Layers of the Earth
The Earth is divided into three chemical layers: the core, the
mantle and the crust. The core is composed of mostly iron and nickel and
remains very hot, even after 4.5 billion years of cooling. The core is
divided into two layers: a solid inner core and a liquid outer core. The
middle layer of the Earth, the mantle, is made of minerals rich in the
elements iron, magnesium, silicon, and oxygen. The crust is rich in the
elements oxygen and silicon with lesser amounts of aluminum, iron,
magnesium, calcium, potassium, and sodium. There are two types of
crust. Basalt is the most common rock on Earth. Oceanic crust is made of
relatively dense rock called basalt.
Continental crust is made of lower density rocks, such as andesite and
The outermost layers of the Earth can be divided by their physical
properties into lithosphere and asthenosphere.
The lithosphere (from the Greek, lithos, stone) is the rigid outermost
layer made of crust and uppermost mantle. The lithosphere is the "plate"
of the plate tectonic theory. The asthenosphere (from the Greek,
asthenos, devoid of force) is part of the mantle that flows, a
characteristic called plastic behavior. It might seem strange that a
solid material can flow. A good example of a solid that flows, or of
plastic behavior, is the movement of toothpaste in a tube. The flow of
the asthenosphere is part of mantle convection, which plays an important
role in moving lithospheric plates.