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. Also called the Moho. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Related Resources. Ocean Currents. View Collection. Plate Tectonics. The Rock Cycle. Seafloor Spreading. View Article. View Video. View More Resources.
The Earth is made up of different layers:. Demonstrating the movement of the Earth's crust and how tectonic plates interact at plate boundaries. The crust is the outer layer of the Earth.
It is the solid rock layer upon which we live. It is either continental or oceanic. Continental crust is typically km thick, whilst oceanic crust is only km thick. Oceanic crust is denser, can be subducted and is constantly being destroyed and replaced at plate boundaries.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Seafloor spreading is a geologic process in which tectonic plate s—large slabs of Earth's lithosphere —split apart from each other.
Seafloor spreading and other tectonic activity processes are the result of mantle convection. Convection current s carry heat from the lower mantle and core to the lithosphere. Seafloor spreading occurs at divergent plate boundaries.
The less-dense material rises, often forming a mountain or elevated area of the seafloor. Eventually, the crust cracks. Hot magma fueled by mantle convection bubbles up to fill these fracture s and spills onto the crust. This bubbled-up magma is cooled by frigid seawater to form igneous rock. Seafloor spreading occurs along mid-ocean ridge s—large mountain range s rising from the ocean floor.
The East Pacific Rise is a mid-ocean ridge that runs through the eastern Pacific Ocean and separates the Pacific plate from the North American plate, the Cocos plate, the Nazca plate, and the Antarctic plate. The Southeast Indian Ridge marks where the southern Indo-Australian plate forms a divergent boundary with the Antarctic plate. Seafloor spreading is not consistent at all mid-ocean ridges.
Slowly spreading ridges are the sites of tall, narrow underwater cliff s and mountains. Rapidly spreading ridges have a much more gentle slopes. The Mid-Atlantic Ridge, for instance, is a slow spreading center. It spreads centimeters. The East Pacific Rise, on the other hand, is a fast spreading center.
It spreads about centimeters inches every year. There is not an ocean trench at the East Pacific Rise, because the seafloor spreading is too rapid for one to develop!
The newest, thinnest crust on Earth is located near the center of mid-ocean ridge—the actual site of seafloor spreading. The age, density, and thickness of oceanic crust increases with distance from the mid-ocean ridge.
The magnetism of mid-ocean ridges helped scientists first identify the process of seafloor spreading in the early 20th century. Basalt, the once- molten rock that makes up most new oceanic crust , is a fairly magnetic substance, and scientists began using magnetometer s to measure the magnetism of the ocean floor in the s.
Scientists determined that the same process formed the perfectly symmetrical stripes on both side of a mid-ocean ridge. The continual process of seafloor spreading separated the stripes in an orderly pattern. Oceanic crust slowly moves away from mid-ocean ridges and sites of seafloor spreading. As it moves, it becomes cooler, more dense, and more thick. Eventually, older oceanic crust encounters a tectonic boundary with continental crust.
In some cases, oceanic crust encounters an active plate margin. An active plate margin is an actual plate boundary, where oceanic crust and continental crust crash into each other. Active plate margins are often the site of earthquake s and volcano es.
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