Plate Tectonics
The continental crust and the upper part of the mantle form a rigid layer called the lithosphere. The lower part of the mantle is relatively weak and is known as the asthenosphere. The relative weakness of the asthenosphere allows the lithosphere to move around. The lithosphere is not a single unit, it consists of seven major plates and half a dozen minor ones.
The plates are driven by heat rising from Earth’s core. Each plate moves as a distinct unit and so all major interactions between the plates occur along plate boundaries. Most of Earth’s seismic activity, volcanism and mountain building occur along these dynamic margins. Plate movements have occurred throughout the planet’s history and the moving continents changed climates by altering meteorological conditions and oceanic circulation.
Rock Cycle
Rocks are commonly thought of as hard but the term includes soft or unconsolidated materials such as clay, sand and gravel. Igneous rock forms when molten material called magma cools and solidifies. This process is called crystallisation and may occur either beneath Earth’s surface or, following a volcanic eruption, at the surface.
Igneous rocks exposed on Earth’s surface undergo weathering and erosion. Weathering breaks down exposed rocks by the influences of the atmosphere. Erosion removes rock and minerals by water, wind and ice. Weathering and erosion operating together lowers the surface of the land and their combined effects are known as denudation.
When this unconsolidated material (sediment) collects, it will undergo lithification (conversion into rock) either by compaction due to the weight of overlying layers, or by groundwater filling the pores with material matter.
If the resulting sedimentary rock is buried deep within Earth or involved in the dynamics of mountain building, it will be subjected to great heat and pressure and will turn into metamorphic rock. Metamorphic rock subjected to still greater heat and pressure will melt to create magma that will eventually complete the cycle by solidifying into igneous rock.
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