Geoscience Reference
In-Depth Information
Earth's Inner Structure
than hotter zones because cooler regions are more rigid. Areas that
are denser absorb seismic waves, whereas other variations in den-
sity cause seismic waves to be reflected or bent. These differences
have allowed scientists to determine the nature of Earth's interior
and map its basic structure, somewhat like the explorers in the
Jules Verne novel. The following discussion outlines that character
of the layers associated with the Earth's structure.
Have you ever wondered what the inside of Earth is like?
Science fiction writers have often pondered this question, with
the most famous example being Jules Verne's famous book
Journey to the Center of the Earth
. In this story, a professor
leads a party of explorers on a wild adventure to the center of
Earth through a volcano in Iceland. Although such an adventure
is pure fiction, most geologists would certainly like to make
the trip. Unfortunately, the only way to actually see a portion
of the Earth's interior is to core into it with a large drill rig
and collect samples. Although such studies are informative, the
deepest geologists have ever been able to drill into Earth is only
about 12 km (7.5 mi). The distance to the center of Earth is
thousands of kilometers, however, which means that geologists
must rely on indirect evidence to construct models about the
Earth's interior.
Much of what is known about the Earth's structure is based on
how
seismic waves
travel through Earth after earthquakes occur.
Cooler areas within Earth transmit these waves at higher speeds
The Major Layers
With a cross-sectional view of Earth, you can see that the inner
Earth contains several major layers (Figure 12.1). Because it
is difficult to imagine the depths and distances associated with
these layers, it is useful to compare them to the distances be-
tween places on a map of North America (Figure 12.2). For
example, the distance to the center of the Earth's core—in other
words, to the center of Earth—is about 6370 km (3963 mi),
which happens to be the approximate distance from Anchorage,
Alaska, to Miami, Florida.
The Earth's innermost layer is the
inner core
. This
part of Earth has a radius of about 1220 km (760 mi),
Lithosphere
250 km
420 km
Asthenosphere
Upper mantle
Viscous nickel
670 km
Lower mantle
Solid oxides of iron,
magnesium, silicon
1300°C
Continental crust
Granite
2230 km
Oceanic crust
Basalt
8 km
2250 km
m
40 km
Lithosphere
70 km
Outer core
Liquid iron
2250 km
Mohorovicic
discontinuity
Asthenosphere
Molten rock
5150 km
250 km
Inner core
Solid iron, nickel
3200-5200°C
1220 km
Upper mantle
6370 km
Center of
the Earth
Figure 12.1 Earth's interior structure.
The Earth's interior structure consists of several major layers, each with a distinct mineral compo-
sition and density. Note that the middle image is a blown-up diagram of the pie-shaped cross section in the left-hand diagram. The right-
hand image, in turn, is a blown-up diagram of the pie-shaped cross section in the upper part of the middle image.
Seismic waves
Vibrations that travel through the Earth when
stress is released in an earthquake.
Inner core
The inner part of the Earth's core. This area is about
1220 km (760 mi) thick and consists of solid iron and nickel.
