Erosion

F

The Earth’s Magnetic Field

The Earth is a giant magnet.  There are several theories of how a planetary body produces a magnetic field.  The current favorite is called a self-sustaining dynamo.

The molten minerals & metals of the outer core flow around the inner core, possibly at a rate of more than 10 kilometers per year, which is 100,000 times faster than the currents moving the plates of the lithosphere.  The core, being mostly iron, has a magnetic field and produces electric currents within the fluid, and these generate a second magnetic field.  The new field is added to the original, reinforcing it and continuing the cycle.

The principal energy source of the magnetic dynamo is the production and loss of heat.  Our planet produces heat by the friction in its rotation, radioactive elements, and crystallization of iron.  Most of the heat lost by the outer core is from its surface, called the “D” layer.  The change in temperature between the bottom and top of this narrow (100 kilometer) layer is an estimated at 1000°C.  The width of the lay fluxgates because of temperature currents. Hotter, wider areas become less dense and less viscous.  Overlying material becomes unstable triggering a magnetic field reversal.  This process is still being studied.

Plate Tectonics and Asteroids

Francis Bacon first noticed how continents of Africa, South America, Antarctica, and Australia fit together like puzzle pieces in the 17th century.  It wasn't until 1912 that Alfred Wegener, a German meteorologist, proposed the revolutionary concept of continental drift.  His book, On the Origin of Continents and Oceans, proposed drifting continents where all the worlds land masses were once joined in a super-continent named Pangaea (meaning "all lands").  He also went against poplar belief in stating that lunar craters are the result of impacts not volcanism.  In the 1920’s the Swiss geologist Emile Argand believed the Himalayan Mountains range resulted from a collision between India and Asia.

More and more geologic discoveries of similar fossils from similar stone beds, the climates they lived in, and an apparent wandering polar region made them realize that the world was not static.  Even large land masses move.  As you might guess, it took over 50 years for such preposterous concepts to catch on.  Evidence collected from multiple fields mounted, and now we know they were correct.

Plate Tectonics (the study of how plates move) developed during the mid-1960s  Twenty years later scientists had instruments that could finally measure how much India was still pressing into Asia.  Core samples taken from the Atlantic seabed revealed a spreading seafloor.  Today, satellites track the continents, the expansion of volcanoes, and melting glaciers.  Telescopes show us pictures of close misses with large asteroids, and astronauts did not find lava on the moon.

Tuzo Wilson, a Canadian geologist, made the next breakthrough in the workings of plate-tectonics.  His hypothesis was so radical it took several years to get it published, and not in a geology journal but an obscure physics journal in 1963.  He realized the three volcanic island chains in the Pacific were in parallel lines.  Each chain's active volcano is in the south east corner (last island) and the oldest was at the far northern end.  All three chains also take a sharp course change at the same time of 43 million years ago.  From this he formed the "Hotspot" Theory.  This is a kind of volcano that is not related to a plate boundary, instead the plates slide over the area of activity.  Periodic episodes of volcanic activity produce a new crack in the crust and a new island.

A third kind of plate boundary was also introduced by Tuzo Wilson in 1965.  The first two were ridges (crumpling) and trenches (subduction).  This was transform faults where a horizontal slip of the plate does not destroy the crust, like in the San Andreas Fault.

"In 1971 W. Jason Morgan added to the hot spot theory. When the rising solid rock (mantle plume) reaches the plates it splits and spreads horizontally. This split or flow causes the plates to drift. Morgan proposed that there are 20 different hot spots in the world. Most hot spots are located at mid-ocean ridges, but there are a few located in the middle of plates, like Hawaii and Yellowstone"

"Most hotspot volcanoes are basaltic because they erupt through oceanic lithosphere (e.g., Hawaii, Tahiti). As a result, they are less explosive than subduction zone volcanoes, which have high water contents. Where hotspots occur under continental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts for form rhyolites. These rhyolites can be quite hot and form violent eruptions, despite their low water content. For example, the Yellowstone Caldera was formed by some of the most powerful volcanic explosions in geologic history"

over 50 active hotspot volcanoes have been located

Oceanic Ridge

When lava cools, iron oxide crystals orient with the Earth's magnetic poles.  This permanent compass lasts until the stone is altered or reheated.  On either side of mid-oceanic ridges, where new ocean floor is being made, the magnetic orientation within the basalt crust alternates in parallel, symmetric positive and negative bands.  This shows the Earth’s magnetic field has reversed periodically throughout the life of the planet.  These bands give us a precise timeline reaching back 160 million years.  Beyond the mid-Jurassic other forms of measurement must be used because all older ocean floors have been shifted by plate tectonics.

During the Carboniferous and Permian, the magnetic field did not reverse polarity for 70 million years.  Again in the Cretaceous, another extremely long interval of 35 million years occurs.  No mass extinctions can be directly linked to field reversals, but it is noteworthy that soon after the two stable time periods ended and the field started reversing again the two largest mass extinctions coincided with two of the largest volcanic hot-spot eruptions of all time.  A third long interval has been suggested during the Ordovician; it is being researched.

Hot-Spot Theory

(according to Dr. Vincent Courtillot's book Evolutionary Catastrophes)

Most volcanoes occur where continental plates collide.  Pressure produces heat; heat melts stone and turns water to gas.  Everything expands and becomes less dense; less dense liquids rise to the surface, often explosively.  All this happens in the lithosphere, which is 16-26 miles deep.

The lithosphere is the Earth’s crust made up of continental and ocean floor plates riding on the upper mantle's molten rock.  Occasionally some of the upper mantle is transported to the surface with the lava giving us an idea of its composition.

Hot-spot volcanoes are rarer and originate much deeper in the Earth, possibly 1800 miles below the surface.  The thin “D” layer that divides the core from the mantle becomes unstable.  Occasionally this heavy, very hot division lava bubbles into a plume, just like a lava lamp.  It rises only a couple feet per year through the lower and upper mantle.  This journey can take as long as 15 million years. 

The extra hot lava accumulates under the lithosphere, melts and buckles the plate, until the overlaying rocks crack in a violent release of lava and gas.  The reduced pressure allows the ground to relax and seal the fissure.  The lithosphere continues to move above the lava pool.  When enough pressure builds up again the lava cracks a new spot in the crust.  The Hawaiian Islands are a good example of the trail of mounds this kind of volcano produces.  In fact, a chain of old eroded underwater islands reach all the way to Russia to be lost in a subduction zone.

Oceanic plates are thinner and softer than continental plates.  Underwater Hot-spot volcanoes are not as explosive because their pressure is reduced sooner and they impact climate less because their dust and gasses are absorbed by the deep oceans.

Continental plates are more difficult to crack.  The buildup of lava and gasses can cause whole continents to rise.  When lithosphere finally gives way the eruption is tremendous.  But even here, once the gasses dissipate, it settles down.  Years to thousands of years can pass before the next huge eruption.  This can go on for half a million years.

A large continental hot-spot eruption can have a lava flow of 2 cubic miles per year, with convulsive phases of more than 240 cubic miles in a few weeks.  Violent eruptions darkened the Earth every few hundred years with ash and sulfur compounds, dropping the average air temperatures by 20˚F (1˚F is considered a significant event).  In the atmosphere, sulfur combines with water vapor forming sulfuric acid, or acid rain.  The acid dissolves limestone releasing carbon dioxide, triggering the greenhouse effect and raising temperatures.  Warmer oceans escalate the release.  Algae, which normally clean the air and oceans of carbon dioxide, are killed by the acidity.  The calcium shells of Plankton also dissolve, disrupting the food chain and starving the ocean.  Within 25 years, the average temperature of the lower atmosphere would increase 18˚F.  The Earth would slowly recover only to have the cycle repeated with another eruption.

The eruptions also expel 10 million metric tons of sulfur dioxide that combine with water in the atmosphere producing sulfuric acid.  The acid stays suspended much longer then dust, destroying the ozone layer and circling the globe with acid rain.  The amount of sulfur in an eruption dictates how much of a climatic impact occurs. More gasses mean the greenhouse effect occurs quickly.

Such extreme changes in light, temperature, and the percentage of carbon dioxide, within a few years, devastates animal population and a series of mass extinctions occur.

  A large continental hot-spot eruption can have a lava flow of 10 cubic kilometers per year, with convulsive phases of more than 1000 cubic kilometers

Hot-spots influence plate tectonics, often resulting in continental breakup.

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Reactions

1. Volcano releases sulfur into atmosphere

2. sulfur + water => sulfuric acid

3. sulfuric acid + limestone => carbon dioxide

4. carbon dioxide triggers a greenhouse effect heating the oceans

5. warmer oceans helps sulfuric acid dissolve limestone quicker => carbon dioxide =>more heat

6. carbon dioxide + water => lowers the pH of water, acidifying the water

7. carbon dioxide is usually absorbed by algae but high acidity kills the algae

8. skyrocketing acidity dissolves planktons shells => disrupting food chain => mass extinction

Hot-Spot and Continental Breakup

Hot-spot volcanoes drive plate tectonics, often resulting in continental breakup.

1. The eastern seaboard of the US was once connected to Africa and split at the end of the Triassic.

2. Antarctica split from South Africa during the Jurassic.

3. South America split from Africa at the end of the Jurassic.

4. Madagascar is split from Africa in the Cretaceous.

5. India splits from Africa at the end of the Cretaceous.