Interior of the Earth
Interior of the Earth
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This section addresses, in whole or in part, the following National Science Education Standards:
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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.
| 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.
(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.
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?
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.
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.
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)
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