Geologists have distinguished three main internal
subdivisions of the Earth, based on the behavior of seismic waves and
laboratory experiments. These divisions are based on the composition of each
layer. The outermost layer is the crust. The Earth has two types of crust.
Continental crust underlies much of the Earth’s land surface and is what
we live on. The ocean floors are underlain by oceanic crust. These materials
are composed of different kinds of minerals. The continental crust is like
the igneous rock granite, and the oceanic crust is like basalt, another
Underlying the crust is the second layer, the mantle.
It comprises the largest portion of the Earth. Like the crust, it is also
composed of silicate minerals. The innermost portion of the Earth is the
core. It is composed of metallic elements, primarily iron and nickel. The
core is subdivided into two parts. The outer core is liquid metal, while the
inner core is solid.
The plates are composed of the crust and the uppermost
part of the mantle. These two layers are often called the lithosphere
because they are both composed of solid rock. The underlying, partially
molten part of the mantle, on which the plates slide, is called the
asthenosphere. In contrast to the crust–mantle-core division, which was
based on composition, the lithosphere and asthenosphere are separated based
on strength. The lithosphere is strong, rigid rock, while the asthenosphere
is a weaker, very viscous fluid.
Students have learned that the locations of
earthquakes and volcanoes provide clues (or data) that help understand plate
tectonics. The causes of these movements are complicated, but at this level
students should begin to recognize that some of the stresses come from
within the Earth. In particular, one reason for the movement of the plates
may be convection currents within the upper part of the mantle. Convection
is the heat-driven circulation of a fluid. In the mantle, heat from deeper
in the Earth causes the overlying mantle to circulate. The upper mantle can
circulate because it contains a little magma (molten rock) and behaves like
a very hot mush. Mantle convection currents move very, very, slowly. It is
possible that as the mantle convects, it drags the overlying plates along
with it. It is possible that as the mantle convects, it drags the overlying
plates along with it. Where convection currents come together, a converging
plate boundary is present. Where they move apart, a diverging plate boundary
- Show students how geologists divide the interior of the Earth. Make
sure you go over the vocabulary and contrast lithosphere and
asthenosphere with the crust, mantle and core. Draw a picture on the
board of the different divisions.
- On the worksheet, have the students label and color in each layer of
the Earth. Next, instruct students to measure the thickness of each of
the unit in millimeters and record their answers. Finally, have them
calculate the approximate thickness of each layer.
ANSWERS (Please note that these figures are simplified. The
amount calculated by scientists is written in parenthesis.) 1. crust:
1mm/82 km (71 km); 2. mantle: 37mm/3034 km (2900 km); 3. outer core:
26mm/2132 km (2300 km); 4. inner core: 14mm/1148 km (1200 km).