from continental crust in several ways; it is thinner, denser, younger, and of
different chemical composition. Like continental crust, oceanic crust is destroyed
in subduction zones.
Mantle
Most of Earth’s mass is in the mantle, which is composed of iron, magnesium,
aluminum, and silicon-oxygen compounds. At over 1,800°F (1,000°C), most of
the mantle is solid. However, the upper third (known as the asthenosphere) is
more plastic-like in nature.
Lithosphere
The lithosphere is the solid, outer part of Earth. Earth consists of three main
layers: (1) the core, or the inner layer; (2) the mantle, in the middle; and (3) the
crust, which includes the continents and the ocean floor. The lithosphere, which
is about 60 miles (100 kilometers) deep in most places, includes the brittle upper
portion of the mantle and the crust. The lithosphere is always moving, but very
slowly. It is broken into huge sections called tectonic plates. The extreme heat
from the mantle part of the lithosphere makes it easier for the plates to move.
The movement of the lithosphere, called plate tectonics, is the reason behind
many of Earth’s most dramatic geologic events. When one plate moves beneath
another, or when two plates rub together, they can create earthquakes and
volcanoes.
Core
The core is composed mostly of iron and is so hot that the outer core is molten.
The inner core is under such extreme pressure that it remains solid.
Basalt—a common igneous (volcanic) rock that is formed from the rapid
cooling of lava exposed at or near the surface. Basalt makes up most of the
ocean floor and is the most common type of lava.
Granite rock—contains mostly the minerals quartz and feldspar which
solidified (crystallized) from magma that cooled far below Earth’s surface.
Metamorphic rock—a type of rock that has been changed by extreme heat
and pressure. Two examples are marble, which can be formed from limestone,
and slate, which is formed from shale.
Sedimentary rock—formed through deposition and the solidification of
sediment that was transported by water, ice, and wind.
Volcanic rock—formed from magma erupted from a volcano.