A laccolith is a body of
intrusive rock
Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: Marc ...
with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when
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 natura ...
(molten rock) rising through the
Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
begins to spread out horizontally, prying apart the host rock
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...
. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith its dome-like form.
Over time,
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 di ...
can expose the solidified laccolith, which is typically more resistant to
weathering
Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...
than the host rock. The exposed laccolith then forms a hill or mountain. The
Henry Mountains of
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
, US, are an example of a mountain range composed of exposed laccoliths. It was here that geologist
Grove Karl Gilbert carried out pioneering
field work
Field research, field studies, or fieldwork is the collection of raw data outside a laboratory, library, or workplace setting. The approaches and methods used in field research vary across disciplines. For example, biologists who conduc ...
on this type of
intrusion. Laccolith mountains have since been identified in many other parts of the world.
Description
A laccolith is a type of
igneous intrusion
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
, formed when
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 natura ...
forces its way upwards through the
Earth's crust
Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
but cools and solidifies before reaching the surface. Laccoliths are distinguished from other igneous intrusions by their dome-shaped upper surface and level base. They are assumed to be fed by a conduit from below, though this is rarely exposed.
When the host rock is volcanic, the laccolith is referred to as a
cryptodome. Laccoliths form only at relatively shallow depth in the crust, usually from
intermediate composition magma, though laccoliths of all compositions from
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...
-poor
basalt
Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90% of a ...
to silica-rich
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained ( aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The min ...
are known.
A laccolith forms after an initial
sheet-like intrusion has been injected between layers of
sedimentary rock
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...
. If the intrusion remains limited in size, it forms a
sill
Sill may refer to:
* Sill (dock), a weir at the low water mark retaining water within a dock
* Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma
* Sill (geostatistics)
* Sill (river), a river in Austria
* Sill plate, a ...
, in which the strata above and below the intrusion remain parallel to each other and the intrusion remains sheetlike. The intrusion begins to lift and dome the overlying strata only if the radius of the intrusion exceeds a critical radius, which is roughly:
:
where
is the pressure of the magma,
is the
lithostatic pressure (weight of the overlying rock),
is the thickness of the overlying rocks, and
is the shear strength of the overlying rock. For example, in the
Henry Mountains of
Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
, US, the geologist
Grove Karl Gilbert found in 1877 that sills were always less than in area while laccoliths were always greater than 1 square kilometer in area. From this, Gilbert concluded that sills were forerunners of laccoliths. Laccoliths formed from sills only when they became large enough for the pressure of 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 natura ...
to force the overlying strata to dome upwards. Gilbert also determined that larger laccoliths formed at greater depth.
[ Both laccoliths and sills are classified as ''concordant'' intrusions, since the bulk of the intrusion does not cut across host rock strata, but intrudes between strata.
More recent study of laccoliths has confirmed Gilbert's basic conclusions, while refining the details. Both sills and laccoliths have blunt rather than wedgelike edges, and sills of the Henry Mountains are typically up to thick while laccoliths are up to thick. The periphery of a laccolith may be smooth, but it may also have fingerlike projections consistent with Rayleigh-Taylor instability of the magma pushing along the strata. An example of a fingered laccolith is the Shonkin Sag laccolith in ]Montana
Montana () is a U.S. state, state in the Mountain states, Mountain West List of regions of the United States#Census Bureau-designated regions and divisions, division of the Western United States. It is bordered by Idaho to the west, North ...
, US. The critical radius for the sill to laccolith transition is now thought to be affected the viscosity of the magma (being greater for less viscous magma) as well as the strength of the host rock. A modern formula for the shape of a laccolith is:
:
where is the height of the laccolith roof, is the acceleration of gravity, is he elastic modulus of the hos rock, is the horizontal distance from the center of the laccolith, and is the ouer radius of the laccolith. Because of their greater thickness, which slows the cooling rate, the rock of laccoliths is usually coarser-grained than the rock of sills.
The growth of laccoliths can take as little as a few months when associated with a single magma injection event, or up to hundreds or thousands of years by multiple magmatic pulses stacking sills on top of each other and deforming the host rock incrementally.
Over time, erosion can form small hills and even mountains around a central peak since the intrusive rock is usually more resistant to weathering than the host rock. Because the emplacement of the laccolith domes up the overlying beds, local topographic relief
Terrain or relief (also topographical relief) involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin w ...
is increased and erosion is accelerated, so that the overlying beds are eroded away to expose the intrusive cores.
Etymology
The term was first applied as laccolite by Gilbert after his study of intrusions of diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...
in the Henry Mountains of Utah
Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
in about 1875. The word ''laccolith'' derived in 1875—1880, from Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
, ''lákko(s)'', meaning "pond", plus ''-lith'', meaning "stone".
Where laccoliths form
Laccoliths tend to form at relatively shallow depths and in some cases are formed by relatively viscous magmas, such as those that crystallize to diorite
Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...
, granodiorite, and granite
Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies und ...
. In those cases cooling underground may take place slowly, giving time for larger crystals to form in the cooling magma. In other cases less viscous magma such as shonkinite may form phenocrysts of augite at depth, then inject through a vertical feeder dike that ends in a laccolith.["Beall, Joseph J." "Pseudo-Rhythmic Layering in the Square Butte Alkali-Gabbro Laccolith." ''American Mineralogist.'' 57:7-8 (July–August 1972).]
Sheet intrusions tend to form perpendicular to the direction of least stress in the country rock they intrude. Thus laccoliths are characteristic of regions where the crust is being compressed and the direction of least stress is vertical, while areas where the crust is in tension are more likely to form dikes, since the direction of least stress is then horizontal. For example, the laccoliths of the Ortiz porphyry belt
The Ortiz porphyry belt is a cluster of small mountain ranges in Santa Fe County, New Mexico. The mountains are laccoliths formed by intrusion of magma into the upper layers of the Earth's crust. This took place during the late Eocene through ea ...
in New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe, New Mexico, Santa Fe
, LargestCity = Albuquerque, New Mexico, Albuquerque
, LargestMetro = Albuquerque metropolitan area, Tiguex
, Offi ...
likely formed during Laramide compression of the region 33 to 36 million years ago. When Laramide compression was later replaced by extension, emplacement of sills and laccoliths was replaced by emplacement of dikes. Dating of the intrusions has helped determine the point in geologic time when compression was replaced with extension.
Examples
In addition to the Henry Mountains, laccolith mountains are found on the Colorado Plateau in the La Sal Mountains
The La Sal Mountains or La Sal Range are a mountain range located in Grand and San Juan counties in the U.S. state of Utah, along the border with Colorado. The range rises above and southeast of Moab and north of the town of La Sal. This range ...
and Abajo Mountains.
The filled and solidified magma chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...
of Torres del Paine (Patagonia
Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and ...
) is one of the best exposed laccoliths, built up incrementally by horizontal granitic and mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks includ ...
magma intrusions over 162 ± 11 thousand years. Horizontal sheeted intrusions were fed by vertical intrusions.
The small Barber Hill
A barber is a person whose occupation is mainly to cut, dress, groom, style and shave men's and boys' hair or beards. A barber's place of work is known as a "barbershop" or a "barber's". Barbershops are also places of social interaction and publi ...
syenite
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proport ...
-stock laccolith in Charlotte, Vermont
Charlotte is a town in Chittenden County, Vermont, United States. The town was named for Queen Charlotte, though unlike Charlotte, North Carolina, Charlottesville, Virginia, and other cities and towns that bear her name, the town's name is pronou ...
, has several volcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposed laccolith. In Big Bend Ranch State Park, at the southwesternmost visible extent of the Ouachita orogeny
The Ouachita orogeny was a mountain-building event that resulted in the folding and faulting of strata currently exposed in the Ouachita Mountains. The more extensive Ouachita system extends from the current range in Arkansas and Oklahoma so ...
, lies the Solitario. It consists of the eroded remains of a laccolith, presumably named for the sense of solitude that observers within the structure might have, due to the partial illusion
An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people.
Illusions may ...
of endless expanse in all directions.
One of the largest laccoliths in the United States is Pine Valley Mountain in the Pine Valley Mountain Wilderness area near St. George, Utah.
A system of laccoliths is exposed at Elba
Elba ( it, isola d'Elba, ; la, Ilva) is a Mediterranean island in Tuscany, Italy, from the coastal town of Piombino on the Italian mainland, and the largest island of the Tuscan Archipelago. It is also part of the Arcipelago Toscano Nation ...
, which form a "Christmas tree" laccolith system in which a single igneous plumbing system has produced multiple laccoliths at different levels in the crust.
Problems reconstructing shapes of intrusions
The original shape of intrusions can be difficult to reconstruct. For instance, Devils Tower in Wyoming
Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the sou ...
and Needle Rock in Colorado
Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...
were both thought to be volcanic necks, but further study has suggested they are eroded laccoliths. At Devils Tower, intrusion would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite porphyry seen today. However, erosion has stripped away the overlying and surrounding rock, so it is impossible to reconstruct the original shape of the igneous intrusion, which may or may not be the remnant of a laccolith. At other localities, such as in the Henry Mountains and other isolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have the classic shapes of laccoliths.
Extraterrestrial laccoliths
There are many examples of possible laccoliths on the surface of the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
. Some are centered in impact crater
An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...
s and may form as part of the post-impact evolution of the crater. Others are located along possible faults or fissures. Laccoliths on the Moon are much wider but less thick than those on Earth, due to the Moon's lower gravity and more fluid magmatism.
Possible laccoliths have also been identified on Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin atmos ...
, in western Arcadia Planitia.
Gallery
See also
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References
Further reading
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External links
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Petrology
Igneous rocks
Sheet intrusions
Geologic domes
Volcanic landforms