Earth's Interior
Welcome! Thanks for visiting Poorna's pages at the Glendale Community College web-site. These pages and the related links tell you about the courses I teach and the topics that I am interested in.
Home My Book Physical Geology Environmental Geology Oceanography
					Updated on 05/05/2015

	A discussion of the earth’s interior is the basic prerequisite for understanding Physical Geology partly because of the role of planetary dynamics in creating and sustaining the planetary processes and partly because the tools that inform us of the planetary interior are also the ones we need in our understanding of the surface effects.
	Access this USGS publication “Interior of the Earth” at http://pubs.usgs.gov/gip/interior

— Crust is the earth's thin (0-70 Km) outer skin, averaging ~30 Km beneath the continents and ~15 Km beneath the oceans.

— Mantle is the earth's ~2,900 Km thick and rocky outer shell that underlies the crust.

— Core is the earth's ~3,500 Km thick metallic interior, comprising the (a) solid inner core (1,250 Km radius), and (b) liquid outer core (2,250 Km thick).

(Inner core and crust have similar volumes of ~8.2×10⁹ Km³)

n Earth is a multi-layered body

— Crust is the earth's thin (0-70 Km) outer skin, averaging ~30 Km beneath the continents and ~15 Km beneath the oceans.

— Mantle is the earth's ~2,900 Km thick and rocky outer shell that underlies the crust.

— Core is the earth's ~3,500 Km thick metallic interior, comprising the (a) solid inner core (1,250 Km radius), and (b) liquid outer core (2,250 Km thick).

(Inner core and crust have similar volumes of ~8.2×10⁹ Km³)

	Earth is a largely spherical body, with an average radius of 6,378 Km, an equatorial bulge and polar flattening, the equatorial radius being ~21 Km greater than the polar radii.
	Earth has a multi-layered structure, with:

Earth's equatorial radius (r_eq) is ~21 km greater than the polar radius (r_pole).

Crust is the earth's thin (0-70 Km) outer skin, averaging ~30 Km beneath the continents and ~15 Km beneath the oceans.

Based on satellite mapping, the earth is now known to be truly pear-shaped, in that, added to facts of earth’s equatorial bulge and polar flattening, the south pole is ~40 m closer to the earth’s center than the north pole.

Mantle is the earth's ~2,900 Km thick and rocky outer shell that underlies the crust.

Core is the earth's ~3,500 Km thick metallic interior, comprising the (a) solid inner core (1,250 Km radius), and (b) liquid outer core (2,250 Km thick).

Curiously, for whatever its significance, the inner core and the crust have similar volumes of ~8.2×10

		Whole Earth	Crust
Oxygen	O	29.8%	46.6%
Silicon	Si	15.6%	27.7%
Magnesium	Mg	13.9%	2.1%
Iron	Fe	33.3%	5.0%
Aluminum	Al	1.5%	8.1%
Calcium	Ca	1.8%	3.6%
Nickel	Ni	2.0%	—
Sodium	Na	0.2%	2.8%
Potassium	K	—	2.6%
Others		1.9%	1.5%

Of the 9 elements that dominate the Earth's chemical composition,

crust carries most of the Earth's Si, O, Al, Ca and Na;

mantle is, in effect, a compositional replica of the whole earth; whereas

most of the earth's Fe, Mg and Ni occur in the core.

Information on the Earth's internal structure comes from (a) gravity, (b) seismic, and (c) geomagnetic studies.

Of the 9 elements that dominate the Earth's chemical composition,

n Earth has a multi-layered structure:

Earth’s Interior

Text Box: n Of the 9 elements that dominate the Earth's chemical composition,

—

Isostasy

G.B. Airy's 1855 model of “isostacy”, illustrated here, assumed that the low density crust floats over a denser substratum. The root ‘R’ of crust beneath a mountain (or its antiroot beneath an ocean) is:

R = (h´σ_crust)/(σ_crust – σ_subcrust)

Here h is height (or depth) relative to mean sea level while σ_crust and σ_subcrust are densities of crust and subcrust respectively.

Text Box: Isostasy
G.B. Airy's 1855 model of “isostacy”, illustrated here, assumed that the low density crust floats over a denser substratum. The root ‘R’ of crust beneath a mountain (or its antiroot beneath an ocean) is:
R = (h´σcrust)/(σcrust – σsubcrust)

Here h is height (or depth) relative to mean sea level while σcrust and σsubcrust are densities of crust and subcrust respectively.

— c

—

n The Gravity Picture:

— Because of earth’s equatorial bulge and polar flattening, gravitational acceleration on the surface increases from equator to the poles.

— The whole earth density (~5.5 g/cm³) is about

twice the average crustal density (~2.7 g/ cm³): Clearly, density increases with depth.

— The continental crust is lighter and thicker

than the oceanic crust, as “mountains have their own roots” (i.e., isostasy).

n The Seismic Evidence

— Seismic waves are of two types: the surface waves (Love and Rayleigh) and

The undulations of the geoid, or the equipotential surface, reveal inhomogeneous mass distribution. For instance, notice in this picture of the earth geoid from NASA how the geoid is depressed in South Asia-Indian Ocean and North America regions, and is raised in the North Atlantic and West Pacific regions and in the region immediately south of Africa.

the body waves (the P and S waves). Of these, P-wave
velocity in the crust averages ~6 Km/s, S-wave ~4 Km/s.

— Earthquake focal depths are usually <250 Km.

— The “shadow zone”: No direct P-waves from an earthquake.

arrive between 103° and 142° from the epicenter, whereas
no direct S-waves from an earthquake are seen beyond 103° from the epicenter. Since S-waves do not traverse a fluid layer, where the P-waves slow
down, this suggests that the outer core is a fluid layer.

The Geomagnetic Field

To learn about the geomagnetic field, visit the National Geophysical Data Center at http://www.ngdc.noaa.gov/seg/potfld/geomag.shtml

n Why is the Outer Core fluid?

Because temperatures there exceed the melting
points of its constituents.

Help available 'Online'

Help available at this site

Practice Exercise/Quiz at Book's Website:

Answers to the End-of-Chapter Questions

Internet Exercises

'Multiple-Choice' Quiz

'True/False' Quiz

'Matching' Quiz

Chapter Links at the Book's Website:

Overview

Learning Objectives

Glossary

Web Links

Related Readings

'Virtual Vista'

Animations and Movies

Boxed Readings

Quizzes etc:

Information on Quiz-1, Test-1 and other Quizzes, Tests, Class Meetings, Final Examination

Video Episodes:

Episode 103: Earth's Interior

n‹…‚Ì“à•” EARTH'S INTERIOR(’n‹…‚ð’T‚é EARTH REVEALED@3)@–{‘Ì‰¿Ši18,000‰~

Video Study Guide:

Mike Strickler's Study Guide for the Video Episode 103

Ruth Lebow's Study Guide for Earth Revealed

Poorna's Outlines:

Earth's Interior

You may need to download the Adobe Acrobat Reader to view this PDF file

Click on this
image to access the USGS online publication "Interior of the Earth"

Some Dictionaries and Glossaries ONLINE:

McGraw-Hill 'Access Science' Dictionary

Dictionary of Scientific Quotations

Harcourt Dictionary of Science and Technology

Interesting WWW sites about science

Enchanted Learning Science Dictionaries

Glossarist Earth Science Glossary

USGS Glossary

USGS Photo Glossary of Volcanic Terms

USGS Water Science Glossary of Terms

GeoMan's Glossary of Earth Science Terms

Glossary of Natural History Terms

Desert Environment and Geology

Textbook Chapter: Chapter 2 (The Earth's Interior)

The only rocks that geologists can study directly are those of the crust, earth's thin skin of rock that makes up less than 1% of the earth's total volume. Geologists therefore use the tools of geophysics — the measurement of gravity, heat flow, geomagnetic field and seismic wave propagation — in order to learn about the earth's interior.

The evidences suggests that the earth is a multi-layered body, its three principal layers being the crust, the mantle and the core. Of these, crust and mantle are rocky whereas core, comprising a solid inner core and a fluid outer core, is metallic in composition.

(based on the chapter introduction)

How Thick is the Earth's Crust?	Access these USGS sites to find out
This USGS (http://quake.usgs.gov/research/structure/CrustalStructure/) contour map of the thickness of the Earth's crust shows that, to a first approximation, the continents and their margins are outlined by the 30 km contour. That portion of the continental interior enclosed by the 40 km contour, and regions with crustal thickness of 45 to 50 km are found on all well surveyed continents (i.e., North and South America, Australia, and Eurasia). Continental crust with a thickness in excess of 50 km is exceedingly rare and accounts for less than 10% of the continental crust. The contour interval is 10 km; with the 45 km contour included for greater detail on the continents.	The figure on the right is the USGS crustal thickness map of North America, derived from a comprehensive compilation of seismic refraction and reflection data, earthquake studies, and surface wave analyses. Notice the extension of the thin crust of the Basin and Range Province into Western Canada. The contour interval is 10 km (solid line) (dashed line shows the 45 km contour. The average thickness of the North American continental crust, including margins, is 36.5 km ranges from 14 km to 60 km.

This USGS map of North America alongside shows the average p-wave velocity of consolidated or crystalline crust (Pcc) without sediments.

Notice

	the close correspondence between high average crustal velocity and accretionary and magmatic orogenies (e.g., Trans-Hudsonian, Grenvillian, Acadian, Appalachian, the Cascades, and the Alaskan Range); and
	in contrast, the anomalously low average crustal velocities in the Basin and Range Province, the Snake River Region, and the Northwest Territories of Canada.

Many of these latter regions are regions of active or recent extension or hotspot activity.

Source:

http://quake.usgs.gov/research/structure/
CrustalStructure/nam/index.html

The Geomagnetic Field

Earth's magnetic field generally behaves as one produced by a bar magnetic aligned to the spin axis, as can be seen from these world magnetic charts for epoch 2000 produced by the USGS, jointly with the British Geological Survey. Its precise mechanism, and the reason why it occasionally flips over and reverses its polarity, remain far from clear as yet, however.

Try the following URLs to learn more about this subject:

http://www.earth.rochester.edu/ees201/Rudin/rudin.html

http://www.psc.edu/science/Glatzmaier/glatzmaier.html#References

http://es.ucsc.edu/~glatz/geodynamo.html

Note: Rotating globe at the top left corner of this page is from http://adinet.net/Globe_HTML/frames/scientific/f2p3a7_78kb2.htm

This site was last updated 05/05/15

Earth’s Interior

Because temperatures there exceed the melting points of its constituents.

Because temperatures there exceed the melting
points of its constituents.