Recent research has led to important new insights into how these most magnificent of Earths formations came to be. Mountains are created and shaped, it appears, not only by the movements of the vast tectonic plates that make up Earths exterior but also by climate and erosion. In particular, the interactions between tectonic, climatic and erosional processes exert strong control over the shape and maximum height of mountains as well as the amount of time necessary to build--or destroy--a mountain range.
Paradoxically, the shaping of mountains seems to depend as much on the destructive forces of erosion as on the constructive power of tectonics. In fact, after years of viewing erosion as the weak sibling of tectonics, many geologists now believe erosion actually may be the strong one in the family.
In the words of one research group, "Savor the irony should mountains owe their [muscles] to the drumbeat of tiny raindrops. Because of the importance of mountain building in the evolution of Earth, these findings have significant implications for earth science.
To a geologist, Earth's plains, canyons and, especially, mountains reveal the outline of the planets development over hundreds of millions of years. In this sprawling history, mountains indicate where events in or just below Earth's crust, such as the collisions of the tectonic plates, have thrust this surface layer skyward.
Thus, mountains are the most visible manifestation of the powerful tectonic forces at work and the vast time spans over which those forces have operated. The effort to understand mountain building has a long history. One of the first comprehensive models of how mountains evolve over time was the Geographic Cycle, published in This model proposed a hypothetical life cycle for mountain ranges, from a violent birth caused by a brief but powerful spasm of tectonic uplift to a gradual slide into "old age" caused by slow but persistent erosion.
The beauty and logic of the Geographic Cycle persuaded nearly a century of geologists to overlook its overwhelming limitations. In the s the plate tectonics revolution explained how mountain building is driven by the horizontal movements of vast blocks of the lithosphere--the relatively cool and brittle part of Earth's exterior.
According to this broad framework, internal heat energy shapes the planet's surface by compressing, heating and breaking the lithosphere, which varies in thickness from kilometers or less below the oceans to kilometers or more below the continents.
The lithosphere is not a solid shell but is subdivided into dozens of plates. Driven by heat from below, these plates move with respect to one another, accounting for most of our worlds familiar surface features and phenomena, such as earthquakes, ocean basins and mountains.
Earth scientists have by no means discarded plate tectonics as a force in mountain building. Over the past few decades, however, they have come to the conclusion that mountains are best described not as the result of tectonics alone but rather as the products of a system that encompasses erosional and climatic processes in addition to tectonic ones and that has many complex linkages and feedbacks among those three components.
Mountain building is still explained as the addition of mass, heat or some combination of the two to an area of Earths crust the crust is the upper part of the lithosphere. Thicker or hotter crust rises upward, forming mountains, because the crust is essentially floating on the mantle under it, and crust that is either thicker or hotter less dense floats higher.
Plate tectonics contributes to the thickening of the crust by either lateral convergence between adjacent plates or through the upward flow of heat and magma molten rock. Convergence of tectonic plates generally occurs in one of two ways.
One plate may slide down, or subduct, below the other, into the mantle. At a subduction zone boundary, the upper plate is thickened as a result of the compression and from magma being added by the melting of the descending plate. Many mountains, including almost all the ranges that surround the Pacific Ocean in a geologically active area known as the ring of fire, formed by subduction.
With continental collision, on the other hand, neither plate subducts into the mantle, and therefore all the mass added as a result of the collision contributes to the building of mountains. Such collisions have created some spectacular topography, such as the Tibetan Plateau and the Himalayas, the mountain range that includes the worlds 10 highest peaks. The flow of magma and heat to Earth's crust--during volcanic activity, for example--can also drive mountain building.
Earth's longest mountain chains--the mid-ocean ridges--are the result of magma welling up as adjacent plates move apart, forming new crust under the ocean. These ridges run through the Atlantic, eastern Pacific and Indian oceans like the seam on a baseball; the Mid-Atlantic Ridge alone is more than 15, kilometers long, rising as much as 4, meters above the surrounding abyssal plains of the ocean floor.
On land, heat associated with the flow of magma can also help uplift large areas by making the lithosphere less dense and more buoyant on the underlying mantle. Erosion includes the disaggregation of bedrock, the stripping away of sediment from slopes and the transport of the sediment by rivers.
The mix of erosional agents active on a particular landscape--gravity, water, wind and glacial ice--depends on the local climate, the steepness of the topography and the types of rock at or near the surface. Climate is inextricably linked with erosion because it affects the average rate of material loss across a landscape.
In general, wetter conditions favor faster rates of erosion; however, more moisture also promotes the growth of vegetation, which helps to "armor" the surface. Mountains in polar latitudes are the least vulnerable to erosion, partly because of the aridity of cold climates and partly because continental ice sheets such as those on Greenland and Antarctica commonly are frozen to the underlying rock and cause little erosion.
In contrast, mountain glaciers such as those of the European Alps and the Sierra Nevada in California aggressively attack the subsurface rock, so that this type of glacier may be Earth's most potent erosional agent. There are many other links among erosion, climate and topography.
For example, mountains lift the winds that flow over them, causing increased precipitation on the range's windward slopes, intensifying erosion as a result.
Known as orography, this effect is also responsible for the "rain shadow" that creates deserts on the leeward sides of many mountain ranges [ see photograph on opposite page ]. Elevation can also affect erosion, because average temperature decreases with altitude, so that higher peaks are less likely to be protected by vegetation and more likely to be eroded by glaciers. In temperate regions the rate of erosion is proportional to the average steepness of the topography, apparently because gravity- and water-driven processes are more effective on steeper slopes.
Taken together, all these facts suggest that mountains evolve their own climates as they grow--becoming typically wetter, colder and characterized by more intense erosion. The links described above demonstrate that mountain ranges are best viewed as a system. To understand the behavior of any such system, it is necessary to identify both its components and the interactions among those components. Because these interactions are so important, simple system inputs can lead to surprisingly complex outputs.
Such complexities include feedback--stabilizing or destabilizing links between component processes. In the simple example we have outlined, the system is forced by tectonic collision, which adds mass to the mountain belt, and the response is an increase in the average height of the mountain range.
All questions are welcome — serious, weird or wacky! Hello Astrid. You may not believe this but when I was about your age my teacher Mr Rouve explained to the class how mountains get made.
He took a sheet of paper and put it flat on the table. Try to do it and you will see that the middle part of the paper will lift off the table to form a nice fold. My teacher explained that mountains form in a similar way, when flat layers of rocks are pushed toward each other they move upward forming tall mountains.
My teacher was really excited by a discovery made by geologists at that time, when I was a kid. These geologists had figured out that the surface of the Earth was, like a giant jigsaw puzzle, made of pieces. That is indeed what happened on Mars, where mountains loom much taller than on our planet, McQuarrie added. Mars' Olympus Mons, the tallest known volcano in the solar system, extends 82, feet 25, m high, nearly three times taller than Mount Everest. Most likely because Mars has low gravity and high eruption rates, mountain-building lava flows continued on Mars for much longer than they ever have or ever will on Earth, according to NASA.
What's more, Mars' crust isn't divided up into plates like that of our planet. On Earth, as plates move around and over hotspots — areas of the mantle that shoot out hot plumes — new volcanoes form and existing volcanoes become extinct.
Activity in Earth's mantle distributes lava across a larger region, forming multiple volcanoes. On Mars, the crust doesn't move so the lava piles up into a single, massive volcano. The second limiting factor for mountain growth on Earth is rivers. At first, rivers make mountains appear taller — they carve into the edges of the mountains and erode material, creating deep crevices near a mountain's base. But as rivers erode material, their channels may become too steep.
This can trigger landslides that carry material away from the mountain and limit its growth, she added. A group of researchers recently suggested that rivers reach a "threshold steepness" after which their impact on a mountain's growth through erosion is limited in a study published Sept.
0コメント