Living at the foot of Pikes Peak brings a sense of awe and wonder. The massive rise of the Front Range of the Rocky Mountains is and omnipresent symbol of the forces of geology. The colossal summit of these mountains, more than a mile above a city already more than a mile above sea level inspired Katherine Lee Bates to write about “purple mountains majesty” in her song “America the Beautiful”.
But, just what was it that created these majestic peaks? What were the actual forces that are at work here?
The majority of the actions that create mountains are due to plate tectonics. The crust of the Earth is made up of numerous plates that move across the surface of the planet. Where these plates come into contact various forces and reactions create mountain ranges.
When two plates of continental material collide they buckle against one another creating massive ranges, like the Himalayas. The same event that created the Himalayas is believed to have also created the Alps, the Pyrenees and most all of the major mountain ranges of Europe, the Middle East and Southern Asia. In the case of the Himalayas the plates of Africa and India moved northward into the massive Eurasian plate. Since both plates were of similar makeup neither could be forced under the other causing the edges to buckle, just like if you push the edges of a tablecloth together towards the middle it will fold and bunch up.
This process is continuing today in some of the mountainous regions of Europe and Asia. However, the changes are so slow that we are unable to measure them. The building of mountains takes millions, or tens of millions, of years. Imagine the tallest mountain on Earth, Mt. Everest. At 8,848 meters above sea level, even if the mountain started at sea level, over ten million years it would have risen less than a millimeter per year to reach its present height.
When an oceanic plate comes into contact with another plate, whether it’s a continental plate or another oceanic plate, one of the plates goes under the other, creating a subduction zone. Oceanic plates are made up of basalt, a fairly dense rock. Continental plates are made up of granite, and the sedimentary and metamorphic products of granite, which are considerably less dense than basalt. Therefore, oceanic plates will always be driven under continental plates. In the case of contact between two oceanic plates there is no certainty as to which will go over and which will go under, but if they’re not sliding alongside one another, one has to go under the other.
Near the places where oceanic plates are driven underneath other plates another form of mountain building occurs. The oceanic plates that are driven deep into the Earth heat and some of the rocks and minerals that make up these plates are melted. The molten rock, being lighter than any of the solid rocks above, forces its way through fissures and weak spots in the overlying crust. This is how the Rocky Mountains and volcanic mountains such as the islands of Japan were formed.
In the case of most of the Rocky Mountains the molten rock was unable to break through to the surface creating lava flows. Instead, the molten rock, or magma, was forced upward and bulged the overlying sedimentary rocks lifting them. As the magma cooled slowly underneath the existing rocks, granite batholiths formed. A batholith is a large mass of igneous rock formed in a pocket deep in the Earth. In the Rocky Mountains, the massive granite peaks were once covered with other rocks, and traces of those rocks still appear throughout the mountain range. Over time, including during the millions of years it took for the mountains to form, much of the sedimentary and metamorphic rock on top of these batholiths was worn away to expose the peaks we see today.
Volcanic mountains, such as Mt. Fuji in Japan and the volcanoes in the American Northwest, are formed when the magma forces its way to the surface. Magma that has reached the surface is called lava. When this occurs the rock cools more quickly than in the case of the formation of intrusive batholiths and the crystalline structure of the rocks is much finer, creating basaltic type rocks like those that form the oceanic plates.
Volcanic mountains can be of several different types. Two of the most common types are shield volcanoes and cinder cones. Shield volcanoes are those with low sloping sides, like a shield laid flat on the ground. These form when thin, quick flowing magma flows repeatedly from the Earth in fairly even layers all the way around the vent of the volcano. Cinder cones, spatter cones and ash and tuff cones are formed when gasses inside the magma force the lava to be thrown into the air falling around the vent. The force of the ejection, the temperature of the lava and how the ejecta cools determines what form the material that makes up the cone takes.
Another type of mountain building, nearly always forming volcanic mountains, seems to be completely independent of plate tectonics. The prime example of this is the Hawaiian Islands in the Pacific. In the Hawaiian Islands there appears to be a particularly hot spot in the mantle, the layer of the Earth directly under the crust. This hot spot forces magma to flow through to the ocean floor forming volcanoes. Since all of the plates of the Earth are moving, and the hot spot doesn’t appear to be, as the Pacific Plate moves across this spot in the mantle islands are formed in turn creating the chain of islands we are familiar with.
Of course, as implied in the discussion of the Rocky Mountains, above, there are other forces at work creating what we see on the surface of the Earth. Remember how it was mentioned that the rocks overlying the batholiths that formed the Rocky Mountains were eroded away, exposing the granite peaks? That’s because there are forces that wear away the mountains, even as they’re being formed. These forces act even slower than the forces creating the mountains, if they didn’t the mountains would wear away, or erode, before they could be formed.
Erosion is the removal of soil and rocks through numerous causes. Many of the dramatic features we see in the mountains were caused by glacial erosion; the wearing down of the rocks through massive slabs of ice slowly flowing, like rivers, downhill. And rivers, too, cause erosion. As water flows over rock it can dissolve small amounts of the rock. And water also can carry particles that bounce over the rock further increasing the rate of erosion. Ice can form in crevices in the rock, expanding and wedging the rock apart breaking small and large chunks away. And wind can, like water, carry small particles that can further break down rock.
Even life causes erosion. Fungi and lichens create mild acids that eat away at rock. The roots of trees growing in crack in rocks can have the same effect, although at a much slower rate, as the expansion of ice in crevices, wedging rocks apart from one another.
Many of the most spectacular features in mountain ranges today were created by erosion. But, over time this erosion will wear away even the greatest of mountains. Some geologists believe that the Appalachian Mountains of the Eastern United States were at one time greater than the Rocky Mountains. Today there are few peaks in the Appalachians that surpass 6000 feet above sea level, while the Rocky Mountains has 25 peaks above 14,000 feet above sea level. The difference is the age of the mountains, and the amount of erosion experienced over time. The Rocky Mountains were formed much more recently than the Appalachians and, over time, the Appalachians have just worn away.
With nearly 25% of the Earth’s land mass covered by mountains, and considering that mountains form most all of the islands in the oceans, understanding how mountains are created, and destroyed, is an important part of understanding the Earth. And, beyond the majesty of the mountains themselves, the awe inspiring forces at work which created these geologic features are impressive in and of themselves.
