In geology, a chonolith is a type of

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

intrusion (also known as pluton). Igneous rock intrusions are bodies of

that are formed by the crystallization of cooled

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 ...

below the Earth’s surface. These formations are termed intrusive rocks due the magma intruding rock layers but never reaching the earth’s surface. However, sometimes portions of plutons can become exposed at the Earth’s surface and thus the minerals can be observed since they are large enough. The different plutonic formations are named based on the different shapes that the cooled crystallized magma takes. However, all plutonic formations that have irregular shapes and do not share the same characteristics as other plutonic structures are termed chonoliths. Other plutonic structures that have specific shapes include: dikes, sills,

laccolith A laccolith is a body of intrusive rock with a dome-shaped upper surface and a level base, fed by a conduit from below. A laccolith forms when magma (molten rock) rising through the Earth's crust begins to spread out horizontally, prying apar ...

s and sheets. Another unique characteristic of chonoliths is that there is a floor or base present which is typically absent in other types of intrusions.

How they are formed

The rocks on Earth’s surface are continuously subject to different types of forces, including tensional, compressional and

shear stress Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. ''Normal stress'', on the ...

. When these rocks are under this kind of stress, they are forced to deform causing them to take on different types of shapes. For example, as

tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...

are pushed together, rocks undergo compressional stress due to the shortening and thickening of the crust thus forming mountain formations. Whereas, tensional stress due to tectonic plates being pulled apart causes rock formations to become faulted creating

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear d ...

s. In the occurrence of different types of rock deformations, there are oftentimes cavities left behind. The

that rises out of the lithosphere, the rigid outermost shell of the Earth, fills these cavities up from either below, the side or from above. This magma then slowly cools down since the surrounding rocks act as an insulator. Once it completely cools down and crystallizes it turns into a large igneous body. This process can often take tens of thousands years to complete due to the slow cooling of the magma. Igneous rock bodies can be injected either passively by filling in spaces between rocks or actively by forcing apart rock formations and filling it in. The plutons are then classified based on the shape and characteristics it has. Any type of igneous rock intrusion that does not have a shape that can be classified as the other classifications, including dikes, sills and laccoliths, and are irregular in shape are said to be chonoliths.

Examples of Chonoliths

The Spring Gulch chonolith composed of
aplite Aplite () is an intrusive igneous rock in which the mineral composition is the same as granite, but in which the grains are much finer, under 1 mm across. Quartz and feldspar are the dominant minerals. The term ''aplite'' or ''aplitic'' ...
, in Elko County in Nevada, USA.

The outcrops of the Spring Gulch reveal a complex shape and there is an exposure to Missipian shale. For this reason, this igneous rock intrusion is termed a chonolith. The Spring Gulch is composed of fine grained leucocratic granite. This is also known as aplite. Altogether, the Spring Gulch chonolith forms hypabyssal intrusive bodies which are mainly centered around the Spring Gulch. The granite found in the Spring Gulch chonolith is a two-mica granite, meaning that it is high in potassium and low in plagioclase. Due to its finely grained characteristic and its intense bleach, the Spring Gulch chonolith displays a prominent argillic alteration. The estimated age of the Spring Gulch chonolith is about 156.4±0.23 million years

The Nebo-Babel chonolith of
gabbronorite Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite, orthopyroxene, and olivine. The name ''norite'' is derived from ''Norge'', the Norwegian name for Norway. Norite also known as orthopyroxen ...
in the West Musgrave area of Western Australia.

Within the last 10 years, the Nebo-Babel chonolith became the largest nickel sulfide discovery in the West Musgrave Block, Western Australia. This pluton is a large Ni-Cu-platinum-group element (PGE) sulphide deposit and is mainly made up of gabbronorite. This specific chonolith has a tube-like shape and extends for about 5 km

Chonoliths in Norilsk, Russia.

The Siberian continental flood basalt (CFB) province was formed at the Permo-Triassic boundary when a superplume was surrounded by subducting oceanic crust slabs. It is thought that the igneous rock intrusions and the flood basalt events lasted around 2 to 4 million years. There are substantial Cu-Ni-PGE sulfide deposits found within the mafic intrusive igneous rock formations Summit_of_Mount_Shavano

located under the flood basalts. This specific pluton is 20km in length.

Chonolith of
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 ...
of
Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological 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 finally confirmed in ...
age in Chinkuashih, Taiwan.

The chonolith found in Chinkuashih, Taiwan, dates back to the Pleistocene age. It is thought that this specific chonolith intruded the water-saturated Miocene sediments. This pluton is composed of dacite, a type of volcanic rock that is formed by lava. Ore can be found in the altered argillized dacite. As there is already a copper-gold producing mine located in this area, the chonolith provides a good source of arsenical copper-gold ores.

Chonolith of
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
age in the Cortez Mountains, near Carlin in Nevada, USA.

Chonolith of
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 predo ...
near Shavano in Colorado, USA.

{{Cite journal, last=Kantor, first=Tedral, date=1961-01-01, title=Geology of the east-central portion of the Nelson quadrangle, Clark County, Nevada, url=https://scholarsmine.mst.edu/masters_theses/2763, journal=Masters Theses

References

Igneous rocks Igneous petrology Igneous intrusions

How they are formed

Examples of Chonoliths

Chonoliths in Norilsk, Russia.

Chonolith of 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 predo ... near Shavano in Colorado, USA.

See also

References

Chonolith of
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 predo ...
near Shavano in Colorado, USA.