Why does the thickness of the earth crust vary

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. In some instances mantle clearly drives changes in the crust, as in the Hawaiian Islands. However, there is ongoing debate whether oceanic crust subduction and mid ocean ridge spreading is driven by a push or pull mechanism. In very broad terms, oceanic crust is made up of basalt and continental crust is made up of rocks similar to granite. Below the crust is a solid relatively cooler portion of the upper mantle that is combined with the crust to make the lithosphere layer.

The lithosphere is physically distinct from the below-lying layers due to its cool temperatures and typically extends km in depth. Below the lithosphere is the asthenosphere layer, a much hotter and malleable portion of the upper mantle. The asthenosphere begins at the bottom of the lithosphere and extends approximately km into the Earth. The asthenosphere acts as the lubricating layer below the lithosphere that allows the lithosphere to move over the Earth's surface.

The mantle starts at the Mohorovicic Discontinuity , also known as the Moho. The Moho is defined as the density contrast from less dense crust to denser mantle and where seismic wave velocities increase.

The mantle acts similar to plastic and at very high temperatures and pressures the rock is deformable at geologic timescales. This deformation causes a convection like process in the mantle where you have larige-scale upwelling and downwelling zones.

The mantle extends down to 2, km into the Earth's surface Temperatures that range from to degrees Celsius in the upper portion to over 4, degrees Celsius near the core boundary. Earth's mantle is believed to be composed of bulk mineralogy similar to peridotite. The video above provides a glimpse into the global circulation of mantle magma around the Earth.

Of course, this is greatly simplified but provides a schematic of the process creating mid ocean ridges, volcanoes, and mountains. 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.

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. Although those are deep mines, they are shallow crust. A sophisticated air conditioning system lowers the temperature to allow miners to work. 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. The Earth itself is about 4. Silicates, Silicates Everywhere. Conrad discontinuity. Also called a collision zone. Also called the geosphere. Mohorovicic discontinuity.

Out of them, the mantle is the thickest layer, while the crust is the thinnest layer. The core has, in total, a radius of km, but it is generally viewed as two distinct parts:.

It is divided into several layers, based on different seismological characteristics as a matter of fact, much of what we know about the mantle comes from seismological information — more on that later in the article. The upper mantle extends from where the crust ends to about km. Even though this area is regarded as viscous, you can also consider it as formed from rock — a rock called peridotite to be more precise.

A peridotite is a dense, coarse-grained igneous rock, consisting mostly of olivine and pyroxene, two minerals only found in igneous rocks. But it gets even more complicated. The crust is divided into tectonics plates, and those tectonic plates are actually thicker than the crust itself, encompassing the top part of the mantle.

The crust and that top part of the mantle going 00 to kilometers below surface, is called the asthenosphere. Scientific studies suggest that this layer has physical properties that are different from the rest of the upper mantle.

Namely, the rocks in this part of the mantle are more rigid and brittle because of cooler temperatures and lower pressures. This is the area with the highest temperatures and biggest pressures, reaching all the way to the outer core. Join the ZME newsletter for amazing science news, features, and exclusive scoops.