Earth ’ s Layered Structure

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 interior consists of three major zones defined by its chemical composition:

– Crust– Mantle– Core


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 2.7-

g.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 3.0-g/cm3.


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.


3. Core:

• The core is a sphere composed of an iron-nickel 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).


Most of the information about Earth’s interior was obtained

by studyingA. Earthquake waves.

B. Rocks of the ocean crust.

C. Meteorites.

D. Rocks in deep wells.

Earth’s thin, rocky outer layer is its

A. Core.

B. Mantle.

C. Outer core.

D. Crust.

Earth’s core is made of an alloy of

A. Iron and nickel.

B. Copper and iron.

C. Zinc and magnesium.

D. Iron and zinc.

The continental crust has the average composition of

A. Gneiss.

B. Granite.

C. Basalt.

D. Limestone.


• 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 can be divided into 4 additional layers based on physical properties:

– The lithosphere.– The asthenosphere.– The outer core.– The inner core.


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.


• 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.


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.


What layers of Earth make up the lithosphere?

A. The crust and lower mantle.

B. The crust and upper mantle.

C. The continental crust and oceanic crust.

D. The upper and lower mantle.

Earth’s inner core is solid because of

A. The composition of its rock.

B. Its great diameter.

C. Extreme temperatures.

D. Immense pressure.


• 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.


• 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.



• 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.



• 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. Continental crust.

B. Ocean crust.

C. Inner core.

D. Outer core.

Evidence that Earth’s core has a high iron content comes

from

A. Deep wells.

B. Deep-sea drilling.

C. The study of earthquake waves.

D. Meteorites.

The greatest concentration of metals occurs in Earth’s

A. Oceanic crust.

B. Continental crust.

C. Core.

D. 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.

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Earth ’ s Layered Structure