[MUSIC] When portions of the Earth's crust move, crumple and dive, mountains are created. But the shaping of mountains can also depend as much on the constructive power of tectonics and on the destructive agents of erosion, water, ice and gravity. Together these forces conspire over great spans of time to produce four main types of mountains. Volcanic mountains, convergent mountains, fault-block mountains and dome mountains. Let's first take a look at volcanic mountains. Volcanoes occur on all continents and make up a substantial percentage of the world's mountains. The number becomes even more impressive if you include those hidden by oceans. Most islands, in fact, are nothing more than the summits of underwater volcanoes. Volcanic mountains are formed when gas-rich molten rock, or magma, from deep within the Earth moves forcefully to the surface, erupts, and accumulates, and cools in various sizes and forms. Magma is called lava when it breaks through the Earth's crust. Successive eruptions, usually separated by long periods of inactivity, combine to build the structure of the volcano. Volcanoes are associated with three basic tectonic regions. The first is rift-valley spreading centers, like Iceland, along the mid-Atlantic ridge, or Mount Kilimanjaro, on the east Africa rift system. The second is along convergent boundaries, where one plate is subducting beneath another. Like Japan's Mount Fuji, or others along the Ring of Fire. Lastly, volcanoes can also be found above intraplate hotspots, where plumes of solid, yet mobile mantle rock, rise to the surface, kind of like blobs in a 1970s lava lamp. Hotspot volcanism is unique in that it occurs within tectonic plates and is generally not related to plate boundaries. While a plume that feeds hotspot volcanos remains stationary relative to the mantle and the crust above is always in motion, a chain of regressively older volcanoes is created on the overlying plate. A striking example of these hotspot tracks is the string of Hawaiian Islands in the Pacific Ocean. The youngest of the islands, Hawaii, now overlies the hotspot. The island hosts one of the largest mountains in the world, Mauna Kea, rising 10,000 meters from its base deep in the ocean, Mauna Kea is the tallest mountain in the world, as measured from base to peak. We'll talk more about volcanoes in a later lesson on hazards. A second type of mountains are convergent mountains, or orogens. These are actually the most common type of mountains. In fact, the world's largest mountain ranges are all convergent. Examples include the Himalaya, and the Andes, European Alps. These are active origins meaning the processes that built them up are presently ongoing. In contrast, the Rocky Mountains of western North America and their eastern continental cousins, the Appalachians. These are no longer active, but they were all formed as a result of the same basic process, by the slow collision of two plates at convergent boundaries. When plates and the continents riding on them collide, the continental crust is too buoyant to be subducted and ends up being shortened instead. The accumulated layers of rock crumple, fold, and fault causing extraordinary uplift. Much of the folding occurs underground, where rock is hot enough to become more flexible. Nearer the surface, cold brittle rock cracks and forms faults. Faults can be found at the surface too. This is usually because the erosion layer has removed the upper, more brittle surface layers. As we look around the Canadian Rockies, we see lots of evidence of tilting and folding. Rock layers that were once level now rest at almost every conceivable angle. Geologists give formations of tilted and folded rock special names. A fold where younger rock is in the middle, between layers of older rock that have been uplifted on either side, is called a syncline. Folds where older rock have been brought up the middle are called anticlines. Folds occur in a variety of orientations and anticline or a syncline lying on its side is said to be recumbent. Faults also have special names. A gently sloping fault, where originally lower older rock have been pushed over younger, higher rocks, is called a thrust fault. In some places, multiple thrust faults have stacked slabs of rock, like overlapping shingles. These thrust faults are said to be imbricate. The shaping of mountains, remember, depends on both the constructive power of tectonics and the destructive agents of erosion. Water, wind, gravity, and ice. Consider for example the Appalachian Mountains in eastern North America. Here's University of Alberta geologist John Waldron. >> So at one time these mountains were probably as high as the present-day Himalaya. The crust that's now in the Appalachians began deforming around 419 million years ago, when an ancient ocean that lay to east began to close due to the plate tectonic process that we call subduction, where one plate moves under another. Subduction continued until a second continent, roughly equivalent to today's northern Europe, collided with ancient North America. This happened, we think about 430 million years ago. About a hundred million years later, a third, enormous continent, Gondwana, collided with North America. By this point, most of the Earth's continental crust, formed a single super continent, Pangea, which had a huge range of mountains through the middle. Plate tectonics eventually tore Pangea apart, starting in the Jurassic period, about 118 million years ago. Parts of the mountain range got left behind in North America as the Appalachians. Other pieces can be found in Scotland, Norway, and Greenland. Millions of years of erosion have taken their toll on the mountains. And the highest peaks of the Appalachians are today less than a third of the height of Mount Everest. >> Today the Appalachians are just one remnant of Pangea's mountains, stretching from Newfoundland in eastern Canada through to the southern state of Alabama in the US. They're related to the mountains of the Scottish Highlands, and the mountains of Norway and Sweden. And these were all part of a single enormous mountain range that once ran through the middle of Pangea. A third type of mountains are fault-block mountains. Fault-block mountains form when the faults in the Earth's crust allow portions of the surface to drop and others to rise as opposed to earth folding over and bending under pressure and heat. The Teton Range in the American Rocky Mountains was formed in this manner. Less than 10 million years ago, the Earth's crust here cracked or faulted. West of the fault line, a block of rock about 40 kilometers long tilted upwards to form the Teton Range. Meanwhile, east of the fault, a block fell to form the valley called Jackson Hole. And the action resembles a pair of swinging doors, with one swinging up, while the other down. The fault was caused by a swelling of the Earth's crust, not far below. Of course the movement is generally very slow, taking place in short jerks over thousands to millions of years. As the Tetons rose and the valley sank, the area was stricken by severe earthquakes. While earthquakes are often part of the mountain building process, no major quakes had been recorded on the Teton fault for at least a century. This range continues to rise almost a foot every 300 years. In the meantime, erosion from glaciers and running water carve the fault block into a series of scenic peaks. The Grand Teton is the highest mountain in the rage and nearly 4,200 meters. The last of our four principal mountain types are dome mountains. Dome mountains are the result of a great amount of magma pushing itself up under the Earth's crust. Without actually erupting onto the surface, the molten rock pushes out the overlaying rock layers. Now at some point, the magma cools and hardens. The rock layers over the hardened magma are warped upwards to form the dome while the rock layers of the surrounding area remain flat. In some cases, over long periods of time, erosion can wipe away the outer layers of the mountain exposing the dome shaped cool magma now hard rock. A dramatic example of dome mountains comes from the flat expanse of North America's Northern Great Plains. Rising high above the prairie grassland near the Alberta-Montana border. West Butte is among the highest of the Sweetgrass Hills. And it was formed about 48 million years ago when molten rock welled up deep within the earth. The molten rock uplifted the surface sedimentary layers, but solidified before ever reaching the surface. Millions of years of erosion have stripped away the top softer layers leaving the harder rock behind as an isolated landform above a flat plain. The area's Indigenous Peoples, the Blackfoot Nation, often use the top of west Butte to look for bison herds. In fact much of the Blackfoot's traditional territory can be seen from its summit. A powerful presence in the center of their world, the Dome Mountains of the Sweetgrass Hills remain sacred to the Blackfoot people. Younger tribal members often climb the hills in order to undertake vision quests, an important ritual fast resulting in dreams of the spirit world and revelations of the origins of all things.
