Which trench contains the oldest rock

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The oldest crust is at edges of the ocean. One place where the crust is the oldest is at edge of a subduction zone. It is here that the oldest ocean crust is pushed under a continental crust and destroyed. As you can see, the newest crust red is adjacent to where there is seafloor spreading.

The continental shelves like the Eastern coast of the United States is another edge. The continents of Europe and North America are thought to have been connected at one time.

A rift valley split the two continental crust apart forming an ocean between them. 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.

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.

Remnants of crust from Earth's infancy are hard to come by because most of that material has been recycled into Earth's interior several times by the plate tectonics that continue to shape our planet's surface. In , geologists found an expanse of bedrock, known as the Nuvvuagittuq greenstone belt, exposed on the eastern shore of Hudson Bay in northern Quebec.

Suspecting that the rocks there could be from one of the earliest periods of Earth's history, geologists took samples to try and determine their age.