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The Earth’s Interior
Most scientists agree that soon after its formation, the Earth was a large ball of molten rocky
material. As time passed, the molten material cooled, hardened, and separated into layers. By
studying such things as seismic waves that are sent out by earthquakes, scientists have found
that the Earth is made of several layers that include the core, mantle, and crust.
When earthquakes occur, vibrations are sent out through the Earth. These vibrations are called
seismic waves. As we learned earlier, if the earthquake occurs underwater, it can cause
destructive water waves called tsunamis. Seismic waves can travel along the surface of the
Earth or through its interior. The waves speed up, slow down, or completely stop depending upon
the properties of the medium they are passing through. The study of the behaviors of seismic
waves has allowed geologists to piece together information about the parts of the Earth that
humans have never seen directly.
The core of the Earth is the central part of the planet. It is made of hot, dense metals that
sank, due to gravity, after the Earth formed. Iron and nickel are the two main elements that
make up the core. The core can be subdivided into the inner and outer cores based upon distinct
differences. Within the inner core, the pressure is so great that the iron and nickel cannot melt.
The enormous pressure at this depth pushes the particles of the elements so tightly together
that they remain in their solid state. However, in the outer core the two metals are in the liquid
state.
The mantle is the layer that surrounds the core and is made mainly of silicon, oxygen, iron, and
magnesium. About 80 % of the Earth’s volume is the mantle. By studying the behavior of seismic
waves, scientists believe that the mantle is a solid that has the ability to flow. This special
physical property of the mantle is called plasticity.
The crust is the thin, outermost layer that covers the planet and floats on the plastic mantle. In
relation to the rest of the Earth, this layer is about as thin as the shell on an egg. The crust is
divided into two distinct parts called the continental crust and the oceanic crust. The continental
crust is made mostly of an igneous rock called granite and makes up the continents. Running
beneath the continents and the oceans is the oceanic crust, made of a dense, igneous rock called
basalt.
The thickness of the Earth’s layers varies. Approximately 2900 km thick, the mantle is the
thickest layer. The thicknesses of the other layers include the outer core at 2250 km, the inner
core at 1300 km, and the crust ranging from 32 km (continental) to 8 km (oceanic).
Name: ______________________
Follow the shapes and logical reasoning to fill in the graphic organizer. Use each choice once.
Names
Phases
Continental, core, Earth’s layers, inner, mantle, oceanic, outer
Liquid, solid with plasticity, solid, solid
Composition
Avg
Thicknes
s
Basalt, Fe & Ni Fe & Ni, Si & O & Fe & Mg, granite
8 km, 1300 km, 2250 km, 2900 km