Earth’s Layered
Structure
Section 8.4
Earth’s layered structure
• Most knowledge of the interior of the Earth comes
from the study of earthquake waves.
• If the Earth were made of the same materials
throughout, seismic waves would spread through it
in straight lines at constant speed.
• Seismic waves travel at different speeds due to the
differences in the composition of the Earth.
• The general increase in speed with depth is due to
increased pressure, which changes the elastic
properties of deeply buried rock.
• As a result, the paths of seismic waves are
refracted, or bent, as they travel.
Earth’s layered structure
Earth’s layered structure
• Earth’s interior consists of three
major zones defined by its
chemical composition:
– Crust
– Mantle
– Core
Earth’s Layered Structure
1. Crust:
•
•
•
•
•
•
The crust is the thin, rocky outer layer of Earth, which is
divided into oceanic and continental crust.
The oceanic crust is roughly 7-kilometers thick and
composed of the igneous rocks basalt and gabbro.
The continental crust is 8 – 75-kilometers thick (average
40-km), and consists of many types of rock.
The average composition of the continental crust is granitic
rock called granodiorite.
Continental rocks have an average density of about 2.7g.cm3 and some are over 4 billion years old.
The rocks of the oceanic crust are younger (180 million
years or less) and have an average density of about 3.0g/cm3.
Earth’s layered structure
2. Mantle:
• The mantle is a solid, rocky shell that extends to a
depth of 2890-km.
• Divided into upper and lower mantle.
• Over 82% of Earth’s volume is contained in the
mantle.
• The boundary between the crust and mantle
represents a change in chemical composition.
• The dominant rock in the uppermost mantle is
perioditite, which has a density of 3.4-g/cm3.
Earth’s layered structure
3. Core:
• The core is a sphere composed of an ironnickel alloy.
• Divided into the inner and outer core.
• Generates the Earth’s magnetic field.
• At the extreme pressures found in the center
of the core, the iron-rich material has an
average density of almost 13-g/cm3 (13 times
heavier than water).
Earth’s layered structure
Most of the information about Earth’s interior
was obtained by studying
A.
B.
C.
D.
Earthquake waves.
Rocks of the ocean crust.
Meteorites.
Rocks in deep wells.
Earth’s thin, rocky outer layer is its
A.
B.
C.
D.
Core.
Mantle.
Outer core.
Crust.
Earth’s core is made of an alloy of
A.
B.
C.
D.
Iron and nickel.
Copper and iron.
Zinc and magnesium.
Iron and zinc.
The continental crust has the average
composition of
A.
B.
C.
D.
Gneiss.
Granite.
Basalt.
Limestone.
Earth’s layered structure
• Scientists have no direct way to measure
temperatures deep within the earth.
• Analysis of seismic waves and heat flow near the
earth’s surface and computer modeling allow
scientists to estimate those temperatures.
• The following graph estimates the earth’s inner
temperatures and pressures.
• The graph shows, the combined temperature and
pressure in the lower part of the mantle keep the
rocks located there below their melting point.
Earth’s layered structure
Earth’s layered structure
•
Earth can be divided into 4 additional layers
based on physical properties:
– The lithosphere.
– The asthenosphere.
– The outer core.
– The inner core.
Earth’s Layered Structure
1. Lithosphere and Asthenosphere:
• Earth’s outermost layer consists of the crust and
uppermost mantle and forms a relatively cool, rigid
shell called the lithosphere.
• This layer averages 100-km in thickness.
• Beneath the lithosphere lies a soft, comparatively
weak layer known as the asthenosphere.
• The asthenosphere has temperature/pressure
conditions that may result in a small amount of
melting.
• The lower lithosphere and asthenosphere are both
part of the upper mantle.
Earth’s layered structure
• From a depth of about 660-km down to near
the base of the mantle lies a more rigid layer
known as the lower mantle.
• The rocks in the lower mantle are still very hot
and capable of gradual flow.
• The bottom few hundred kilometers of the
mantle, laying on top of the hot outer core,
contains softer, more flowing rock like that of
the asthenosphere.
Earth’s layered structure
2. Inner and Outer Core:
• The core is divided into two regions with
different physical properties.
• The outer core is a liquid layer 2260-km thick.
• The flow of metallic iron within this zone
generates Earth’s magnetic field.
• The inner core is a sphere having a radius of
1220-km.
• The material in the inner core is compressed
into a solid state by the immense pressure.
Earth’s layered structure
What layers of Earth make up the lithosphere?
A.
B.
C.
D.
The crust and lower mantle.
The crust and upper mantle.
The continental crust and oceanic crust.
The upper and lower mantle.
Earth’s inner core is solid because of
A.
B.
C.
D.
The composition of its rock.
Its great diameter.
Extreme temperatures.
Immense pressure.
Earth’s layered structure
• In 1909 Andrija Mohorovičić, a Croatian
scientist, discovered that the speed of seismic
waves increases abruptly 32 km to 70 km
beneath the earth’s surface.
• This change in speed of the waves marks the
boundary between the crust and the mantle.
• The boundary is called the Mohorovičić
discontinuity, or the Moho.
• The increase in speed at the Moho indicates
that the earth’s mantle is denser than its crust.
Earth’s layered structure
• Below the Moho, at a depth of about 100 km, a decrease in
seismic waves speed marks the boundary between the
lithosphere and the less rigid asthenosphere.
• Seismic waves then increase in speed until, at a depth of
about 2,900 km, P waves slow down again, while the S
waves disappear entirely.
• These changes in seismic waves mark the boundary
between the mantle and the outer core.
• Because S waves cannot travel through liquids and P waves
slow down in less-rigid materials, scientists think the outer
core may be a dense liquid.
• At a depth of 5,150 km, P waves speed up again, marking
the boundary between the outer core and the inner core.
• This increase in speed suggests that the inner core is a
dense, rigid solid.
Earth’s layered structure
Earth’s layered structure
• Recordings of seismic waves around the world
reveal shadow zones on the earth’s surface.
• Shadow zones are locations on the earth’s surface
where neither S waves nor P waves are detected or
where only P waves are detected.
• Shadow zones occur because the materials that
make up the earth’s interior are not uniform in
rigidity.
• When seismic waves travel through materials of
differing rigidities, their speed changes, causing the
waves to bend and change direction.
Earth’s layered structure
Earth’s layered structure
• Through seismic data and drilling technology, the composition of
the continental and oceanic crust was classified.
– Continental = granitic
– Oceanic = basaltic
• The composition of the rocks of the mantle and core is known from
more indirect data.
– Example: Magma from volcanic activity and lab experiments with the rock
peridotite showed similar properties.
• Earth’s core is thought to be mainly dense iron and nickel, similar
to metallic meteorites.
– Scientists assume that meteorites are composed of the same material from
which Earth was formed (Big Bang Theory). Since iron is a dense element, it
is believed that it sank toward Earth’s center, with less dense elements
floating to the surface of Earth.
• The surrounding mantle is believed to be composed of rocks
similar to stony meteorites.
The Moho is
A. The boundary between the outer and
inner core.
B. The boundary between the crust and the
mantle.
C. The material of which the mantle is
composed.
D. An area of the mantle that will not transmit
seismic waves.
Through which Earth layer are S-waves not
transmitted?
A.
B.
C.
D.
Continental crust.
Ocean crust.
Inner core.
Outer core.
Evidence that Earth’s core has a high iron
content comes from
A.
B.
C.
D.
Deep wells.
Deep-sea drilling.
The study of earthquake waves.
Meteorites.
The greatest concentration of metals occurs in
Earth’s
A.
B.
C.
D.
Oceanic crust.
Continental crust.
Core.
Mantle.
Science Journal
Label the page with Section 8.4 and the date.
Answer the following questions in complete
sentences.
1. List the 3 layers of Earth and then compare
(tell how they are alike) and contrast (tell how
they are different) them.
2. Compare and contrast the lithosphere and
the asthenosphere.