Even though plate tectonics has answered many of the questions dealing with how the surface of Earth changes over time, the theory itself still has many unanswered questions. Perhaps the biggest question is also the simplest one: What makes the tectonic plates move in the first place? The most common answer goes back to Arthur Holmes's idea of convection currents. If you recall, Holmes believed that heat generated by radioactive elements would cause "hot spots" in the mantle. As the rock became hotter, its density would decrease and it would start to rise. If this idea is correct, then convection currents in the asthenosphere would literally push the plates along. In places where two convection cells moved apart, there would be a divergent plate boundary, such as a mid-ocean ridge. In places where two convection cells came together, there would be a convergent plate boundary.
There are a few problems with having convection currents drive plate tectonics, though. If you recall, convection is how heat moves through fluids, like liquid and gas. Convection does not usually occur in solids. Seismic data shows that the asthenosphere
Plate Tectonics Helps Other Scientists Make New Discoveries
Plate tectonic theory has not only helped geologists answer questions about the structure of our Earth, it also has made contributions to other fields. Reconstructions of past continents have helped scientists in the fi elds of climatology and biology solve some of their own mysteries.
One example relates to the way that moving continents have changed the circulation pattern of ocean currents. When Pangaea began breaking up, water in the ocean was free to circulate around the planet at the equator. This made Earth somewhat warmer than today because warm ocean water could travel around the globe freely. But after the Atlantic Ocean opened up, the circulation pattern began to change. When the connection between North and South America, including Central America, was made about 2 million years ago, the equatorial circulation was blocked. Water then
circulated in the Atlantic from pole to pole instead of around the equator. This had a chilling effect on Earth. Some scientists believe this small change is what triggered the last ice age.
Plate tectonics has also played a huge role in explaining the evolutionary history of many animal and plant species scattered throughout the world. Take Australia for instance. It is well known that the "land down under" is home to some interesting animals like the echidna and the duck-billed platypus. These animals belong to a group of mammals called monotremes that are quite primitive and still lay eggs. In all other parts of the world, monotremes have been replaced by placental mammals, which produce young through live birth. Most biologists believe that the live birth process is much more efficient than laying eggs. So far, monotremes have only been found in Australia and some of the surrounding islands like Tasmania. The reason for the existence of these unusual animals is related to the theory of plate tectonics. According to the theory, when Gondwanaland broke up, Australia was the first to separate from the rest of the continents. This caused the primitive animals found there to become isolated from the rest of the mammals in the world. This separation prevented them from following the same evolutionary pathways that the mammals in the rest of the world followed.
is in a semisolid state. There is no proof that convection currents can form in a solid, even one as soft and pliable as the rocks in the asthenosphere. An even bigger problem concerns how the plates move on the asthenosphere. If the plates are simply sliding over the rocks of the asthenosphere, then the motion has to happen below it. Based on the seismic data, it appears that the mantle below the asthenosphere is a true solid, meaning that convection would not be possible there.
Even if convection is the cause of plate motion, then there is the question of why the motion starts, stops, and reverses direction. It is fairly certain that when two plates that carry continental crust collide, the motion between them is slowed because of the enormous forces needed to push the rocks up to form mountains. This may provide part of the answer. When the resisting force gets to be too great in one direction, the motion simply changes direction. As of yet, there is no proof that this occurs, but many geologists continue to explore this question.
Clearly, the theory of plate tectonics is far from complete. Geologists still have a great deal of work to do. In many cases, just when they think they have all the questions answered, a whole bunch of new questions arise. Remember, it took over 50 years for modern plate tectonic theory to come together. In many cases, the answers
had to wait for new technology to be developed. At the present time, plate tectonics is the best way that scientists have to explain the way the surface of our planet behaves. Who knows if, in another 50 years, an entirely new theory may come along to replace it. But that is the beauty of science—the only thing for certain is that it keeps changing!
Absolute Time A time scale based on the radioactive decay of certain elements that presents time periods in years.
Asthenosphere The part of the mantle that slows down earthquake waves. It is thought to be the layer on which the tectonic plates move.
Basalt An igneous rock that is rich in iron and makes up most of the seafloor.
Catastrophism The idea that large-scale Earth changes can only happen during large catastrophic events.
Continental drift The theory developed by Alfred Wegener in the early 1900s that says that the continents have changed their positions over time.
Convection The process where heat is transferred through a fluid.
Convection current The circular path that a fluid takes when it is heated from below.
Convergent boundary The area where two tectonic plates collide.
Core The center of the Earth. Earth's core has two parts: a solid inner core and a liquid outer core.
Crust The outermost layer of the Earth. The solid surface of the planet.
Daughter element The element that is formed when the parent element undergoes radioactive decay.
Decay rate The speed at which a radioactive element changes from parent element to daughter element.
Divergent boundary The area where two tectonic plates move away from each other.
Earthquake The shaking of Earth's surface caused by the movement of the crust along a fault zone.
Epicenter The point on Earth's surface directly above the place where an earthquake occurs.
Fault A crack in the crust of the Earth along which usually some movement occurs.
Focus The point inside the Earth where an earthquake occurs.
Fossil The remains of past life preserved in solid rock.
Gondwana A former supercontinent made up of South America, Africa, Antarctica, Australia, and India.
Granite A type of igneous rock that is rich in quartz and is found mostly on the continents.
Half-life The time it takes one-half of a radioactive parent element to turn into a daughter element.
Hypothesis An explanation for a set of observations that has not been tested yet.
Igneous rock A rock that crystallizes from liquid magma.
Inner core The central part of the Earth thought to be made up of solid iron and nickel.
Isostasy The tendency for low-density rock material to rest higher on Earth's surface than high-density rock.
Lava Hot liquid rock that comes out of a volcano.
Lithosphere The upper 60 miles (100 km) of Earth's surface containing both the crust and upper mantle.
Magma Hot liquid rock located inside the Earth.
Magnetic anomaly An area on the surface of the Earth where the planet's magnetic field is either higher or lower than normal.
Magnetic reversal A point in time when Earth's magnetic field flips, making the north magnetic pole into the south magnetic pole.
Mantle The layer of the Earth below the crust but above the core.
Mid-ocean ridge A divergent boundary where new crust forms. The mid-ocean ridge runs through the North Atlantic Ocean and circles the globe, making it the longest mountain chain on the planet.
Moho (Mohorovicic Discontinuity) The layer between the crust and the mantle; it was discovered by and named for Andrija Mohorovicic.
North magnetic pole The point on Earth where the lines of the magnetic field come together. Compasses point to the north magnetic pole.
Outer core The liquid part of the core that surrounds the solid inner core and lies below the mantle.
P-wave Longitudinal wave generated by an earthquake that travels at high speed and is usually the first to arrive at a seismograph.
Pangaea A supercontinent first described by Alfred Wegener back in 1915 demonstrating how all the continents were once joined together in the distant past.
Parent element A radioactive element that naturally changes or decays into another element called the daughter element.
Polar wandering The apparent motion of the north magnetic pole around our planet over time.
Radiation The energy released by the radioactive decay of different elements.
Radioactive decay The process where one element turns into another while releasing radiation.
Radioactivity The natural breakdown of an atom into a different type of atom.
Relative age date The age of a rock or event when compared to another rock. The exact age in years is not known.
Remnant magnetism Phenomenon where magnetite crystals in Earth's magma harden to form magnetic crystals that act as little microscopic compass needles pointing to Earth's north magnetic pole.
S-wave Secondary or shear wave that is generated by an earthquake and is usually the second type to arrive at a seismograph.
Sediment Small pieces of broken rock.
Sedimentary rock A rock, like a sandstone, that is made from pieces of other rock.
Seismic wave The type of wave produced by an earthquake.
Seismologist A scientist who studies how earthquakes behave.
Seismology The branch of Earth science that studies earthquakes and their causes.
Sial The upper layer of crust on the continents, made up of rocks made from the chemical elements aluminum and silica.
Sima The lower layer of crust on the continents and the only layer of crust under oceans. Composed of rocks rich in the chemical elements silicon and magnesium.
Sonar A device that uses reflected sound waves to locate the sea-floor or an object underwater.
Subduction The process where a piece of oceanic plate is recycled by plunging back down into the mantle.
Subduction zone A trench in the ocean where subduction takes place.
Submarine trench A large rip in the crust of the Earth.
Tectonic plate A large piece of the outer surface of Earth that moves around the planet.
Transform fault A fault found mostly in oceans that cuts across other submarine features like the mid-ocean ridge and trenches.
Uniformitarianism The principle that says big changes are sometimes caused by small changes that take place over a long period of time.
Volcano A surface feature of Earth through which lava and ash come up from below.
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Beyond Discovery: The Path from Research to Human Benefit—When the Earth Moves
An overview of the development of plate tectonic theory and its impacts on our lives.
A Science Odyssey: People and Discoveries http://www.pbs.org/wgbh/aso/databank/index.html
A databank of 120 great scientific discoveries made in the twentieth century.
United States Geological Survey (USGS): Men and Women of Seismology http://earthquake.usgs.gov/learning/topics/people.php
A collection of biographies about famous seismologists. Site also includes links to current information about earthquakes.
USGS: This Dynamic Earth The Story of Plate Tectonics
An informative introduction to the history and impact of plate tectonic theory.
Yale University Department of Geology and Geophysics The Origins and Early History of Earth Sciences at Yale http://www.geology.yale.edu/graduate/history.html
A collection of biographies of some of the famous men and women Earth scientists who either worked at or graduated from Yale University.
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