Wednesday, August 25, 2010

A Brief History of Our Understanding of Planet Earth.

We know surprisingly little about our planet! The reason for this is that we are not able to probe the depths of Earth directly and explore. Also, because geological processes occur on much longer timescales than humans are familiar in dealing with. What little that we do know, we learned in the past couple decades! We learned how to split the atom before we learned how our own planet worked. Here I provide a brief history of our journey in understanding our home, planet Earth.

The age of Earth was of scientific speculation for many centuries. Finally, in 1953, Clair Patterson at the University of Chicago determined the now accepted age of Earth of 4,550 million (plus or minus 70 million) years old. He accomplished this through the Uranium/Lead dating of meteorites, which are the building blocks of planets. But once we discovered how old Earth was, a significant question came into the scientific forefront. If Earth is in fact ancient, then where were all the ancient rocks?

It took quite some time to answer this question. But it all started a few years earlier with Alfred Wegner, a German Meteorologist from the University of Marburg. Wegner developed a theory to explain geologic anomalies such as similar rocks and fossils being located on the East coast of the U.S. and the North West coast of Africa. His theory was that Earth’s continents had once been connected together, in a large landmass known as Pangea, and had since split apart into their contemporary locations. This theory opened up another question, what sort of force could cause the continents to move and plow through the Earth’s crust?

In 1944, Arthur Holmes, an English geologist, published his text Principles of Physical Geology in which he laid out his “Continental Drift” theory, which described how convection currents inside Earth could be the forcing behind the continent’s motion. Many members of the scientific community still could not accept this as a viable explanation for the movement of continents.

At the time, many thought that the seafloor of Earth’s oceans was young and mucky from all the sediment that was eroded off the continents and washed down river into the oceans. During the Second World War, a mineralogist from Princeton, Harry Hess, was on board the USS Cape Johnson. On board the Johnson there was a new depth sounder called the fathomer that was made to aid in shallow water maneuvering. Hess realized the scientific potential of this device and never turned it off. Hess surprisingly found that the sea floor was not shallow and covered with sediment! It was in fact deep and scored everywhere with canyons, trenches, etc. This was indeed a surprising and exciting discovery.

In the 1950’s oceanographers found the largest and most extensive mountain range on Earth, in the middle of the Atlantic Ocean. The mountain range, known as the Mid-Atlantic Ridge, was very interesting, being that it seemed to run exactly down the middle of the ocean and had a large canyon running down the middle of it. In the 1960’s, core samples showed that the seafloor was young at the ridge and got progressively older with distance away from the ridge. Harry Hess considered this and came to the conclusion that new crust was being formed at the ridge and was being pushed away from it as new crust came along behind it. The process became known as seafloor spreading.

It was later discovered that where the oceanic crust met continental crust, the oceanic crust subsided underneath the continental crust and sank into the interior of the planet. These were called subduction zones and their presence was able to explain where all the sediment had gone (back into the interior of the planet) and the youthful age of the seafloor (the older portion of seafloor currently being around 175 million years old at the Marianas Trench).

The term “Continental Drift” was then discarded, once it was realized that the entire crust moved and not just the continents. Various names were used to refer to the giant separate chunks of crust, including “Crustal Blocks,” and “Paving Stone.” In 1968, three American seismologists in the paper in the Journal of Geophysical Research called the chunks “Plates,” and coined the name for the science that we still use today, “Plate Tectonics.”

Finally it all made sense! Plate tectonics were the surface manifestation of convection currents in Earth’s mantle. This explained where all the ancient rocks on Earth’s surface went, that they were recycled back into the interior of the Earth. Plate tectonics gave answers to many questions in geology, and Earth made a lot more sense.

Plate tectonics are the surface manifestation of convection currents in Earth’s mantle. Convection involves upwellings and downwellings like in a boiling pot of water. Subduction zones are the downwellings in Earth’s convection system. Upwellings known as plumes are thought to exist, where hot material rises to the surface of the planet from the very hot interior. These plumes are thought to cause volcanism at the surface that are known as Large Igneous Provinces, such as the Shatsky Rise. We are out here today, continuing our journey of learning and understanding how our planet works. The data collected during this survey will hopefully shed light on what processes produced that Shatsky Rise, and if it was in fact a plume from Earth’s interior.

Note: Most of the information in this blog can be found in Bill Bryson's book, A Brief History of Nearly Everything

1 comment:

  1. Plate tectonics is also helpful in explaining the course of evolution, as the movement of continents either causes species to become isolated or, conversely, to come into contact with a new set of predators/prey.