The Interior of the Earth

Pamela J. W. Gore
Georgia Perimeter College

Objectives

Relate the discovery of the shadow zone to the existence of the core.
List the principle zones of the Earth (crust, mantle, outer core, inner core), and contrast them in terms of their thickness, chemical makeup, and seismic behavior.
Contrast the asthenosphere and lithosphere.
Discuss seismological and other evidence that indicates that the outer core is liquid.
Understand what is meant by the term Mohorovicic Discontinuity.
Understand the basics of how the Earth's magnetic field is generated.
Discuss the importance of meteorites and our understanding of the interior of the Earth.

Overview of Earth's Internal Structure

Size of Earth:

Radius = 6370 km

Diameter = 12,740 km

Layered structure:

Crust

Continental (about 35 km thick; 60 km in mountain ranges)
Granitic composition
Oceanic (about 5 km thick)
Basaltic composition

Mantle (2885 km thick)
Composition: peridotite (Mg Fe silicates), kimberlite (diamonds), eclogite
Solid that flows (rheid); plastic behavior
Low velocity zone at 100 - 250 km (seismic wave velocity decreases); asthenosphere (Outermost 100 km is called lithosphere)

Outer core (2270 km thick)
Molten Fe with some Ni

Inner core (1216 km radius)
Solid Fe with some Ni

How do we know what the Earth's Interior is like?

Drilling
Wells drilled into Earth are mostly in the upper 7 km of the crust

Deepest well = Soviet (Russian) well in northern Kola Peninsula
24 year effort to drill a 12 km (12,000 m) hole, starting in 1970.
History: 5 years to drill 7 km; 9 years to drill the next 5 km; got stuck at 12 km in 1989.
Later drilling re-started, and by 1994 had reached 12,262 m (7 miles deep), the deepest hole ever drilled.
This is about a third of the way through the crust in that region.
The rocks at the bottom are 2.7 billion years old.
Target depth was 15 km.
Costs were more than $100 million.
Bottom hole temperature is 190 º degrees C, almost double the boiling point of water.

Deepest U.S. well was a gas well drilled in Oklahoma (the GHK/Lone Star Bertha Rogers #1-27 well). Drilling starting in 1974 and stopped at 31,441 ft (9,583 m or 9.58 km) when it struck molten sulfur.

The second deepest U.S. well was the Lone Star/GHK #1-28 E.R. Baden well (total depth 30,050 feet). Drilling started in 1970 and ended in 1972.
Both of these two deepest U.S. wells are in the Anadarko Basin, an east-west trending basin in west-central Oklahoma.

Another deep U.S. well is next to San Andreas Fault (Cajon Pass)
Had reached 3.5 km in 1988
Cost was $5 million ($1400 per meter)
Cost overruns and budget cuts suspended drilling in 1988
Other deep holes are planned.
Costs of a German 10 km hole are estimated at $110 million (or $11,000 per meter)
Germans drilled 3.5 km pilot hole and found bottom temperature was 118 º C (instead of the expected 80 º C)
Volcanic activity
Materials are brought up from below.
Xenoliths = foreign rock (pieces of the mantle in lava)
example: coarse-grained olivine (peridotite) xenoliths in basaltic lava
Only useful to depth of about 200 km
High pressure laboratory experiments
Samples of the solar system (meteorites)
Study of seismic waves generated by earthquakes and nuclear explosions
(More below)

Probing the Earth's Interior with Seismic Waves

P and S wave travel times depend on properties of rock materials that they pass through.
Search for differences in travel times which will correspond to differences in rock properties.

Major layers of the Earth were detected before 1950.
Fine details were delineated in 1960's during nuclear testing.

Wave velocity depends on density and elasticity of rock.
Seismic waves travel faster in denser rock.
Speed of seismic waves increases with depth (pressure and density increase downward).

Layer	Density (g/cm³ )	P-wave velocity (km/sec)
Continental crust	2.6 - 2.8	6
Oceanic crust	3.5	7
Mohorovicic discontinuity (Moho)
Mantle	4.5 - 10	8 - 12
Gutenberg discontinuity
Core (average)	12	-
Outer core (liquid)	-	8 - 10
Inner core (solid)	13.5	11 - 12

See diagrams in text about behavior of P- and S-waves as they travel through the Earth.

Note shadow zones and refraction (bending of waves) at boundaries

S-waves cannot pass through the molten (liquid) outer core.

Note magnetic field.

Heat flow

Geothermal gradient in crust is about 25 ° C/ km (75 ° F/mile)
In diamond mines, at 11,788 ft deep, temperature is about 90 ° F.

Source of heat? radioactivity? primordial heat?

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This page copyright Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA