What happens to oceanic crust at a deep-ocean trench? At a deep-ocean trench, the oceanic crust bends downward. In a process taking tens of millions of years, part of the ocean floor sinks back into the mantle at deep-ocean trenches.

Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

Trenches are formed by subduction, a geophysical process in which two or more of Earth’s tectonic plates converge and the older, denser plate is pushed beneath the lighter plate and deep into the mantle, causing the seafloor and outermost crust (the lithosphere) to bend and form a steep, V-shaped depression.

• molten material rises to surface-upwarps/thins crust, volcanic activity produces high density balsaltic rock.
• Plates begin to move apart, linear rift valley is formed/volcanism contiues.
• further spreading apart = rifting, area drops below sea level.

Several types of evidence from the oceans supported Hess’s theory of sea-floor spreading-evidence from molten material, magnetic stripes, and drilling samples. This evidence also led sci- entists to look again at Wegener’s theory of continental drift.

Geologists have hypothesized that the movement of tectonic plates is related to convection currents in the earth’s mantle. Tremendous heat and pressure within the earth cause the hot magma to flow in convection currents. These currents cause the movement of the tectonic plates that make up the earth’s crust.

What do scientists think caused the movement of Earth’s plates? Convection currents in the mantle.

Core, mantle, and crust are divisions based on composition. The crust makes up less than 1 percent of Earth by mass, consisting of oceanic crust and continental crust is often more felsic rock. The mantle is hot and represents about 68 percent of Earth’s mass. Finally, the core is mostly iron metal.

crust

The lithosphere is about 100 km thick, although its thickness is age dependent (older lithosphere is thicker). The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.

Humans live in the biosphere, anywhere on Earth that there is life. In relation to the structure of the Earth which contains the outer crust, the mantle, the outer and inner cores, life is found at the lithosphere, which is the uppermost mantle together with the crust.

biosphere

A special planet: the habitable Earth What makes the Earth habitable? It is the right distance from the Sun, it is protected from harmful solar radiation by its magnetic field, it is kept warm by an insulating atmosphere, and it has the right chemical ingredients for life, including water and carbon.

Beneath that is the mantle, which is itself made of three different sub-layers: the upper mantle, the transition zone, and the lower mantle. Together, they’re about 1,800 miles (2,900 kilometers) thick, and they make up about 84 percent of the planet’s volume.

The temperature of the mantle varies greatly, from 1000° Celsius (1832° Fahrenheit) near its boundary with the crust, to 3700° Celsius (6692° Fahrenheit) near its boundary with the core. In the mantle, heat and pressure generally increase with depth.