day shoreline of
Hawaiʻi Kai
Maunalua (Hawaii Kai) is a largely residential area located in the City & County of Honolulu, in the East Honolulu CDP, on the island of Oahu. Maunalua, (Hawaii Kai) is the largest of several communities at the eastern end of the island ...
was formed by Honolulu Volcanics; the Kuapā Pond is a leftover lake between the new shoreline and the old shoreline of the Koʻolau volcano. The volcanic vents there include Koko Head, the Hanauma Bay craters,
Kahauloa crater bay, a wave-eroded cone,
Koko Crater
Koko Crater (Hawaiian language, Hawaiian: ''Kohelepelepe'' or ''Puʻu Mai'') is an extinct tuff cone located on the Hawaiian Islands, Hawaiian island of Oahu near Hawaii Kai, Hawaii, Hawaii Kai. It is northeast of Hanauma Bay and south of the Ko ...
and
Kalama
Kalama Hakaleleponi Kapakuhaili (1817 – September 20, 1870) was a Queen consort of the Kingdom of Hawaii alongside her husband, Kauikeaouli, who reigned as King Kamehameha III. Her second name Hakaleleponi is Hazzelelponi in Hawaiian.
Ear ...
cinder cone; Koko Head cones are heavily eroded, and the sea has broken into one of the cones, exposing its structure in outcrops. Koko Head is the largest cone of the Honolulu Volcanics and the crater of Koko Crater is about wide. Hanauma Bay close to Kalanianaʻole Highway is located east of Honolulu and is a large, deep compound crater/tuff cone with several associated dykes and lava flows- It was breached by the sea and coral reefs grow within it. Together with Kahauloa and Kalama all these vents form the Koko Rift. Farther northeast lie the Kaupō lava flow and the islands of
Kāohikaipu and Mānana; all these except Mānana are also in the Koko Rift. This long rift includes at least 12 separate volcanic vents and its vents appear to have formed in a single eruption.
Diamond Head is a (rim-to-rim) wide typical tuff cone with a wide and not overly deep crater that forms a prominent promontory east of Honolulu. Inland from Diamond Head lie the Kaimukī and Mauʻumae cones, which appear to come from a shared
fissure
A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes.
Ground fissure
A ...
. Mauʻumae features a lava flow and Kaimukī is an unusual lava cone with a summit crater. Its slopes are gentle and lava ponded against topographical obstacles. The Kaimukī and Kaʻau cinder cones together with Mauʻumae and Diamond Head form the Kaʻau or Kaimukī rift zone; the Kaʻau crater lies close to the crest of the Koʻolau Range and is filled by a
swamp
A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...
that drains into Waimao stream. Punchbowl Crater rises north of and at the centre of Honolulu and is a good outlook to the city and its surroundings.
The Mokapu peninsula was formed by Honolulu Volcanics and includes the three volcanic vents of Puʻu Hawaiʻiloa, Pyramid Rock and Ulapaʻu Head; additional vents form islets off the peninsula, such as
Moku Manu and
Mōkōlea Rock. Puʻu Hawaiʻiloa is a cinder cone in the middle of the peninsula, Pyramid Rock at the northwestern tip is deeply eroded and probably the oldest vent in the peninsula, and Ulapaʻu Head is a crater that was breached by the sea and of which only a crescent-like western part remains.
Salt Lake Crater contains a
salt lake
A salt lake or saline lake is a landlocked body of water that has a concentration of salts (typically sodium chloride) and other dissolved minerals significantly higher than most lakes (often defined as at least three grams of salt per litre). ...
and is located east from Pearl Harbor; the salt lake formed when salty
groundwater
Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
seeped into the crater and was concentrated by
evaporation
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...
.
A cluster of additional older vents known as ʻĀliamanu, Makalapa, ʻĀliamanu School Cone, Moanalua Cone, ʻĀkulikuli Vent and Wiliki Cone are associated with Salt Lake Crater.
The Salt Lake Tuff is associated with these craters and covers an area of at least ;
Honolulu International Airport
Daniel K. Inouye International Airport , also known as Honolulu International Airport, is the main airport of Oahu, Hawaii.[Hickham Air Force Base
Hickam Air Force Base is a United States Air Force installation, named in honor of aviation pioneer Lieutenant Colonel Horace Meek Hickam. The installation merged in 2010 with Naval Station Pearl Harbor to become part of the newly formed Joint B ...](_blank)
lie south and southwest from the vents respectively. Some of these vents have been identified as
maar
A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...
s.
Geology
The Honolulu Volcanics developed on the 2.3 million year old
Koʻolau Volcanic Series, which forms the core of eastern Oahu and extends underwater far from the shore. Like other Hawaiian volcanoes Koʻolau is a
shield volcano
A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...
that grew through lava flows erupted from a
rift
In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.
Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-grabe ...
system with a central
caldera
A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
, although a large section of the volcano has sunk below sea level. This volcano constitutes the
tholeiitic
The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma i ...
stage of Hawaiian volcanism, and developed possibly during
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
to
Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...
time. Before Koʻolau volcano was active, between 3.5 and 2.74 million years ago,
Waiʻanae volcano formed the western part of Oʻahu. Koʻolau volcano appears to be unrelated to the Honolulu Volcanics, which are considered to be a separate volcanic system; sometimes the "Kokohead Volcanics" are split off from the Honolulu Volcanics.
The Honolulu Volcanics constitute a late stage of volcanism which in Hawaiʻi is known as the
rejuvenated stage and the third stage of a typical Hawaiian volcano. They have a much smaller volume than the Koʻolau volcano even though their lava flows are usually thicker; the
unconformity
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
that separates the Honolulu Volcanics from the Koʻolau Volcanic Series was already recognized in the 19th century.
As Hawaiian volcanoes grow, they start to sink under their weight. As volcanism moves along the Hawaiian chain, the
Hawaiian Arch moves behind the volcanism at a distance of several hundred kilometres, and appears to have passed under Oahu in geologically recent times. The tectonic effect of the Hawaiian Arch passing under the island may be responsible for the onset of Honolulu Volcanics volcanism, as well as of the Kōloa Volcanics on
Kauaʻi
Kauai, () anglicized as Kauai ( ), is geologically the second-oldest of the main Hawaiian Islands (after Niʻihau). With an area of 562.3 square miles (1,456.4 km2), it is the fourth-largest of these islands and the List of islands of th ...
and perhaps for future volcanism on
Maui
The island of Maui (; Hawaiian: ) is the second-largest of the islands of the state of Hawaii at 727.2 square miles (1,883 km2) and is the 17th largest island in the United States. Maui is the largest of Maui County's four islands, which ...
or
Molokaʻi
Molokai , or Molokai (), is the fifth most populated of the eight major islands that make up the Hawaiian Islands archipelago in the middle of the Pacific Ocean. It is 38 by 10 miles (61 by 16 km) at its greatest length and width with a us ...
, but also for ongoing
uplift
Uplift may refer to: Science
* Geologic uplift, a geological process
** Tectonic uplift, a geological process
* Stellar uplift, the theoretical prospect of moving a stellar mass
* Uplift mountains
* Llano Uplift
* Nemaha Uplift
Business
* Uplif ...
on Oʻahu. Other proposed mechanisms are a
conductive
In physics and electrical engineering, a conductor is an object or type of material that allows the flow of charge (electric current) in one or more directions. Materials made of metal are common electrical conductors. Electric current is gener ...
heating of the
lithosphere
A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
or ongoing upwelling in the
mantle plume
A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...
.
The terrain that the volcanoes developed on includes both old volcanic rocks of the Koʻolau volcano, sediments of the coastal plains, and soils. Some Honolulu Volcanics have grown on coral deposits, Koko Head developed on
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
for example, and coral reef development was widespread during the activity of the Honolulu Volcanics.
The Honolulu Volcanics are not associated with either
aeromagnetic or
gravimetric
Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest.
Units of measurement
G ...
anomalies; only Salt Lake Crater has an associated
gravity anomaly
The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...
.
There is rejuvenated volcanism on Waiʻanae volcano also, but it appears to be older than the Honolulu Volcanics.
Composition
The
petrology
Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
of the Honolulu Volcanics is well studied. The volcanic rocks of the Honolulu Volcanics are diverse; they include
alkali basalt
Alkali basalt or alkali olivine basalt is a dark-colored, porphyritic volcanic rock usually found in oceanic and continental areas associated with volcanic activity, such as oceanic islands, continental rifts and volcanic fields. Alkali basalt ...
s,
melilite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is ( Ca, Na)2( Al, Mg, F ...
and
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
basalt
Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
s,
basanite
Basanite () is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and plagioclase and forms from magma low in silica and enriched in alkali metal oxides that s ...
s, melilite,
nephelinite
Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may rese ...
and
websterite
Websterite is ultramafic igneous rock that consists of roughly equal proportions of orthopyroxene and clinopyroxene. It is a type of pyroxenite.
Websterite is named after the town Webster in North Carolina
North Carolina () is a stat ...
, and form an alkaline
-nephelinite suite. The variations in composition reflect distinct proportions of melts produced from parent rocks.
Phenocryst
300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...
s include
augite
Augite is a common rock-forming pyroxene mineral with formula . The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.
Characteristics
Augite is a solid solution in the pyroxene group. ...
,
labradorite
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
,
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
and
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prope ...
; additionally
spinel
Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals.
Properties
S ...
s are found in the rocks.
Xenolith
A xenolith ("foreign rock") is a rock fragment (country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusively used to describe inclusions in igne ...
s of
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...
,
calcite
Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
,
clinopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
,
dunite
Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with mi ...
,
garnet
Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives.
All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...
, garnet
peridotite
Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
,
orthopyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
,
phlogopite
Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.
Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O ...
, garnet
pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websteri ...
,
lherzolite
Lherzolite is a type of ultramafic igneous rock. It is a coarse-grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser amounts of calcic chromium-rich clinopyroxene. Minor minerals include chromium and alu ...
and spinel have been described.
The most common ones are dunite, garnet containing rocks and lherzolite and the relative prevalence of the various xenoliths is a function of the position of their source vent relative to the Koʻolau caldera. Their formation was influenced by mantle rocks left over from the Koʻolau volcano.
Coral fragments have been found in Koko and Salt Lake rocks, and
metamorphic
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causin ...
rocks included in the volcanics may be part of the
basement
A basement or cellar is one or more floors of a building that are completely or partly below the ground floor. It generally is used as a utility space for a building, where such items as the furnace, water heater, breaker panel or fuse box, ...
that the
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
s of the volcanoes traversed. Calcite – which in the form of crystals gives Diamond Head its name
– in the volcanic rocks may come from coral reefs, groundwater or even from the magma itself;
isotope ratio
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s of the rocks indicate that groundwater
carbonate
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
s are the most important source, however.
In Punchbowl Crater, where the rocks have been quarried, they have a brown to yellow colour. Cinders have red-black colours which can grade to yellow when they are
hydrothermal
Hydrothermal circulation in its most general sense is the circulation of hot water (Ancient Greek ὕδωρ, ''water'',Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with th ...
ly altered, due to the formation of the glassy rock
palagonite
Palagonite is an alteration product from the interaction of water with volcanic glass of chemical composition similar to basalt. Palagonite can also result from the interaction between water and basalt melt. The water flashes to steam on contact w ...
. Many of the erupted rocks have undergone various degrees of alteration, including the formation of
zeolitic
Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These pos ...
palagonite; minerals included in altered rocks include
analcime
Analcime (; ) or analcite is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is Na Al Si2 O6· H2O. Minor amounts of potassium and calciu ...
,
aragonite
Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...
, calcite,
chabazite
Chabazite ()The New Oxford Dictionary of English (1998) - p. 300 "chabazite /'kabəzʌɪt/ noun "A colourless, pink or yellow zeolite mineral, typically occurring as rhombohedral crystals.". is a tectosilicate mineral of the zeolite group, close ...
,
erionite
Erionite is a naturally occurring fibrous mineral that belongs to a group of minerals called zeolites. It usually is found in volcanic ash that has been altered by weathering and ground water. Erionite forms brittle, wool-like fibrous masses in t ...
,
faujasite
Faujasite (FAU-type zeolite) is a mineral group in the zeolite family of silicate minerals. The group consists of faujasite-Na, faujasite-Mg and faujasite-Ca. They all share the same basic formula by varying the amounts of sodium, magnesium and ...
,
gonnardite
Gonnardite is a comparatively rare, fibrous zeolite, natrolite subgroup. Older papers claim that a complete solid solution exists between tetranatrolite and gonnardite, but tetranatrolite was discredited as a separate species in 1999.American Mi ...
,
gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
,
montmorillonite
Montmorillonite is a very soft phyllosilicate group of minerals that form when they precipitate from water solution as microscopic crystals, known as clay. It is named after Montmorillon in France. Montmorillonite, a member of the smectite group, ...
,
natrolite
Natrolite is a tectosilicate mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula . The type locality is Hohentwiel, Hegau, Germany.
It was named natrolite by Martin Heinrich Klaproth i ...
,
opal
Opal is a hydrated amorphous form of silica (SiO2·''n''H2O); its water content may range from 3 to 21% by weight, but is usually between 6 and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms ...
,
phillipsite
Phillipsite is a mineral series of the zeolite group; a hydrated potassium, calcium and aluminium silicate, approximating to . The members of the series are phillipsite-K, phillipsite-Na and phillipsite-Ca. The crystals are monoclinic, but only c ...
and
thomsonite
Thomsonite is the name of a series of tecto-silicate minerals of the zeolite group. Prior to 1997, thomsonite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a ser ...
. In some vents, such as Diamond Head, the rocks are so heavily altered that their original composition and texture can no longer be reconstructed.
Origin of the rocks
The Honolulu Volcanics rocks originate at greater depths than the rocks from Koʻolau volcano and their composition is fairly dissimilar as well, while there are substantial
geochemical
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the e ...
similarities to young volcanic rocks from
East Molokaʻi, Kauaʻi and
West Maui
The West Maui Mountains, West Maui Volcano, or Mauna Kahālāwai which means "holding house of water," is approximately 1.7 million years old and forms a much eroded shield volcano that constitutes the western quarter of Maui. Since its last eru ...
. Jackson and Wright suggested that the pyroxenite may be the source rock of the magmas, with orthopyroxene-rich rocks being leftovers from the melting process;
strontium
Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...
isotope ratios endorse this origin although none of the xenoliths appears to be entirely representative of the source melts.
Water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
- and
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
-containing
volatiles
Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances.
On planet Earth, the term ' ...
may have altered the source rocks of the Honolulu Volcanics melts before these actually melted.
In terms of the ultimate origin of the magmas, either an origin from a mixing between
MORB
A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diverge ...
mantle with mantle plume rocks, the lithosphere or exclusively from
lithospheric mantle
The lithospheric mantle is the uppermost solid part of Earth's mantle.
The lithospheric mantle is subdivided into the subcontinental lithospheric mantle associated with the continental lithosphere and oceanic lithospheric mantle, associated with ...
have been proposed. Research published in 2007 favoured an origin from a
depleted mantle component along with a mantle plume "Kalihi" component, with additional material contributed from the margins of the mantle plume.
Groundwater content
Groundwater contained in Honolulu Volcanics rocks, while not voluminous, is important in some areas such as Maunawili Valley. Moreover, impermeable
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
layers can hold groundwater in rock layers above them. However, Koʻolau volcanic rocks contain most of the groundwater on Oʻahu, and most Honolulu Volcanics have little significance. Some groundwater in Honolulu Volcanics rocks is saline, and has been used both as a water source for
a sea-life park at Makapuʻu and for the discharge of salty wastewaters.
Eruption history
Chronology
The activity of the Honolulu Volcanics began less than one million years ago during the late Pleistocene and
Holocene
The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...
, after volcanic activity at Koʻolau had ceased and the volcano been substantially eroded. The first eruptions occurred within the Koʻolau caldera and the youngest in the far southeastern part of Oʻahu, coinciding with the Koko Rift. There is otherwise little evidence for a spatial pattern in the volcanic activity, with each rift having eruptions widely spaced in time. The lava flows from Honolulu Volcanics have been used to construct a history of variations of
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
.
Eruptions of the Honolulu Volcanics have been correlated to
shoreline
A shore or a shoreline is the fringe of land at the edge of a large body of water, such as an ocean, sea, or lake. In physical oceanography, a shore is the wider fringe that is geologically modified by the action of the body of water past a ...
s generated by sea level variations, which have left both drowned and emergent platforms and terraces on Oʻahu. Some volcanoes formed when sea level was lower than today and thus part of their structures are now submerged, others formed when it was higher and grew on
reef
A reef is a ridge or shoal of rock, coral or similar relatively stable material, lying beneath the surface of a natural body of water. Many reefs result from natural, abiotic processes— deposition of sand, wave erosion planing down rock out ...
s. These sea level variations are a function of
glacial
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
-
interglacial
An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene in ...
changes, with higher sea levels associated with interglacials when
polar
Polar may refer to:
Geography
Polar may refer to:
* Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates
* Polar climate, the c ...
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
s expand and retreat. Thus four stages of volcanic activity have been defined, a first during the Kahipa highstand, a second during the Kaʻena and Lāʻie highstands, a third during the Waipiʻo and Waimānalo highstands and a fourth after the Waimānalo highstand. In turn, the Waimānalo stage was correlated to the last interglacial/
Sangamonian
The Sangamonian Stage (or Sangamon interglacial) is the term used in North America to designate the last interglacial period. In its most common usage, it is used for the period of time between 75,000 and 125,000 BP.Willman, H.B., and J.C. Frye, 1 ...
interstadial and the Kaʻena highstand to an interglacial 600,000 ± 100,000 years ago.
First dating efforts yielded ages of Pleistocene-Holocene based on
sea level
Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
variations, while
potassium-argon dating has yielded ages ranging between 800,000 and 60,000 years old. However, the presence of excess radiogenic
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
due to the xenoliths makes dates obtained by potassium-argon dating unreliable and dates older than 800,000 years are especially questionable.
Argon-argon dating has been applied to submarine vents of the Honolulu Volcanics and has yielded ages of 700,000 to 400,000 years for submarine vents northeast of Oʻahu while the southwestern submarine vents have ages clustering around 140,000 ± 50,000 years, in line with the ages of the Koko rift.
Volcanic activity occurred in two pulses, one between 800,000 and 250,000 years ago and the other in the last 120,000 years, with a hiatus in between the two pulses. Sometimes the vents of the Sugar Loaf, Tantalus Peak and Koko Rift area are classified separately from the other vents of the Honolulu Volcanics as they are usually less than 100,000 years old. Volcanic eruptions in the Koko Rift occurred between 100,000 and 60,000 years ago; it is not clear whether Koko Rift or Tantalus Rift erupted last, as ages of 35,000 and 85,000 years before present have been obtained on the Koko Rift while the Tantalus Rift has yielded ages of 76,000 ± 1,000 years ago. The average recurrence interval for eruptions in the Honolulu Volcanics is about 35,000 years assuming that the younger ages for the Koko Rift are correct.
Eruption characteristics
Many eruptions were highly
explosive
An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...
, probably due to the magma being rich in gases, and threw tephra high into the air. When the rising magma intercepted water—especially close to the coast—
steam explosion
A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, occurring when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten m ...
s ensued which threw up rock debris including older rocks such as coral fragments. Lava flows were erupted mainly during the late stage of the eruption and were typically of small dimensions; the longest flow is long and the largest one at Kaimukī forms a lava shield or
lava dome
In volcanology, a lava dome is a circular mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on ...
.
Cinder cones form from the fallout of volcanic ejecta material, which is variously described as
cinder
Cinder is an alternate term for scoria.
Cinder or Cinders may also refer to:
In computing
*Cinder (programming library), a C++ programming library for visualization
*Cinder, OpenStack's block storage component
* Cyber Insider Threat, CINDER, a ...
,
lava bomb
A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...
s,
pumice
Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...
and spatter and forms layered deposits. Tuff cones in addition can also contain non-magmatic ejecta. Many of these eruptions, especially that of Diamond Head, probably only lasted for a short time such as a few hours, as has been observed in similar eruptions during historical time.
The field has also seen many
Surtseyan or
phreatomagmatic
Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...
eruptions, especially at nearshore vents, when ascending magma encountered water, such as at Mānana Island, Koko and Punchbowl; these eruptions were explosive and they are the best exposed site of Surtseyan activity in the United States. Some formed small islands when they emerged, and one vent may now be buried beneath coral deposits. At Koko Crater and Diamond Head, the amount of water interacting with the developing volcano varied over the course of the eruption, as more water entered the vent when the eruption enlarged it.
Volcanic rocks of Honolulu Volcanics include
agglomerate
Agglomerate (from the Latin ''agglomerare'' meaning "to form into a ball") is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records flui ...
s,
breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
, breccia-tuffs,
scoria
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...
bombs and tuffs. Tephra takes the form of both
lapilli
Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones".
By definition lapilli range f ...
and
volcanic ash
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...
and some eruptions were followed by
lahar
A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley.
Lahars are extreme ...
s when volcanic ash was swept up by streams; one such deposit is found in Pālolo Valley. During eruptions,
trade wind
The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisph ...
s blew material from the vent; this wind-driven material transport is responsible for the frequently asymmetric shape of the volcanoes such as at Diamond Head where the southeastern rim is the highest part of the crater rim.
Drill core
A modern core drill is a drill specifically designed to remove a cylinder of material, much like a hole saw. The material left inside the drill bit is referred to as the ''core''.
Core drills used in metal are called annular cutters. Core dri ...
s in Hanauma Bay have found ash layers; a black ash known as "black sand" is widespread across Honolulu and originated in volcanic eruptions of the Honolulu Volcanics, and ash layers cover coral reef deposis both onshore and offshore as far as
ʻEwa Beach.
Ash from the Salt Lake Craters was transported by winds to Pearl Harbor and is considered to be responsible for the formation of the
bay
A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean, a lake, or another bay. A large bay is usually called a Gulf (geography), gulf, sea, sound (geography), sound, or bight (geogra ...
by closing off its access to the sea.
Activity at specific vents:
* At Diamond Head, eruptions commenced underwater and first deposited white rocks formed mostly by reworked corals. Proper tuff layers were emplaced on top of this unit.
* The formation of Kaʻau Crater was also accompanied by
phreatic
''Phreatic'' is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption.
Hydrology
The term phreatic (the word originates from the Greek , meaning "well" ...
activity that deposited alluvial tuffs. Some eruptions – such as those of the Kaʻau, Mōkōlea and Training School vents – may have occurred as one sequence over a long line.
* Intense explosive eruptions took place when magma erupted underwater, forming Hanauma Bay.
The eruption took place in several stages; pauses between these stages lasted no more than a few months, and erosion was already underway during the eruption. Hanauma Bay was colonized by coral reefs after it formed, and
marine benches developed within the bay; the exact origin of these benches is often not clear. The eruption of Kahauloa was synchronous to that of Hanauma Bay.
* Koko Head was formed by the Koko Tuff. Later
explosion crater
An explosion crater is a type of wikt:crater, crater formed when material is ejected from the surface of the ground by an explosive event at or immediately above or below the surface.
A crater is formed by an explosive event through the displacem ...
s, such as Hanauma Bay, developed within Koko Head. Koko Crater was also the source of an ash layer that covers the surrounding terrain, and of
pyroclastic
Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...
density flows.
After Koko Head had formed, a second volcanic explosion generated another crater on its eastern foot and filled
gullies
A gully is a landform created by running water, mass movement, or commonly a combination of both eroding sharply into soil or other relatively erodible material, typically on a hillside or in river floodplains or terraces. Gullies resemble lar ...
which had been carved into Koko Head by erosion.
* Punchbowl Crater formed from the fallout of an
eruption column
An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air a ...
. Material from the column fell onto the coral plain where the vent was located, forming the crater.
* The formation of the Sugarloaf flow was accompanied by tephra fallout. The tephra reached over thickness away in what is now downtown Honolulu, landing on a limestone 123,000 ± 2,000 years old. The flow was erupted on a ridge, west from the Manoa Valley. It is a thick
ʻaʻā
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
flow, an unusual trait given its composition which resembles that of
Nyiragongo
Mount Nyiragongo ( ) is an active stratovolcano with an elevation of in the Virunga Mountains associated with the Albertine Rift. It is located inside Virunga National Park, in the Democratic Republic of the Congo, about north of the town of ...
volcano which produces fast flowing lava flows. The eruption of the Tantalus vent was apparently synchronous to that of Sugarloaf; both eruptions were highly explosive due to a high volatile content of the magmas, shedding ash over a large area.
* Ulupaʻu Head crater contained a
lake
A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
, which was once the largest lake of Hawaiʻi with a surface area of . It persisted for a long time during the Middle Pleistocene until the sea breached the crater rim. A number of
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s have been found in the lake deposits.
Rano Kau
Rano Kau is a tall dormant volcano that forms the southwestern headland of Easter Island, a Chilean island in the Pacific Ocean. It was formed of basaltic lava flows in the Pleistocene with its youngest rocks dated at between 150,000 and 210,0 ...
on
Easter Island
Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...
resembles the former Ulupaʻu Head lake.
Many vents of the Honolulu Volcanics are furrowed;
erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...
has cut gullies in the slopes of Diamond Head and Punchbowl Crater.
Wavecut terraces formed in some volcanoes during sea level highstands; it is likely that
wave erosion
Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward ...
breached Hanauma Bay, flooding it,
either during or after the eruption that created it.
Youngest activity and hazards
The youngest eruptions of the whole field took place either 30,000 years ago or 76,000 years ago and there is no evidence in oral tradition for eruptions during historical times. Some of the youngest volcanics of Honolulu Volcanics were once considered 5,000 years old,
with an age of 7,000 years attributed to a volcanic event at Hanauma Bay
and another of 10,000 years to the Kaupō flow. However,
radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
has not yielded ages younger than 31,000–43,000 years old,
and most age estimates of less than 30,000 years ago are misinterpreted.
Future eruptions of Honolulu Volcanics are possible, but the likelihood of a new event in the next hundred or thousand years is considered to be so small as to be negligible; it is probably comparable to the
Kohala peninsula on Hawaiʻi, the lowest risk area of the active island. Additionally, only small areas of Oʻahu are likely to be affected by an eruption.
Any future eruption is likely to occur in the southeastern sector of Oʻahu and will be of small volume, involving the emplacement of cinders, lava flows and mudflows with characteristics similar to those of past Honolulu Volcanics eruptions:
* Tephra fall would be drawn by winds in a most likely southwestward direction, and could depending on the vent location fall over downtown Honolulu and Pearl Harbor; known dangers of tephra falls are collapsing roofs, pollution of the air and of water resources, impaired visibility and damage to machinery and vegetation. Close to the vent
volcanic gas
Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from lava, from volcanic ...
es and falling lava bombs would constitute additional dangers. Tephra deposits can be swept away by
rainfall
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
, forming mudflows which owing to their high speed and density can be hazardous.
* Steam explosions could generate
lateral blast
A lateral eruption or lateral blast is a volcanic eruption which is directed laterally from a volcano rather than upwards from the summit. Lateral eruptions are caused by the outward expansion of flanks due to rising magma. Breaking occurs at the ...
s, which can spread to distances of from vents at high speed and transport dangerous heat and debris. Such lateral blasts may have occurred when Diamond Head, Koko Head and Punchbowl formed.
* Lava flows can cause severe property damage as they can destroy structures in their way, but owing to their slowness are seldom a threat to life; additionally a lava flow would most likely be preceded by other eruptive phenomena which would drive people away from the hazard zone before a lava flow can become a threat.
Geothermal power prospecting
The Mokapu Peninsula has been prospected for the presence of
geothermal power
Geothermal power is electrical power generated from geothermal energy. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. Geothermal electricity generation is currently used in 2 ...
resources, but the presence of such resources was judged to be unlikely. Short-lasting volcanism typical for Honolulu Volcanics is unlikely to leave remnant heat resources. The only anomalously warm springs in the area are found within the Koʻolau caldera and are related to that volcanic system rather than the Honolulu Volcanics. Some evidence of geothermal activity has been found elsewhere on Oʻahu.
List of vents
Notes
References
Sources
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{{Hawaiian volcanism
Volcanoes of Oahu
Pleistocene volcanoes
Honolulu