GED Science Practice Test: Plate Tectonics

The layers of the geosphere influence a number of processes and structures on earth.  We already mentioned that the earth’s outer core is responsible for creating the earth’s magnetic fields.  The layers of the geosphere are also responsible for volcanic activity, earthquakes, and geographic features such as mountains.  In order to understand these processes and structures, we must take a closer look at the lithosphere and the asthenosphere.

Averaging at least 80 km in thickness over much of the Earth, the solid lithosphere is broken up into moving tectonic plates that contain the world’s continents and oceans. Tectonic plates are composed of thinner oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust. These solid plates move by floating on the hot semi-solid material in the asthenosphere, which is slowly moving. The following picture shows the major tectonic plates of the world.

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Plate tectonics is the theory that:

  1. the earth’s lithosphere is broken up into these tectonic plates
  2. that these tectonic plates move slowly over time, and
  3. that the movement of these plates explains geographic features, volcanoes, earthquakes, and even the changing fossil record.

The way that plate tectonics influence so many earth structures and processes can best be understand by looking at plate boundaries, which is where two tectonic plates come together.  These boundaries are classified by the relative motion of the two plates:

  1. Transform Boundaries: The plates slide past each other.
  2. Divergent Boundaries: The plates spread away from each other.
  3. Convergent Boundaries: The plates move toward each other.

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At divergent boundaries, solid tectonic plates move apart from each other, allowing the liquid asthenosphere to rise up in the gap, and solidify to extend the tectonic plate.  Divergent boundaries can occur between continental plates, resulting in rift valleys.  Volcanoes can be one result of a divergent boundary, with hot magma rising up at places where two plates diverge from one another.  However, most commonly, divergent boundaries are found where two oceanic plates meet, at oceanic ridges.  Plate tectonics explains sea floor spreading, and in fact, the scientific community first accepted the theory of plate tectonics after the concepts of seafloor spreading were developed in the late 1950s and early 1960s. The following diagram depicts sea floor spreading between two oceanic plates at a divergent boundary:

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Along convergent boundaries, two different things can occur:  two tectonic plates can collide into each other, or one plate can slide under the other.  In order to predict or explain which of these two actions might occur, it is necessary to contrast continental and oceanic plates.

Continental plates tend to be thicker and less dense.  Oceanic plates tend to be thinner, and more dense.  Density is the characteristic of a material that describes how tightly packed a material is.  For example, Styrofoam is much less dense than metal.  One can think of density as how heavy something is for its size.  Denser materials are more likely to sink, while less dense materials are more likely to float.

Continental and Oceanic Plates: If a continental plate and an oceanic plate meet at a convergent boundary, the denser oceanic plate will slide under the less dense continental plate.  The process of one plate sliding under the other is called subduction;  the oceanic plate is said to subduct.  When the oceanic plate subducts, it sinks down into the hot, liquid asthenosphere, and melts.  At convergent boundaries between continental and oceanic plates, the forces between the plates can create earthquakes, mountain ranges, and volcanoes.  In a volcano, the molten magma of the asthenosphere pushes up and out of the crust

The crust lost due to the subduction of oceanic plates is roughly balanced by the formation of new oceanic crust along divergent plate boundaries.  In this way, the total surface of the globe remains the same.

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Two Continental Plates: If, on the other hand, two continental plates meet at a convergent boundary, they are of the same density, and neither is more likely to sink below the other.  At convergent boundaries between two continental plates, the plates typically collide and push upwards.  Mountain ranges are the result of such convergent boundaries.

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At transform boundaries, plates are not moving toward or away from each other, but rather are sliding past each other.  Transform boundaries commonly produce earthquakes.  One example of a transform boundary is the San Andreas fault in California.  At the San Andreas fault, the North American tectonic plate and the Pacific plate are sliding past each other in opposite directions.  This is why earthquake activity is so prevalent in California.

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When the theory of plate tectonics was first proposed, it was not readily accepted.  The idea that somehow large chunks of the earth’s crust were floating on molten rock, and that those plates moved was pretty radical.  However, there are a number of pieces of evidence to support plate tectonics:

Locations of Volcanic and Earthquake Activity:   If the theory of plate tectonics is true, it would predict that volcanoes would be most prevalent along the boundaries between plates.  Indeed, if one superimposes the location of volcanoes on top of a map showing the location of the plates, you can see that this is indeed the case.  The following image shows the relationship between volcano presence and the plate boundaries.  It also show the “Ring of Fire” which is a ring of volcanoes in the Pacific Ocean:

sci138Plate tectonics also predicts earthquake activity to be highest along the boundaries between plates.  This proves true as well:

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Sea Floor Spreading:  If there are indeed divergent plates at some ocean boundaries, and new oceanic crust is being formed at those divergent boundaries, plate tectonics would predict the the oceanic crust at the plate boundary is the newest in age, while the oceanic crust farther from the plate boundary is the oldest in age.  This pattern holds true when scientists dated the age of the oceanic crust.

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Also, the earth’s magnetic polarity flips over time.  This can be likened to a bar magnet switching its north and south poles.  Interestingly, the sea floor shows these changes in the earth’s magnetic polarity.  Since the oceanic crust in the sea floor is solid, the magnetic polarity of a certain time is “locked in” and can’t change:

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Shape of Continents:  Plate tectonics says that the tectonic plates have moved throughout earth’s history.  For example, there is a divergent boundary in the Atlantic Ocean between North America and Europe.  These two land masses have been moving apart from each other for a long time.  One could predict that if you “rewind the tape” far enough, that North America and Europe might have been joined.  The shape of the continents supports this idea.  If you cut pictures of the continents out and try to fit them together, the shapes match extraordinarily well:

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This image is what would be predicted by plate tectonics if you could see earth 225 million years ago, with all of the major continents we know today forming one giant supercontinent, called Pangaea. Perhaps initiated by heat building up underneath the vast continent, Pangaea began to rift, or split apart, around 200 million years ago. Oceans filled the areas between these new sub-continents. The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy. These continents are still on the move today; the relative movement of the plates typically varies from zero to 100 mm annually.

Fossil Record: When scientists examine the fossil record, they find similar fossils on continents separated by large geographic distances.  Placing the continents together, by their shapes, to simulate the landform of Pangea, the locations of the fossils on different continents begins to make more sense. Large bands can be drawn to show where fossils of a similar type are found.  The fossil record suggests that the continents were indeed, at one time, touching, which is support for the theory of plate tectonics, which explains how the continents move.

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