![Geologic map of Guam (cropped)](https://upload.wikimedia.org/wikipedia/commons/3/38/Geologic_map_of_Guam_%28cropped%29.jpg)
The geology of Guam formed as a result of
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 include ...
,
felsic
In geology, felsic is a modifier describing igneous rocks that are relatively rich in elements that form feldspar and quartz.Marshak, Stephen, 2009, ''Essentials of Geology,'' W. W. Norton & Company, 3rd ed. It is contrasted with mafic rocks, whi ...
and
intermediate composition In igneous petrology, an intermediate composition refers to the chemical composition of a rock that has 5263 wt% SiO2 being an intermediate between felsic and mafic compositions. Typical intermediate rocks include andesite, dacite, and trachyandes ...
volcanic rocks erupting below the ocean, building up the base of the island in the
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
, between 33.9 and 56 million years ago. The island emerged above the water in the Eocene, although the volcanic crater collapsed. A second volcanic crater formed on the south of the island in the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
and
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 ...
. In the shallow water, numerous limestone formations took shape, with thick alternating layers of volcanic material. The second crater collapsed and Guam went through a period in which it was almost entirely submerged, resembling a swampy atoll, until structural deformation slowly uplifted different parts of the island to their present topography. The process of uplift led to widespread erosion and clay formation, as well as the deposition of different types of limestone, reflecting different water depths.
Stratigraphy & Geologic History
Guam began to form in the late middle
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
with the development of the Facpi Formation. The Facpi Formation was associated with submarine volcanism, evidenced by pillow basalt in flow rocks from that series. The Facpi Formation forms the basement volcanic rock of Guam and was deposited when the island was formed in close proximity to the Palau-Kyushu Ridge.
The second major volcanic episode is associated with the Alutom Formation which was formed in the late Eocene. The formation contains
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 ...
and
andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomi ...
, as well as volcanic
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 ...
with component
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
pebbles and cobbles and
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 ...
shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
cemented together with
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 ...
. The presence of the
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
pebbles and correlation with dacitic lava flows on
Saipan
Saipan ( ch, Sa’ipan, cal, Seipél, formerly in es, Saipán, and in ja, 彩帆島, Saipan-tō) is the largest island of the Northern Mariana Islands, a Commonwealth (U.S. insular area), commonwealth of the United States in the western Pa ...
, dacite flows likely preceded andesite eruptions on Guam. Although the Alutom Formation does not contain many carbonates besides the calcite, the presence of limestone fragments in upper units suggests that limestone formations existed close by.
Stabile shallow water reef-forming environments by the end of the Eocene gave way to violent volcanic eruptions in the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
. The island's central caldera collapsed as a result of uplift and faulting.
In the
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
, the Tenjo structural block formed as
Eocene
The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...
volcano eroded.
Miocene (23.3-5.3 million years ago)
A spur of Alutom volcanic rocks forms the
Mount Alifan
Mount Alifan is a peak on the U.S. island territory of Guam. It is located in Agat on the south-west of the island. Alifan is the highest point overseeing Agat Bay and was the site of fierce fighting during the 1944 U.S. invasion of Guam after fo ...
-
Mount Lamlam
Mount Lamlam (meaning ''lightning'' in Chamoru) is a peak on the United States island of Guam. It is located near the village of Agat ( north), in the south-west of the island.
Rising to above sea level, the distance from the peak to the bottom ...
ridge was surrounded on all sides by major lava flows in the early
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 ...
that formed the
Umatac Formation. The formation was first dated using foraminifera fossils in a layer of limestone. Pyroclastic breccias and conglomerates formed in conjunction with the new eruptions.
Eruptions resumed after the deposition of the limestone, generating hundreds of feet of tuff-breccia and volcanic conglomerate, interspersed with periodic lava flows. In fact, the calcareous lenses of the Maemong limestone, which hold the oldest Miocene fossils are underlain by sequences of
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 ...
hundreds of feet thick. Minor faulting complicated the formation of the Facpi volcanic member of the Umatac Formation. Quiet periods in between eruptions allowed the deposition of limestone, from coral reefs, that are now isolated pieces of the Maemong limestone north of the Talofofo River. Higher parts of the Tenjo block were probably eroded, given the presence of early Miocene reefs in central Guam. The Dandan volcanic flow is the final rock unit of the Umatac Formation.
The Bonya limestone overlies the Dandan member on the eastern coast. Parts of the island experienced underwater erosion as they began to uplift above the water surface. When they were exposed to rainfall, they rapidly weathered to clay. Like the Eocene caldera before it, the Miocene volcano collapsed, probably in connection with formation of the Dandan member.
The Alifan limestone began to deposit after the Bonya limestone as Guam became almost entirely submerged, resembling an atoll.
Montmorillinite clay and
lignite
Lignite, often referred to as brown coal, is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat ...
fragments accumulated in the higher elevation swamps near the shore. The current limestone ridge on the Mount Alifan-Mount Lamlam ridge may represent the remnants of an atoll lagoon. In north Guam, there are no early Miocene rocks. A period of uplift and erosion wore down the Mount Santa Rosa volcanic hills means that the Bonya limestone is overlain by the deep water Janum argillaceous limestone.
Pliocene-Holocene (5.33 million years ago-present)
The
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58[laterite
Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content. They develop by ...](_blank)
weathering of underlying volcanic rocks. The Barrigada limestone in the north was uplifted and eroded as well, but parts of the north were resubmerged by the movement of the
Adelup fault forming lagoon coral deposits around the Barrigada limestone. The deposition of the wave-break corals of the Mariana limestone continued through the
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 ...
. Numerous terraces formed around the island in the middle and late Pleistocene.
Hydrogeology
![Water levels in selected wells, Guam (cropped)](https://upload.wikimedia.org/wikipedia/commons/7/7d/Water_levels_in_selected_wells%2C_Guam_%28cropped%29.jpg)
Groundwater supplies about 80% of the island's freshwater needs, with greater reliance on surface water in the south. Streams are only found in southern Guam, where volcanic rock with low-permeability slows the infiltration of rainwater into the ground. In the north, a limestone karst landscape quickly absorbs water. On Guam, freshwater forms a lens, with a transitional zone of brackish water to salt water underneath it.
Throughout the island, the freshwater lens is nearly flat, with water levels less than eight feet. In some cases, the lens extends into fractures in the volcanic rock, but for the most part, it is contained in limestone in the north. Minor areas of perched water occur in limestone, atop nearly impermeable volcanic rocks at different points.
The island has some ability to extract groundwater for drinking water and other uses, but there is a constant risk of saltwater intrusion. Well measurements indicate that further inland, thicker rocks moderate the effect of tide cycles in wells, such that wells within one mile of the coast had a range of half a foot, which dropped to 0.05 feet further inland. The
Guam Waterworks Authority,
Guam Environmental Protection Agency
The Guam Environmental Protection Agency (Guam EPA, ch, Ahensian Pruteksion Lina'La Guahan) is a government agency of the United States territory of Guam.
History
Acting Gov. Kurt S. Moylan Signed Public Law 11-191 on Dec. 7, 1972 that enabled ...
, the Air Force and the Navy manage the 180 wells throughout the island. A combination of high-permeability rock, over pumping and overcrowding of wells resulted in 32% of wells (measured in data from the early 1960s through 1997) exceeding a guideline of 250 mg/L chloride concentration.
References
{{Oceania topic, Geology of
+Guam
Geography of Guam