Fault-block mountains are formed by the movement of large crustal blocks along faults formed when tensional forces pull apart the crust Figure 3. Tension is often the result of uplifting part of the crust; it can also be produced by opposite-flowing convection cells in the mantle see Figure 1.
It breaks up into blocks or chunks. Sometimes these blocks of rock move up and down, as they move apart and blocks of rock end up being stacked on one another.
Often fault-block mountains have a steep front side and a sloping back side. Fault-block mountains often result from rifting, an indicator of extensional tectonics.
These can be small or form extensive rift valley systems, such as the East African Rift zone. Thrust and reverse fault movement are an important component of mountain formation. Mountain formation refers to the geological processes that underlie the formation of mountains. Reverse faults are exactly the opposite of normal faults. If the hanging wall rises relative to the footwall, you have a reverse fault.
On the other hand, continental crust over three billion years old has been found. As this information became available, the theories of sea-floor spreading and plate tectonics were developed. These theories answer most of the objections raised to earlier proposed mechanisms which attempted to account for continental drift. Continental drift, and its supporting theories of sea-floor spreading and plate tectonics, are now accepted as valid by most geologists throughout the world.
These theories help explain the origin and location of mountains. According to the theory of sea-floor spreading, new ocean crust is formed by the injection of liquid and solid rocks from the underlying mantle into fractures along mid-ocean ridges Figure 7.
For example, Iceland is on top of the Mid-Atlantic Ridge. It was formed, and is still growing, in this way. As new crust moves away from the axis of the mid-ocean ridge, it fuses with the part of the mantle that overlies the athenosphere and becomes part of the lithosphere Figures 1 and 7.
While new ocean crust forms along mid-ocean ridges, old ocean crust sinks into the mantle along deep-sea trenches Figure 9. Since there is no evidence that the earth is expanding or shrinking, it follows that along any great circle on the earth's surface, as much crust is consumed back into the mantle along deep-sea trenches as is formed along mid-ocean ridges.
The lithosphere is divided into plates that move horizontally on top of the asthenosphere. These plates interact along three types of boundaries: spreading boundaries, where new ocean crust is formed along mid-ocean ridges Figure 7 ; converging boundaries, where continents collide Figure 8 or where, in ocean basins, old ocean crust sinks into the mantle along deep-sea trenches Figure 9 ; and conservative boundaries, where plates slide past each other along fractures faults Figure Crust is neither formed nor destroyed along conservative boundaries.
Many of earth's mountain ranges are located along plate boundaries, or former plate boundaries. Spreading boundaries produce mountains along rifts like the East African Rift Figure 6 , and along mid-ocean ridges. Volcanoes and fault-block mountains are common along boundaries. Most of the mountains along mid-ocean ridges are underwater, but a few volcanoes along these ridges rise above sea level. Converging boundaries in ocean basins produce volcanoes and complex mountains associated with deep-sea trenches island arcs like those of Japan, the Philippines, and Indonesia; see Figure 6.
Continents are less dense than oceanic crust and the underlying mantle, so when continents collide along converging boundaries, they do not sink into the mantle.
Instead, the edges of the continents crumple and thicken as they are fused together Figure 8. In the process, complexly folded and faulted mountains such as the Alps, Himalayas, and Applachians are formed Figure 6.
Fault-block mountains and complex mountains occur along conservative boundaries where plates slide past each other. This fault actually a series of faults is part of the complex system of folds and faults that occur along the boundary between the Pacific Ocean Plate and the North American Plate.
Volcanoes can occur along any type of plate boundary. They can also occur in the middle of a plate over hot spots, plumes of very hot rock rising through the mantle. The Hawaiian Islands have formed over such a hot spot. The islands are spread out in a line because the Pacific Ocean Plate moves over a hot spot. At present it is not known for sure whether just the plate moves, or both the plate and the hot spot move.
The island of Hawaii, today directly over the hot spot, is still growing at the southeast end of the chain. This results in a circular mountain range. Domes that have been worn away in places form many separate peaks called Dome Mountains. Volcanic Mountains. As the name suggests, volcanic mountains are formed by volcanoes. Volcanic Mountains are formed when molten rock magma deep within the earth, erupts, and piles upon the surface. Magna is called lava when it breaks through the earth's crust.
When the ash and lava cools, it builds a cone of rock. Rock and lava pile up, layer on top of layer. Plateau Mountains Erosion Mountains. Plateau mountains are not formed by internal activity.
Instead, these mountains are formed by erosion. Plateaus are large flat areas that have been pushed above sea level by forces within the Earth, or have been formed by layers of lava. Plateau mountains are often found near folded mountains.
Volcanoes 4th ed. Holmes Principles of Physical Geology 4th ed. American Society of Civil Engineers. Wilson, Aaron Todd An Introduction to Physical Science 12th ed. Cengage Learning. Woodcock, Robin A. Strachan Geological history of Britain and Ireland. In Robert D.
Hatcher Jr. Geological Society of America. Park The environment: principles and applications 2nd ed. CliffsQuickReview Earth Science. Reference cited. Science : —6. Bibcode: Sci…
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