The shape of the continents led early explorers to speculate that once the continents may have fitted together. Wegener, a German meteorologist found evidence that plant fossils from S-America and Africa were very similar, a feature up till then explained by the existence of "landbridges". He also found evidence for periods of glacial climate on both continents: scratches on rocks (from glaciers) and evidence on ice movement suggesting a similar ice cap geometry on both sides of the Atlantic. He then postulated that the continents once formed a giant single continent which he labelled "Pangea". The evidence at the time was compelling, but no mechanism was found that could do such a thing as splitting continents. The theory went down the drain for several decades, until Arthur Holmes, of the great British geologists in the UK proposed that the mantle is in convective motion and that the continents are carried around on these mantle ‘conveyor belts’. This was gradually accepted until the evidence of the "magnetic stripes" on the sea floor was found in the early 1960’s.

Ocean research was in its infancy those days and commercial vessels carried out on a voluntary basis research projects while crossing the oceans. A magnetometer was towed from Europe to the US and the intensity of the signal varied in a peculiar pattern. Later studies revealed that the floor of the oceans is magnetized in either "normal" or "reversed" magnetic directions. The ship magnetometer measured the modern magnetic field (normal) and saw the signal of the ocean floor rocks (like a small buried dipole magnet). When the modern field and the rock field lined up (both normal), a more intense signal was registered, and when the rock signal was reversed, its intensity was subtracted from the modern field giving a slightly weaker signal. These primitive observations were ultimately translated into a pattern of magnetic stripes in the ocean floor, and the concept of Seafloor Spreading was born. In the middle of the oceans, magma is generated which rises, and two plates are ‘sliding to the side’, with the space in-between filled with new magma. This submarine volcanism leads to the Mid Ocean Ridges (MOR), huge mountain chains that rise 2-3 km above the sea floor and consist of active volcanic belts. When the magma forms during a normal magnetic period, we get normally magnetized rocks, they have a reversed signal when the earth magnetic field had its N pole at the South. The age of the ocean floor thus varies from zero at the MOR and getting older away from the MOR in a mirror image. Usually, the ridges are cut by transverse faults, so called transform faults, which gives a zig-zag pattern in ridge geometry. The new ocean floor creates new crust and since the surface of the earth is constant, it has to disappear somewhere again. This happens in Subduction zones, areas where usually oceanic floor with its sediments slides under another section of crust, be it continent or ocean floor.

In the late 1960’s the various concepts were unified in the theory of Plate Tectonics. This model states that the surface of the earth is covered by a limited number of rigid plates (crust) which are created on one side (MOR), destructed on the other end (subduction), and locally grind along each other (e.g., San Andreas Fault). All of this is driven by mantle convection, which is a result of the heat transport in the whole earth (core= hotter than mantle = hotter than crust). The continents drift along on their plates and occasionally run into each other, leading to mountain formation (orogenesis). The Himalayas are the result of the collision of India with Asia (see the animations on the websites) and the Alps result from a collision of Africa with Europe.

This big picture of the earth has many interesting details, some of which we touched upon in class. When the new ocean floor forms we have hot magma (basalt at 1300 ^oC) that comes into contact with cold seawater (usually around 0 ^oC), roughly at a depth of 2.5 km (top of MOR). The magma quenches into glass (remember piece of ocean floor that I showed), and the cooling leads to shrinking. The new ocean floor cracks and seawater sinks into it. This heats up and then rises again (thermal convection cell), and the hot waters (370-400 ^oC) are injected back into the sea. The hot fluids react with the glassy rocks and pick up chemicals (dissolved elements) and alter the ocean floor into new water and Cl-bearing minerals. Much of the sulfate in sea water is reduced to H₂S in these systems and is injected back into the sea. It is around these submarine hot springs that we find the vent faunas, communities of live living in the dark, cold and toxic deep oceans. Photosynthesis can not be the basis of the local food web here (like people on earth all live ultimately from photosynthetic products), and there must be another source of energy than light. It turns out that the vent communities live off bacteria that thrive on H₂S, through the chemosynthetic reaction

O₂ + 4H₂S + CO₂ è CH₂O + 4S + 3H₂O.

The energy that is released from the H₂S to S oxidation is used to convert CO₂ gas to organic matter (CH₂O). This happens in sea water/vent fluid mixtures where Oxygen is available (aerobic process).

The simplified photosynthetic production reaction is (occurs in eukaryotic organisms)

CO₂ + H₂O + light è CH₂O + O₂.

Here the energy of light is used to split the H-O bond of water and make C_org.

A third alternative is anaerobic photosynthesis, used by prokaryotic species

2H₂S + CO₂ + light è CH₂O + 2S + H₂O

These three reactions produce almost all of the food for the living world. The producers are labeled autotrophs (‘self-feeders’) whereas those that eat other things are called "heterotrophs" (e.g., people, groundhogs). The fundamental difference between chemosynthesis and photosynthesis is the origin of the energy source. For chemosynthesis, it comes from the inside of the earth and such a process could conceivably happen on any planet independent of climate. Photosynthesis and bacterial anaerobic photosynthesis depend on the availability of sunlight, a source of energy outside of the earth.

The H₂S consuming bacteria support a whole ecosystem around the geothermal chimneys, structures made of minerals that precipitate during the mixing of hot fluids and cold sea water (see readings). The strangest creatures abound here, like spaghetti-worms, giant tube worms, vent clams and ugly pale fishes. Some of the giant tube worms keep the bacteria in an organ (trophosome) which is their whole body. The H₂S diffuses through their skin, is bound to an enzym (to protect the worm), and transported to the ‘holding cell’, where the H₂S is delivered to the captives. They do their job and then are "eaten" by the worm. These beasties have no mouth, anus (or brains) and are living "guts-only" in symbiosis with the bacteria. Read the descriptions on these unique colonies in the various web links.

The life cycle of the vents is limited because precipitation of minerals leads to ~100 m high chimneys that vent the hot fluids but ultimately get congested. The system dies out, the H₂S no longer appears and the colony of beasties will die. It is very puzzling how these organisms then move to another hot spring field and settle again for a few 100 years until that is spent. There is evidence that the many chemical reactions create diffuse light which the beasties follow. When the light is flipped off, they move on! The MOR systems are dotted with these vent systems (a whole chimney was put on display in the AMNH) and are a major mechanism of cooling the newly formed ocean floor as well as cooling the whole earth slowly.

The cooling of the new plate, which is about 8 km thick, has 3 effects:

• The cooling process shrinks the plate, which then also gets denser
• The denser plate sinks a little bit more into its mantle substrate
• The ‘shrinking and sinking’ leads to a deeper ocean floor, and the larger column of water also pushes the plate a bit deeper into the mantle.