Seismology What is a seismic wave? What is the difference between body waves and surface waves, and between P-waves and S-waves? Why can't S-waves travel through liquids? How far can seismic waves reach? Why do P-waves travel faster than S-waves? Why is the interior of the Earth hot?
What is the magnetic field of the Earth? Earthquakes and Faults Why do tectonic plates move? Brief history of the plate tectonics theory Before colliding with Asia, where was India? What is an earthquake? What is the highest magnitude an earthquake can reach?
What are the biggest historical earthquakes? Why do earthquakes happen in clusters? Where are earthquakes expected in the world, especially in Asia? What is a supercontinent? Are all the faults on Earth active? How can human activities cause climate change?
Why do urbanisation and deforestation make flooding more likely? Earthquake Hazards Is Singapore threatened by earthquakes? Can we predict earthquakes? This means that Oceanic crust can grow seriously old as it will be destroyed at the convergent boundaries. How very old sedimentary layers end up in horizontal layer on the continents is a mystery given what is now known about plate tectonics.
It is clear that continental crust is rarely distroyed at convergent boundaries. This means that continental crust is almost always older than Oceanic crust. Why do you think oceanic crust is young while continental crust is mostly old? David Drayer. Jun 9, Explanation: Mid Ocean ridges are the source of new crustal material. Related questions What happens at a divergent plate boundary?
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.
Map by USGS. Mining Temperature.
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