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Earthquakes, Volcanoes, and Mountains Study Guide (page 3)

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Updated on Sep 25, 2011

Mountains

Continents are part of the crust but are much thicker than the crust of the ocean floor. Furthermore, the crustal material that makes continents is much lighter than the oceanic crust. Continental material forms when relatively light magma bursts from below to the surface, solidifying as rock. Later, this is sometimes reworked into sedimentary and metamorphic rocks.

Plate movements that rub bits of crust together can cause continents to grow as the lightest, most buoyant material ends up staying on the surface. Thus, continents have generally been growing throughout time because once the light rock reaches the surface, it tends to stay there. Unlike the oceanic crust, which is about 200 million years old at most (because it continues to go back down into subduction zones), continents can contain rocks that are billions of years old. Rocks in parts of Canada, for example, are more than three billion years old.

Wind and water erode the continents, attacking the highest lands and carrying sediments into the ocean. These sediments tend to stay on the parts of the continental crust that are underwater, the continental shelves. So erosion takes from the highlands and gives to the lowlands. Without other forces that lift the land up, the continents would be extremely flat.

Figure 6.2

The forces of continental uplift are the forces that make mountains. What are they? Volcanoes are one answer, because some volcanoes are mountains. Many mountain systems of the world have formed when volcanism proceeded over great areas and over long periods of time.

When ocean crust of one plate subducts under a continent that sits at the edge of another plate, it is called a subduction collision margin. Mountains are born from the resulting volcanism; they appear in giant arcs on the continent above the subducting ocean crust, which becomes intensely hot and melts as it descends into the mantle. The molten rock, as pressurized liquid, pushes up into the volcanic range of mountains.

An example of an impressive mountain range formed by this subduction collision margin is the Andes Range, along the western coast of South America. Another example is the Cascade Range of the northwest U.S. coast.

But other kinds of mountains are not volcanic—the Himalayas, for example. How did the Himalayas form? What about other nonvolcanic systems of mountains? The answer, again, mostly involves the margins of the geological plates.

The Himalayas have formed as a result of the type of a convergent plate margin called a continental collision margin, or continental collision zone. In this type of collision, one plate with ocean crust subducts under another. But continental crust is also carried on the same plate as the one with the ocean crust that is subducting. The continental crust, being lighter than the ocean crust that is going down, cannot also go down. It smashes into the continental crust of the other plate. Thus, two masses of continental crust smash into each other. The result is a huge uplift, a massive system of mountains.

In the case of the Himalayas, they began when the plate that carried the land that is now most of India collided with the plate that had the continental crust that is today's Tibet. That was about 40 million years ago. By 20 million years ago, serious uplift of Tibet had begun, resulting in the tallest mountain range on Earth.

Another example of a mountain range formed by a continental collision zone is the mighty Alps in Europe. Yes, another example is the Appalachians, in the eastern United States. The Appalachians? The mountains of Smoky Mountain National Park in North Carolina, a part of the Appalachian chain, are very pretty mountains, but they are no towering Alps or Himalayas. What is their story?

A very long time ago, the Appalachians, it turns out, were like the Himalayas. They were lifted by collision activities around 400 million years ago. That occurred during a closure of a prior "Atlantic" Ocean, which created a gigantic supercontinent called Pangaea by geologists. Then, when Pangaea split about 200 million years ago, today's Atlantic Ocean started to form (which is widening year by year, as we have seen). Erosion by wind and water has taken the Appalachians down to a height that is perhaps a third of their former glory 400 million years ago. The Urals in eastern Europe are another example of a relatively humble mountain range that had been formed in a very ancient continental collision margin.

 

Practice problems for this concept can be found at -  Earthquakes, Volcanoes, and Mountains Practice Questions

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