Mountain building
Mountain building, or orogenesis, is a geological process primarily driven by plate tectonics, involving the movement of the Earth's lithosphere, which comprises large rock plates. These tectonic plates interact at boundaries—divergent, convergent, and transform—leading to various mountain formations. At convergent boundaries, plates collide, causing the crust to fold and uplift into mountain ranges, such as the Himalayas, which are still growing today. Divergent boundaries result in the separation of plates, allowing magma to rise and create new mountainous formations, as seen in the Mid-Atlantic Ridge. Transform boundaries involve lateral sliding of plates, producing deformations and mountains like the Sierra Nevada. Erosion also plays a significant role in shaping mountains over time through natural forces such as wind and water. The study of mountain building is enriched by advanced technologies, including computer modeling and satellite observations, which help scientists understand the geological history and ongoing processes of these majestic landforms. Historically, mountains have held cultural significance, inspiring awe and reverence across various civilizations, further highlighting their importance beyond mere geological phenomena.
Mountain building
A diversity of geological and geodynamic factors contribute to the formation of the Earth's mountains, a process also known as orogenesis. Plate tectonics—the movement of massive stone plates beneath Earth's outer crust—is central to this process. As these plates contact and then pull away from each other, the outer crust is pushed outward, forming mountains. In some cases, this push is dramatic, while in others, it is more gradual and “gentle.” Many mountains are remnants of much larger, ancient mountains that have since been eroded by geodynamic forces.
Basic Principles
Mountains can be formed in a variety of ways, but most scientists believe that it is primarily done through plate tectonics. The theory of plate tectonics, which has been overwhelmingly accepted in the scientific community, states that beneath the Earth's outer crust lies a layer of seven massive rock plates called the lithosphere. These continental plates (the North American, African, South American, Eurasian, Antarctic, Australian, and Pacific), along with other smaller plates, are constantly in motion and moving in different directions.
The tectonic plates occasionally collide with one another at plate boundaries. In the case of convergent boundaries, the force of these direct collisions causes the boundaries to fold; the upward fold pushes outward in the form of mountains and mountain ranges. When plates move away from one another (to create divergent boundaries), parts of the lithosphere break away and fall into the superheated asthenosphere beneath, causing hot magma to flow into the empty spaces. The magma then pushes against the outer crust, creating cone- and dome-shaped mountains. The mountain becomes an active volcano if lava is pushed through the surface of the domes. When plates sideswipe each other (to create transform boundaries), the shearing effect produces mountainous deformations on the surface.
Mountains are also formed on the Earth's surface through erosion rather than plate tectonics. In this manner, wind, ice floe, and running water carve lower-level mountains into the Earth's crust. These elements are also responsible for sculpting the shape of mountains through millions of years.
Background and History of Study
Throughout history, humans have been awed by the grandeur and powerful visage of mountains. Ancient Tibetans, from 20,000 years BCE, believed that gods resided atop the Himalayas. Ancient Greeks later shared that sentiment, believing that their gods resided atop Mount Olympus. Many other cultures similarly placed importance on mountains as important spiritually or otherwise.
By the early nineteenth century, scientists increasingly examined the nature of higher elevations and mountain ranges. In 1829, French geologist Jean-Baptise Élie de Beaumont theorized that mountains formed as the Earth cooled and contracted. In 1842, American scientist Henry Darwin Rogers argued that mountains were the result of wavelike undulations caused by molten rock flowing beneath Earth's crust.
In 1912, German climatologist Alfred Wegener moved the field of geology to an entirely new level. Wegener noticed that fossils of certain species of plants and animals were unearthed thousands of miles apart, even across oceans. He expanded upon the observations of others, who believed that each continent seemed to fit together with other continents like pieces of a puzzle. Wegener further argued, in developing his theory of continental drift, that the continents were once connected and that they had later broken apart and were now drifting away from one another. Continental drift theory remained relevant until the 1960s, when advances in seismological and volcanic studies seemed to show the outlines of tectonic plate boundaries. Plate tectonics has since been overwhelmingly embraced by the scientific community, with clear applications in orogenesis, the formation of mountains on the Earth's surface.
Convergent Boundaries and Orogenesis
The concept of plate tectonics is central to the process of orogenesis. The shape and configuration of mountains and mountain ranges depend on how tectonic plates converge. For example, the Himalayan range, which includes the world's tallest mountain (Mount Everest), was formed at a convergent boundary of the Eurasian and Indian plates, which first collided 25 million years ago. The force by which the plates collided caused a crumpling effect, pushing rock outward in the form of mountain peaks. The collision is ongoing, which means that the Himalayas continue to form and grow.
Scientists frequently use the Himalayan example to study other mountain ranges. In one study, scientists attempting to understand the orogenesis of the Caledonide mountain range in Scandinavia have noticed similarities between that range and the composition of the Himalayas. This comparison has led them to argue that the Caledonides were formed by a convergent boundary plate collision. Such information is useful for scientists who are surveying this range for geothermal energy and certain minerals within its bedrock.
Divergent Boundaries and Orogenesis
In the second form of orogenesis, known as divergent boundaries, plates move from one another and leave an opening between them. This opening is filled with magma from the Earth's inner regions; the magma pushes outward, creating mountains. One of the best-documented examples of divergent boundary orogenesis is the Mid-Atlantic Ridge, which rests on the floor of the Atlantic Ocean, spreading from the Arctic regions to beyond the Cape of Good Hope off the southern coast of South Africa.
The divergent boundary of the Mid-Atlantic Ridge provides scientists with a rare ability to monitor the spread of the two plates in question (the North American and Eurasian plates). Scientists have placed a number of observation stations in Iceland, which lies along the boundary of the two plates. In addition to the volcanic activity that occurs frequently from this divergence, seismic activity enables scientists to use a wide range of scientific technologies (including global positioning systems) to monitor the continuing rate of divergence and the continuing formation of mountains.
Transform Boundaries and Orogenesis
The third form of orogenesis involves transforming boundaries. This manifestation of mountain building occurs when two plates “sideswipe” each other rather than collide head-on. The San Andreas fault is one of the most famous of these transform boundaries. In this case, the North American and Pacific plates are grinding alongside each other (the Pacific plate in a northwesterly direction and the North American plate in a southeasterly heading). The Sierra Nevada range in California provides an example of the mountains that form at transform boundaries. The Sierra Nevada was formed (and continues to be formed) by the shearing effects of the plate motions. The mountains are fragments of the plates, pushed upward and outward by the encounter between the two tectonic plates.
Methods of Study
Because the subterranean movement of tectonic plates cannot be directly observed, research relies heavily on computer models. In a similar vein, scientists studying orogenesis (which for even the youngest mountain ranges took place millions of years ago) rely on computer models to help create a profile of a region's mountain-building history. For example, scientists estimate that the creation of Wyoming's Laramie Range, which spans from Wyoming to the Black Hills of South Dakota, took place about 70 to 80 million years ago through subduction (a process whereby the weight of an ocean or glacier pushes plates downward, allowing for magma to fill the empty space).
In the early twenty-first century, scientists compiled thermal data taken from various points in the Laramie Range and entered them into a computer database. Based on the models they generated, researchers created a profile of the degree of magma flow that contributed to the range's formation.
One of the most invaluable tools for geologists and geophysicists studying orogeny is satellite technology. Remote-sensing satellites can compile large volumes of photographs and geodynamic data each time they pass over a target. For example, researchers have used data from satellite flyovers to assess the elevations of certain major mountain ranges. The satellite images showed scientists how natural elements such as water and wind erode mountains and contribute to a reduction in mountain elevation. Satellites have also been called into service to observe the many different features of the Alps. The information collected by the ERTS-1 satellite revealed a complex system of fault lines, folding, and shearing.
Geologists and geophysicists commonly use ground-based sensory equipment to better understand the forces involved in building mountains. For example, the rock that is brought to the surface in a divergent boundary scenario maintains a certain magnetic charge. Magnetometers, in one case, detected the magnetic rocks introduced along the Mid-Atlantic Ridge's divergent boundary. (During World War II, scientists used magnetometers to detect German submarines.) Today, magnetometers continue to prove invaluable for surveying the orogeny of mountain ranges.
Scientists also use thermal equipment to observe the different minerals that help form mountains. Such equipment can help differentiate between the elements that foster orogenesis. For example, researchers used thermal imaging equipment to reveal the heat radiating from the minerals found in different locations in the Adirondack Mountains. The use of this thermal equipment helped researchers create a chronology of the orogenesis of mountains in this area. Thermal detection equipment has also helped scientists determine the effects of glaciation on prehistoric mountain ranges. By examining the temperatures of certain components of the bedrock, researchers can determine the rates of glacial development and erosion in a given mountain range.
Principal Terms
convergent boundary: the geodynamic process in which two tectonic plates directly collide
divergent boundary: the geodynamic process whereby two tectonic plates separate and create a space for magma to fill
folding: the effect of the collision of two plates pushing the Earth's outer crust outward
lithosphere: the outermost geological layer of the Earth that consists of massive plates of rock
orogenesis: the process of mountain building
plate tectonics: the theoretical concept in which the Earth's lithosphere, comprising continental and smaller plates, is in constant motion
transform boundary: the geodynamic process in which two tectonic plates collide in a side-swiping motion
Bibliography
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