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Earths Crust: outermost shell of a terrestrial planet

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Crust

Earths crust The crust is the outermost layer of Earth.

Earth’s Crust

For upon |“Crust” describes the outermost shell of a terrestrial planet. Earth’s crust is generally divided into older, thicker continental crust and younger, denser oceanic crust. The dynamic geology of Earth’s crust is informed by plate tectonics.

Earth’s Size

The brittle crust is just one of Earth’s three major layers. The thin, 40-kilometer (25-mile) deep crust—just 1% of Earth’s mass—contains all known life in the universe.

Earth’s Chemicals

The most abundant chemicals in Earth’s crust are silicates—minerals with high silicon and oxygen levels.

Earth’s Movement

Islands, ocean trenches, volcanoes, and mid-ocean ridges are all formed by the movement of molten material between the Earth’s mantle and crust.

A More Complicated Look

Oceanic Crust

The oceanic crust is constantly formed by seafloor spreading at mid-ocean ridges, where tectonic plates are tearing apart from each other.  The age and density of oceanic crust increase from sites of seafloor spreading.

Pillow Lava

The rocks of the oceanic crust are nicknamed “sima” for their most characteristic elements (silicon and magnesium). Basalts, like this pillow lava near the Galapagos Islands, are the most abundant sima rocks.

Mariana Trench

Just as the oceanic crust is formed at mid-ocean ridges, it is destroyed in subduction zones. As oceanic crust sinks into the mantle, and maybe ultimately “recycled” as it surfaces again as crust-making lava at mid-ocean ridges and volcanoes. Largely due to subduction, oceanic crust is much, much younger than continental crust.

Ophiolite

Oceanic crust is often studied through ophiolites, remnants of oceanic crust forced above sea level by tectonic activity. This pillow lava, formed by tectonic activity on the seafloor billions of years ago, is now a part of the French Alps.

Continental Crust

The oceanic crust is created by divergent plate boundaries. Continental crust is created by convergent plate boundaries, where plates crash into each other. The crustal movement here is often associated with orogeny, the process of mountain-building. The massive Himalayas and Andes mountain ranges are the thickest and oldest parts of our continental crust.

Granite

The rocks of continental crust are nicknamed “sial,” after their most characteristic elements (silicon and aluminum). Granites, like these sheets in Yosemite National Park, California, are the most abundant seal rocks.

Tvashtar

All terrestrial planets and moons have a crustal upper layer. The crusts of most planets and moons in our solar system lack tectonic activity and are much more brittle and rigid than Earth’s. Io, a moon of Jupiter, is an exception: Io is the most volcanically active celestial body in the solar system. This volcano, Tvashtar, is ejecting material 200 miles (322 kilometers) into Io’s atmosphere.

“Crust” describes the outermost shell of a terrestrial planet. Our planet’s thin, 40-kilometer (25-mile) deep crust—just 1% of Earth’s mass—contains all known life in the universe.

Earth has three layers: the crust, the mantle, and the core. The crust is made of solid rocks and minerals. Beneath the crust is the mantle, which is also mostly solid rocks and minerals but punctuated by malleable areas of semi-solid magma. At the center of the Earth is hot, dense metalcore.
Earth’s layers constantly interact with each other, and the crust and upper portion of the mantle are part of a single geologic unit called the lithosphere. The lithosphere’s depth varies, and the Mohorovicic discontinuity (the Moho)—the boundary between the mantle and crust—does not exist at a uniform depth. Isostasy describes the physical, chemical, and mechanical differences between the mantle and crust that allow the crust to “float” on the more malleable mantle. Not all regions of Earth are balanced in isostatic equilibrium. Isostatic equilibrium depends on the density and thickness of the crust and the dynamic forces at work in the mantle.
Just as the depth of the crust varies, so does its temperature. The upper crust withstands the ambient temperature of the atmosphere or ocean—hot in arid deserts and freezing in ocean trenches. Near the Moho, the temperature of the crust ranges from 200° Celsius (392° Fahrenheit) to 400° Celsius (752° Fahrenheit).

Crafting the Crust

Billions of years ago, the planetary blob that would become the Earth started out as a hot, viscous ball of rock. The heaviest material, mostly iron, and nickel sank to the center of the new planet and became its core. The molten material that surrounded the core was the early mantle.
Over millions of years, the mantle cooled. Water trapped inside minerals erupted with lava, a process called “outgassing.” As more water was outgassed, the mantle solidified. Materials that initially stayed in their liquid phase during this process, called “incompatible elements,” ultimately became Earth’s brittle crust.
From mud and clay to diamonds and coal, Earth’s crust is composed of igneous, metamorphic, and sedimentary rocks. The most abundant rocks in the crust are igneous, which are formed by the cooling of magma. Earth’s crust is rich in igneous rocks such as granite and basalt. Metamorphic rocks have undergone drastic changes due to heat and pressure. Slate and marble are familiar metamorphic rocks. Sedimentary rocks are formed by the accumulation of material at the Earth’s surface. Sandstone and shale are sedimentary rocks.
Dynamic geologic forces created Earths crust, and the crust continues to be shaped by the planet’s movement and energy. Today, tectonic activity is responsible for the formation (and destruction) of crustal materials.
Earths crust is divided into two types: oceanic crust and continental crust. The transition zone between these two types of crust is sometimes called the Conrad discontinuity. Silicates (mostly compounds made of silicon and oxygen) are the most abundant rocks and minerals in both oceanic and continental crusts.

Oceanic Crust

The oceanic crust, extending 5-10 kilometers (3-6 kilometers) beneath the ocean floor, is mostly composed of different types of basalts. Geologists often refer to the rocks of the oceanic crust as “sima.” Sima stands for silicate and magnesium, the most abundant minerals in the oceanic crust. (Basalts are sima rocks.) The oceanic crust is dense, almost 3 grams per cubic centimeter (1.7 ounces per cubic inch).
The oceanic crust is constantly formed at mid-ocean ridges, where tectonic plates are tearing apart from each other. As magma that wells up from these rifts in Earth’s surface cools, it becomes a young oceanic crust. The age and density of oceanic crust increasing with distance from mid-ocean ridges.
Just as the oceanic crust is formed at mid-ocean ridges, it is destroyed in subduction zones. Subduction is the important geologic process in which a tectonic plate made of dense lithospheric material melts or falls below a plate made of the less dense lithosphere at a convergent plate boundary.

At convergent plate boundaries between continental and oceanic lithosphere, the dense oceanic lithosphere (including the crust) always subducts beneath the continental. In the northwestern United States, for example, the oceanic Juan de Fuca plate subducts beneath the continental North American plate. At convergent boundaries between two plates carrying oceanic lithosphere, the denser (usually the larger and deeper ocean basin) subducts. In the Japan Trench, the dense Pacific plate subducts beneath the less-dense Okhotsk plate.
As the lithosphere subducts, it sinks into the mantle, becoming more plastic and ductile. Through mantle convection, the rich minerals of the mantle may be ultimately “recycled” as they surface as crust-making lava at mid-ocean ridges and volcanoes.
Largely due to subduction, oceanic crust is much, much younger than continental crust. The oldest existing oceanic crust is in the Ionian Sea, part of the eastern Mediterranean basin. The seafloor of the Ionian Sea is about 270 million years old. (The oldest parts of continental crust, on the other hand, are more than 4 billion years old.)
Geologists collect samples of the oceanic crust by drilling at the ocean floor, using submersibles, and studying ophiolites. Ophiolites are sections of oceanic crust that have been forced above sea level through tectonic activity, sometimes emerging as dikes in the continental crust. Ophiolites are often more accessible to scientists than oceanic crust at the bottom of the ocean.

Continental Crust

The continental crust is mostly composed of different types of granites. Geologists often refer to the rocks of the continental crust as “sial.” Sial stands for silicate and aluminum, the most abundant minerals in the continental crust. Sial can be much thicker than sima (as thick as 70 kilometers (44 miles)), but also slightly less dense (about 2.7 grams per cubic centimeter (1.6 ounces per cubic inch)).
As with oceanic crust, continental crust is created by plate tectonics. At convergent plate boundaries, where tectonic plates crash into each other, continental crust is thrust up in the process of orogeny, or mountain-building. For this reason, the thickest parts of continental crust are at the world’s tallest mountain ranges. Like icebergs, the tall peaks of the Himalayas and the Andes are only part of the region’s continental crust—the crust extends unevenly below the Earth as well as soaring into the atmosphere.
Cratons are the oldest and most stable part of the continental lithosphere. These parts of the continental crust are usually found deep in the interior of most continents. Cratons are divided into two categories. Shields are cratons in which the ancient basement rock crops out into the atmosphere. Platforms are cratons in which the basement rock is buried beneath overlying sediment. Both shields and platforms provide crucial information to geologists about Earth’s early history and formation.
The continental crust is almost always much older than the oceanic crust. Because continental crust is rarely destroyed and recycled in the process of subduction, some sections of continental crust are nearly as old as the Earth itself.

Extraterrestrial Crust

Our solar system’s other terrestrial planets (Mercury, Venus, and Mars) and even our own Moon have crusts. Like Earth, these extraterrestrial crusts are formed mostly by silicate minerals. Unlike Earth, however, the crusts of these celestial bodies are not shaped by the interaction of tectonic plates.
Despite the Moon’s smaller size, the lunar crust is thicker than the crust on Earth. Lunar crust is not a uniform thickness and in general tends to be thicker on the “far side,” which always faces away from Earth.
Although Mercury, Venus, and Mars are not thought to have tectonic plates, they do have dynamic geology. Venus, for instance, has a partly-molten mantle, but the Venusian crust lacks enough trapped water to make it as dynamic as Earths crust.
The crust of Mars, meanwhile, features the tallest mountains in the solar system. These mountains are actually extinct volcanoes formed as molten rock erupted in the same spot on the Martian surface over millions of years. Eruptions built up enormous mountains of iron-rich igneous rocks that give the Martian Earth crust its characteristic red hue.
One of the most volcanic Earth crusts in the solar system is that of Jupiter’s moon Io. The rich sulfide rocks in the Ionian Earths crust paint the moon a dappled collection of yellows, greens, reds, blacks, and whites.

crust

Earth’s Earths crust is made of young oceanic material and older, thicker continental material.

Map by USGS

Mining Temperature

The TauTona and Mponeng gold mines of South Africa are the deepest in the world, descending about 4 kilometers (2.5 miles) below the surface of the Earth. Although those are deep mines, they are shallow in Earths crust. Still, temperatures at the bottom of the mines can climb to 55° Celsius (131° Fahrenheit). A sophisticated air conditioning system lowers the temperature to allow miners to work.
Old Earth crust
The oldest rocks yet identified on Earth were discovered in the Jack Hills of Western Australia, part of the Yilgarn Craton, a shield formation. The Jack Hills zircons are about 4.4 billion years old. (The Earth itself is about 4.6 billion years old!)
Silicates, Silicates Everywhere
Silicate minerals, mostly feldspars, and quartz are the most abundant minerals in Earth’s rocky Earth crust.
The article was originally published here.

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