The most abundant rocks in the crust are igneous, which are formed by the cooling of magma. Metamorphic rock s have undergone drastic changes due to heat and pressure. Slate and marble are familiar metamorphic rocks. Sandstone and shale are sedimentary rocks. Today, tectonic activity is responsible for the formation and destruction of crustal materials. The transition zone between these two types of crust is sometimes called the Conrad discontinuity. Silicate s mostly compounds made of silicon and oxygen are the most abundant rocks and minerals in both oceanic and continental crust.
Oceanic crust , extending kilometers kilometers beneath the ocean floor, is mostly composed of different types of basalts. Basalts are a sima rocks. Oceanic crust is dense, almost 3 grams per cubic centimeter 1. Oceanic crust is constantly formed at mid-ocean ridge s, where tectonic plate s are tearing apart from each other. The age and density of oceanic crust increases with distance from mid-ocean ridges. Just as oceanic crust is formed at mid-ocean ridges, it is destroyed in subduction zone s.
Subduction is the important geologic process in which a tectonic plate made of dense lithospheric material melts or falls below a plate made of 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.
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 million years old. The oldest parts of continental crust, on the other hand, are more than 4 billion years old. Geologists collect samples of oceanic crust through drilling at the ocean floor, using submersible s, and studying ophiolites. Ophiolite s are sections of oceanic crust that have been forced above sea level through tectonic activity, sometimes emerging as dike s in continental crust.
Ophiolites are often more accessible to scientists than oceanic crust at the bottom of the ocean. Continental crust is mostly composed of different types of granites. Sial can be much thicker than sima as thick as 70 kilometers kilometers 44 miles , but also slightly less dense about 2.
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. Craton s 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. Shield s are cratons in which the ancient basement rock crops out into the atmosphere.
Platform s are cratons in which the basement rock is buried beneath overlying sediment. Continental crust is almost always much older than 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. 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 tectonic plates. Although Mercury, Venus, and Mars are not thought to have tectonic plates, they do have dynamic geology.
The crust of Mars, meanwhile, features the tallest mountains in the solar system. These mountains are actually extinct volcano es formed as molten rock erupt ed 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 crust its characteristic red hue. The rich sulfide rocks in the Ionian crust paint the moon a dappled collection of yellows, greens, reds, blacks, and whites.
Earth's crust is made of young oceanic material and older, thicker continental material. I hope this guide will walk you through the layers of the Earth, provide a general sense of our understanding and our current gaps. Keep in mind that this is an area of ongoing research and is likely to become more refined in the coming years and decades. During my second year at Edinburgh [] I attended Jameson's lectures on Geology and Zoology, but they were incredible dull.
The sole effect they produced on me was the determination never as long as I lived to read a book on Geology. The Earth has layers not unlike an onion and can be dissected to understand the physical and chemical properties of each layer and its influence on the rest of the Earth. Generally speaking, Earth has 4 layers:. When differentiating the layers, geologists lump subdivisions into two categories, either rheologically or chemically. Rheological differentiation speaks to the liquid state of rocks under tremendous pressure and temperature.
For instance, rock will respond very differently to strain under normal atmospheric temperatures and pressures as compared to fewer than thousands of kilometers of rock. If we subdivide the Earth based on rheology, we see the lithosphere, asthenosphere, mesosphere, outer core, and inner core. However, if we differentiate the layers based on chemical variations, we lump the layers into crust, mantle, outer core, and inner core.
To understand the difference in various portions of the mantle or outer versus inner core you must understand phase diagrams, which I will speak on below. The crust is what you and I live on and is by far the thinnest of the layers of earth. The thickness varies depending on where you are on earth, with oceanic crust being km and continental mountain ranges being up to km thick. Thin oceanic crust is denser than the thicker continental crust and therefore 'floats' lower in the mantle as compared to continental crust.
You will find some of the thinnest oceanic crust along mid ocean ridges where new crust is actively being formed. In comparison, when two continents collide as in the case of the India Plate and Eurasia Plate, you get some of the thickest sections of crust as it is crumpled together. The temperatures within Earth's crust will vary from air temperatures at the surface to approximately degrees Celsius in deeper sections.
At this temperature, you begin to melt rock and form the below-lying mantle. Geologists subdivide Earth's crust into different plates that move about in relation to one another.
Given that Earth's surface is mostly constant in area, you cannot make crust without destroying a comparable amount of crust. With convection of the underlying mantle, we see insertion of mantle magma along mid ocean ridges, constantly forming new oceanic crust. However, to make room for this, oceanic crust must subduct sink below continental crust.
Geologists have studied extensively the history of this plate movement, but we are sorely lacking in determining why and how these plates move the way they do. Earth's crust "floats" on top of the soft plastic-like mantle below. The evolution of the crust would refer to the gradual development of the crust over time. Continental crust transforms into oceanic crust in a cyclic and dynamic process [ 23 ].
Where the old crust is being destroyed at convergent boundaries, new crust is being created at divergent boundaries. When rifting first occurs at divergent boundaries, the crust-mantle system transforms due to the temperature, and a rift forms.
Subduction of the low-velocity zone in the upper part of the crust is the main mechanism overlooking the beginning of crustal attenuation. Intruding magma, originating from the mantle under the rift, modifies the intermediate and lower crustal layers.
Before the new oceanic crust is created, the intermediate crust disappears completely, and the underneath crustal layer is critically modified by bouts of magma from the mantle sources. New oceanic crust is then produced from the ridge and spreads out from the spreading centre towards the subduction zone where the crust is eventually destroyed.
Components of the crust will return to the upper crust in different forms such as igneous intrusions and contribute to the formation of new continental crust [ 21 ].
Depending on the type of plate boundary and the types of plates involved, the resultant processes and landforms formed differ. The different phenomena that occur contribute to the evolution of the crust.
Another example of the evolution of the crust due to endogenous processes is volcanism, where material from the mantle or the deep crust is deposited onto the surface where it contributes in renewing the crust surface with new igneous rock and landforms.
In some places the crust is weaker such as along plate boundaries, the magma forces its way through the rock, extruding rock and releasing pressure, which is why volcanic activity tends to occur near the borders of tectonic plates, for example, the Pacific Ring of Fire [ 22 ]. The composition and origin of the lava determine the type of volcanic landform created, with more fluid mafic lava forming structures such as shield volcanoes and more viscous felsic lava forming structures such as stratovolcanoes from the accumulation of ejecta.
However, in cases where magma does not breach the surface, the magma in horns or magma chambers may solidify to form intrusive or plutonic rocks. Over time, the surrounding softer rock erodes away, revealing the harder plutonic rock beneath, which creates landforms such as plutons, batholiths, dykes, sills, laccoliths and volcanic necks. The evolutionary processes mentioned above were all a result of forces originating from within the Earth. However, the crust is also shaped by a multitude of processes from external forces such as climate and extraterrestrial material.
An overt example of an extraterrestrial force on the crust would be an impact crater, in which materials from space such as asteroids, meteoroids or comets collide with the Earth, leaving scars on the surface.
The size of the impactor and extension diameter of the resultant impact crater is a decisive factor on the type of crater formed, with crater diameters above 2 km for sedimentary rocks and 4 km for crystalline rocks having a more complex impact structure as opposed to a simple bowl shape [ 25 ]. Climate and weathering are also significant drivers in the continued evolution of the crust.
And while the parameters that control climate are complex and not fully understood, its effects can be seen widely. These processes can be observed in many forms, such as the exposure of batholiths by the erosion of soft rock, the carving of the Grand Canyon or the deposition of sediment by fluvial processes to create river deltas [ 26 ].
Additionally, biological processes also play a role in weathering and erosion. For example, plant roots hold the soil together, providing resistance to erosion [ 25 ]. Plants and burrowing animals also contribute to the mechanical breakdown of rock through wedging caused by growth and burrowing, respectively. And while we may be unable to observe all geological evolutionary phenomena in the span of a human lifetime, we have more than enough examples and evidence to show that truly drastic changes occur in geological time.
While the crust may only comprise the superficial layer of the Earth, it is truly a dynamic and fascinating thing to learn about. Superficially appearing to be a solid immutable covering of rock on our world, it is actually a collection of gargantuan rock plates of heterogeneous composition floating upon an equally colossal ocean of magma that is the outer mantle. It is fortunate that the Earth would coincidentally have the perfect chemical composition to form a crust suitable for life forms to exist, all in accordance with the physical laws that govern the formation of worlds.
The crust is truly an amazing and astonishing thing to learn and behold. Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Help us write another book on this subject and reach those readers. Login to your personal dashboard for more detailed statistics on your publications. Edited by Muhammad Nawaz. Edited by Imran Ahmad Dar. We are IntechOpen, the world's leading publisher of Open Access books.
Built by scientists, for scientists. Our readership spans scientists, professors, researchers, librarians, and students, as well as business professionals. Downloaded: Introduction Earth crust is the thinnest and the most rudimentary layer that makes up the Earth, and yet, everything that has ever lived on Earth has called it home.
Table 1. Difference in characteristics.
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