Earth image Interior of the Earth

Dr. Pamela Gore
Georgia Perimeter College

Objectives

  1. Identify the various layers of the Earth, and contrast them in terms of their thickness, composition, density, and seismic behavior.
  2. Discuss the characteristics of the crust.
  3. Discuss the properties and composition of the mantle.
  4. Discuss the characteristics of the inner and outer core.
  5. Discuss seismological and other evidence that indicates that the outer core is liquid.
  6. Understand what is meant by the term Mohorovicic Discontinuity.
  7. Contrast the asthenosphere and lithosphere.
  8. Use common objects and materials (such as a boiled egg or a piece of fruit), to model and discuss the Earth's interior. Evaluate the usefulness of your model.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S6E5a. Compare and contrast the Earth's crust, mantle, and core, including temperature, density and composition.
  • S6CS5b. Identify several different models (such as physical replicas, pictures, and analogies) that could be used to represent the same thing, and evaluate their usefulness, taking into account the model's purpose and complexity.

This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
  • The interior of the earth is hot. Heat flow and movement of material within the Earth cause earthquakes and volcanic eruptions and create mountains and ocean basins. Gas and dust from large volcanoes can change the atmosphere.
  • The slow movement of material within the Earth results from heat flowing out from the deep interior and the action of gravitational forces on regions of different density.

This section addresses, in whole or in part, the following National Science Education Standards:
  • The solid Earth is layered with a lithosphere; hot, convecting mantle; and dense, metallic core.

Size of Earth

Radius = 6370 km
Diameter = 12,740 km


Basic Structure of the Interior of the Earth

  1. Crust - brittle, rocky outer layer of the Earth (5-40 km thick)
    There are two major types of crust:
    1. Continental Crust- thicker (30-40 km thick; 60 km in some mountain ranges), granitic (sialic) composition
    2. Oceanic Crust - thinner (about 5 km), basaltic (mafic) composition

  2. Mantle - solid rocky layer, dense, high pressure. A solid that flows.
    (2885 km thick)
    Composition: peridotite (Mg Fe silicates like olivine), kimberlite (diamonds), eclogite.

  3. Outer Core - molten Fe with some Ni

  4. Inner Core - solid Fe with some Ni


This diagram shows the internal structure of the Earth. The circular diagram in the lower left is drawn to scale, illustrating the extreme thinness of the crust. Sometimes the structure of the Earth is compared to that of a boiled egg, or of a nectarine, to emphasize the thinness of the crust. The pie-shaped wedge on the right enlarges the crust and upper mantle to show more detail. Image courtesy of U.S. Geological Survey.


Table courtesy of U.S. Geological Survey.


Summary of the Major Layers of the Earth
and the Names of the Boundaries or Discontinuities
which separate them

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

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


Magnetic Field

Note that the Earth's magnetic field is generated in the core by processes which include convection in the liquid outer core and the rotation of the solid inner core.

(Note that convection refers to hot material rising and cold material sinking.)
Circular convection cells develop in materials that are heated from the bottom.

The Earth's magnetic field behaves as if there were a giant bar magnet buried within the Earth.

Note that the Earth has a north magnetic pole and a south magnetic pole.
The magentic poles are NOT THE SAME as the north and south poles which correspond to the rotational axis of the Earth and the site where the longitude lines converge (latitude = 90o). The north magnetic pole is somewhere in northern Canada.

The position of the Earth's magnetic poles changes slightly from year to year (we call this polar wandering). Polar wandering is probably due to fluctuations in the convection patterns in the liquid outer core.



Heat Flow and Geothermal Gradient

Recall that the Earth is hot inside.
The deeper you go into the Earth, the higher the temperature.
This is called the geothermal gradient.
(Geo means "Earth" and thermal means "heat".)
The 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 or more.

Source of heat? radioactivity? primordial heat?


Temperature in the Interior

We don't really know the temperature in the center of the Earth. We cannot measure or observe it directly. We do know that temperatures and pressures increase with depth. We know the rate at which temperature increases with depth (the geothermal gradient), and we know the thickness of the various layers of the Earth, so the temperature in the interior can be calculated mathematically. We can also model in the laboratory the high pressures and temperature like those in the Earth's interior, and see how mineral properties and structures change, and compare these results to the changes that occur within the Earth, as determined by seismic studies. So we can get a pretty good idea of what the temperatures are like inside the Earth.

The temperature of the mantle is calculated to be about 870 degrees Celsius.

The temperature of the outer core ranges from about 4,400 degrees Celsius to about 6,100 degrees Celsius.

The temperature of the inner core of the Earth is estimated to be about 7000 degrees Celsius.

References:
Temperature at the Center of the Earth.
Earth's Core Temperature.


Earth's Layered Structure and Plate Tectonics

The Earth's lithosphere is divided into a dozen or so rigid plates that move relative to one another. These plates "float" or ride along on the underlying asthenosphere, which flows and undergoes convection, causing the plates to move.

Plate tectonics is driven by the convection in the asthenosphere (part of the Earth's mantle).


Conceptual drawing of assumed convection cells in the mantle. Below a depth of about 700 km, the descending slab begins to soften and flow, losing its form.


Sketch showing convection cells commonly seen in boiling water or soup. This analogy, however, does not take into account the huge differences in the size and the flow rates of these cells.


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

  1. Drilling

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

    Deepest well = Soviet (Russian) well in northern Kola Peninsula
    20 year effort to drill a 12 km hole. Stopped in 1989.
    History: 5 years to drill 7 km; 9 years to drill the next 5 km; got stuck at 12 km.
    Target depth is 15 km.
    Costs are more than $100 million.
    Bottom hole temperature is 190 º degrees C
    Current status??

    Deepest US 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)

  2. 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 (the depth from which lava comes)

  3. High pressure laboratory experiments to determine which minerals can exist at high pressures like that of the great depths of the mantle.

  4. Samples of the solar system (meteorites) - click here to see web notes on types of meteorites

  5. Study of seismic waves generated by earthquakes and nuclear explosions -
    This is the most important method of determining the internal structure of the Earth.


Probing the Earth's Interior with Seismic Waves

P-waves and S-wave velocities (or travel times) depend on the properties of the rocks through which they travel. (Review "Earthquake" notes if you need to review what P-waves and S-waves are.) Differences in velocities (or travel times) correspond to differences in rock properties. Wave velocity depends on the density and elasticity of the rock. Seismic waves travel faster in denser rock. The velocity of seismic waves increases with depth. (This is because pressure and density increase with depth in the Earth).

P-waves can travel through all of the layers of the Earth because they can travel through both solids and liquids.

On the other hand, S-waves can travel through the Earth's crust and mantle, but they STOP when they reach the Earth's core. This causes a shadow zone (or area in which no S-waves are detected by seismographs) on the side of the Earth away from the earthquake epicenter.

The presence of the S-wave shadow zone tells us that there is an area in the center of the Earth that is molten (liquid).

The size of the shadow zone gives us a good idea of the size and depth of the Earth's outer liquid core.

The size and depth of the solid inner core was determined from P-waves that reflected (or bounced) off the inner core following nuclear detonations.


Diagram showing how P and S waves travel through various layers of the Earth. Note that S waves do not travel through the outer core. Notice the bending or refraction of both P and S waves. Image courtesy of U.S. Geological Survey.


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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 4, 2005