2.6 - Plate Tectonics

Instructor Video Links to an external site.

Physical Geography

Physical Geography is the study of the processes that shape Earth's landforms, climate, and vegetation. Like all Geographers, Physical Geographers look at problems spatially. Understanding the Physical Geography of the Regions we will be studying is essential to understanding how and why these regions developed the way they have, the resources they have available, the nature of the environment that people living in those regions interact with.

Plate Tectonics

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The processes that create the world's landforms (mountains, continents, valleys etc) are some of the most powerful and slow-moving forces on the planet.  Land forming process fall into two broad categories: internal land forming process (plate tectonics) and external land forming processes (running water, glaciers, wind etc). Two key ideas to internal processes are the Pangea Hypothesis and the Theory of  Plate Tectonics. In 1912, a geophysicist named Alfred Wegener, first proposed the Pangea Hypothesis. As soon as we had maps that were fairly accurate, people started to observe that the continents looked like that they fit together like the pieces of a jig saw puzzle. Wegener applied science to what had been casual observation. He looked at the fossil records and the geologic record and saw that they matched what would have been adjacent continents.  Then the records diverge supporting the idea that the continents eventually drifted apart from each other.

It appears today, that Wegener was correct about so many things but he didn't have access to all the information which we do today. His theory was criticized and eventually rejected. Wegener couldn't explain how the continents moved. It wasn't until the late 1960's and 1970's that we finally had that answer and Wegener's theory was reborn as the modern theory of Plate Tectonics. What Wegener could not have known, was that within the Earth there is a layer of molten rock called the "asthenosphere Links to an external site.". Geophysicists learned about its existence by observing the speed and direction of seismic waves as they move through the Earth's interior.

The modern theory of Plate Tectonics proposes that the continents move because of giant convective cells within the asthenosphere. New material rises at the mid-oceanic ridges and older crust is destroyed through "subduction Links to an external site." where plates collide. Plates pull apart (diverge), collide (converge) and slide by one another (transform).  On average, plates move at a rate of about 2 to 4 inches per year. Many of the Earth's landforms are explained by this theory. Volcanoes across the planet, mountain ranges, many island chains, deep oceanic trenches, all exist due to the movement of these large "lithospheric plates Links to an external site.". Earthquake activity around the world is also closely correlated with plate boundaries. Scientists believed that this is a continuous cycle and that the plates have collided and moved apart many times in the Earth's history.

We know today that the internal structure of the Earth includes the rigid lithosphere, the molten asthenosphere and other layers. The lithosphere is composed of two types of crust: Continental and Oceanic.

Oceanic crust  found under oceans is about four miles thick in most places. At the mid-ocean ridges new oceanic crust is being created. Magma moves up through gaps in the ocean’s floor here. As it cools, it hardens into new rock, which forms oceanic crust. Oceanic crust is more dense than continental crust, it is constantly sinking (or being subducted) and moving under continental crust. The most common rock associated with Oceanic Crust is basalt.
 
Continental crust averages about miles deep. Continental crust tends to be less dense and much older than the oceanic crust, and rocks found on this kind of crust are often the oldest in the world. Examples of such rocks are those in Quebec, Canada which are estimated to be about 4 billion years old. The most common rock associated with Continental Crust is granite.

Evidence

Alfred Wegener provided us with the first evidence of this theory: continental shape, the fossil record, the geologic record (rocks matching like fingerprints with rocks on what were adjacent continents). Other evidence has been added to Wegener's discoveries including:

• evidence of glaciation from 380 to 250 million years ago  in Antarctica, southern South America, southern Africa, India, and Australia suggesting they were all clustered around the South Pole.
• the age of the sea floor - youngest rocks near the mid-oceanic ridges, older rocks and the margins of the oceans
• the correlation between mountains, volcanos, and earthquakes, sea floor trenches and plate boundaries
• the formation of the Hawaiian Islands Links to an external site.

 

The Ring of Fire

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Worth a special mention (especially since we live here) is the Ring of Fire.  The Ring of Fire,  runs along the margins of the Pacific Ocean and is characterized by active volcanoes and frequent earthquakes. Its length is approximately 24,900 miles. It traces boundaries between several tectonic plates—including the Pacific, Juan de Fuca, Cocos, Indian-Australian, Nazca, North American, and Philippine Plates.

Seventy-five percent of Earth’s volcanoes—more than 450 volcanoes—are located along the Ring of Fire. Ninety percent of Earth’s earthquakes occur along its path, including the planet’s most violent and dramatic seismic events.The abundance of volcanoes and earthquakes along the Ring of Fire is caused by the  movement of tectonic plates in the area. Along much of the Ring of Fire, plates collide at convergent plate  boundaries (subduction zones). That is, dense oceanic plates are subducted, beneath other plates they collide with.  As rock is subducted, it melts and becomes magma. The abundance of magma so near to Earth’s surface gives rise to conditions ripe for volcanic activity. A significant exception is the border between the Pacific and North American Plates. This stretch of the Ring of Fire is a transform boundary, where plates slide past one another. This type of boundary generates a large number of earthquakes as tension in Earth’s crust builds up and is released. The San Andreas Fault complex is perhaps the world's best example of a transform boundary and  earthquake fault.