The dynamic Earth (Introduction to Geophysics)

Most geophysical processes stem from the transfer of heat from the Earth's core to its surface.
 

Why is the Earth's core hot?
1.  The radio active decay of Uranium (U), Thorium (Th) and Potassium (K). Each radio active decay (the loss of some neutrons and protons) releases very little energy. However, all the countless events acting together release a large sustained amount of energy overtime. In the core of the Earth this energy is trapped and so the Earth's core is heated up.

Since the radioactive elements eventually decay into stable (non-radioactive) elements this
heating source is very slowly becoming less important.  Billions of years ago, when elements
like uranium were more common in the Earth, there was actually enough that natural nuclear
reactors formed deep under the earth where uranium deposits reached critical mass.  No such
natural reactors have existed for more than one billion years.

2.  As the solid inner core grows latent heat is released as the molten outer core freezes to solid rock.  Eventually the whole Earth will be solid and there will be no magnetic field.

3.  Residual formation heat. Some of the kinetic energy ((1/2)*mv²) of the impacting planetesimals would have been converted to heat. This residual formation heat helped melt the core initially.

4.  Another early heat source was the heat produced as the heavy elements (like Iron (Fe) and Nickel (Ni)) "sinking" into the core. This process also generated heat from friction.
 

On some other bodies in the solar system, another important source of heat is tidal heating.  This occurs when an object, such as a moon, feels varying forces from multiple nearby objects.   Due to the
forces from Jupiter and its moons, Jupiter's moon Io  has tides of about 100 meters!  The stretching
and squeezing on its interior keep the core molten, producing dramatic volcanos.

The exchange of heat from the hot core to the cool surface  is called convection (heat rises, cold sinks). In this manner the whole Earth has a series of big convective cells in its mantel. The result is a complex series of movements of the crust of the Earth as it "rides" on top of the convective cells below.

Plate Tectonics
 

In the 1950s and 60s geophysicists started to develop the concept of Plate Tectonics. Plate tectonics is the theory that describes the motion of the continental plates "riding" the tops of these massive convective cells in the Earth (like a conveyor belt).
 

Here is a movie showing how the plates have moved the continents
 

Here is another site showing how the Earth looked at different times in the past.

Today these plates move by about 10 cm/yr

• the rate of seafloor spreading can be measured from dating rocks
• comparing rocks and fossils on different continents can tell us when they were together and when they were apart
• now geologists can use GPS receivers to make very precise measurements of continental motions

• when these plates stick, and then suddenly slip, an Earthquake occurs
• when the heavier ocean crust sinks below the lighter (granite) continental crust (at subduction zones) there will be Earthquakes and Volcanos -the ring of fire around the Pacific is built this way. The Continental crust will also be crumpled, and as a result it is typical to see mountain ranges along the edges of these faults (for example the rocky mountains and the Andes).

• Seamount Island Chains -like the Hawaiian Islands- are made when one hot spot in the Earths mantle leads to continuous eruptions in the same spot. But as the crust moves along the ocean floor a chain of new islands appear.
• Sometimes (but not often) two continental plates collide. In this case neither plate is heavier and so they both "crumple". This is occurring today as the Indian plate collides with the Asian plate. The result of this collision is the Himalayas which are the highest mountains on Earth.

 

 

Why is a hot core important for life on Earth?
1.  the surface temperature is higher
2.  active vulcanism can out gas the atmosphere and oceans
3.  vulcanism helps form land masses above the ocean
4.  hot spots in the sea floor can be "safe" habitats for life
5.  hot springs and even hot water deep in the Earth can harbor life, including exotic species that do not require sunlight
6.  volcanoes play a role in the Earth's carbon cycle
7.  the Earth's magnetic field helps keep out potentially dangerous cosmic rays

Both Mars and the Moon show evidence of past "geological" activity, but
neither is still active.

Basin and Range

Tucson is located in a unique part of the world. The area where we live is called "Basin & Range" geography. This denotes that in Eastern California, Arizona, and New Mexico the terrain is dominated by short (often parallel) mountain ranges with large dry basins between them. This is a highly unusual land form caused by a unique event in the Earth's history.

• About 20 million years ago the continental plate of the Southwest became "attached" somehow to the pacific coast plate which was moving northwest at the time.
• Added to this was intense heat from magma close to the surface.
• The end result was the unique "Basin & Range disturbance" where the coast of California was pulled away from Arizona by some 38% of its original size.
• The hard cold rock on the top splintered into dozens of parallel ranges, while huge basins over 1 km deep were opened up between the ranges.
• The whole stretching event took a few million years. Then due to erosion the valleys filled in and the ranges wore down --further filling the valleys.

The geology of Tucson is very interesting.   The Catalina and Tucson mountains
were once a single range, home to a massive volcano,  which collapsed about 70
million years ago.  The basin that formed then filled with sediment to become the
present-day Tucson valley.  This is the same basin-and-range phenomenon that
happened all across the western United States.
 

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