Category The World Around us

River water does not always carve out wide valleys. In some areas, where there are fairly soft rocks, for instance, very deep, narrow valleys with vertical sides called canyons are formed. In places where continents are drifting apart, very wide rift valleys and flat areas known as plateaux can appear. The Great Rift Valley in Africa is the biggest example of these.

The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the long-standing idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on Earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the Earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.

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Over time, rainfall has the effect of eroding the land to form valleys and other features. At mountain peaks, the rainwater flows quickly to form narrow gullies. Slowing down as it moves further downhill, the water forms a wide valley.

A valley is an extended depression in the Earth’s surface that is usually bounded by hills or mountains and is normally occupied by a river or stream. Since valleys are usually occupied by a river, they can also slope down to an outlet which can be another river, a lake or the ocean.

Valleys are one of the most common landforms on the Earth and they are formed through erosion or the gradual wearing down of the land by wind and water. In river valleys?, for example, the river acts as an erosional agent by grinding down the rock or soil and creating a valley. The shape of valleys varies but they are typically steep-sided canyons or broad plains, however, their form depends on what is eroding it, the slope of the land, the type of rock or soil and the amount of time the land has been eroded.

There are three common types of valleys which include V-shaped valleys, U-shaped valleys, and flat-floored valleys.

V-Shaped Valleys

A V-shaped valley is a narrow valley with steeply sloped sides that appear similar to the letter “V” from a cross-section. They are formed by strong streams, which over time have cut down into the rock through a process called downcutting. These valleys form in mountainous and/or highland areas with streams in their “youthful” stage. At this stage, streams flow rapidly down steep slopes.

An example of a V-shaped valley is the Grand Canyon in the Southwestern United States. After millions of years of erosion, the Colorado River cut through the rock of the Colorado Plateau and formed a steep-sided canyon V-shaped canyon known today as the Grand Canyon.

U-Shaped Valley

A U-shaped valley is a valley with a profile similar to the letter “U.” They are characterized by steep sides that curve in at the base of the valley wall. They also have broad, flat valley floors. U-shaped valleys are formed by glacial erosion as massive mountain glaciers moved slowly down mountain slopes during the last glaciation. U-shaped valleys are found in areas with a high elevation and in high latitudes, where the most glaciation has occurred. Large glaciers that have formed in high latitudes are called continental glaciers or ice sheets, while those forming in mountain ranges are called alpine or mountain glaciers.

Due to their large size and weight, glaciers are able to completely alter topography, but it is the alpine glaciers that formed most of the world’s U-shaped valleys. This is because they flowed down the pre-existing river or V-shaped valleys during the last glaciation and caused the bottom of the “V” to level out into a “U” shape as the ice eroded the valley walls, resulting in a wider, deeper valley. For this reason, U-shaped valleys are sometimes referred to as glacial troughs.

One of the world’s most famous U-shaped valleys is Yosemite Valley in California. It has a broad plain that now consists of the Merced River along with granite walls that were eroded by glaciers during the last glaciation.

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Mountains form over many millions of years, and due to the continual movement of the Earth’s plates, they are still being formed. Young mountain ranges are those that have formed in the last 50 million years or so, such as the Himalayas in Asia. Older mountain ranges, such as the Urals in Russia or the Scottish Highlands, were formed many more millions of years ago and have eroded significantly.

It’s true. Mountains are seriously old. They date back millions and millions of years. As for how they formed, it happened over time. For instance, some mountains formed when volcanoes erupted over and over. The lava that spewed from the volcanoes built up over time. And eventually, this formed a mountain. In other cases, mountains formed when two layers beneath the Earth’s crust moved, collided and pushed against each other. This caused the land to crumple and rise up to form the shape of a mountain.

According to most scientists, the oldest mountain range on Earth is called the Barberton Greenstone Belt and is found in South Africa. It’s estimated that the range is at least 3.2 billion (yes, billion!) years old. As for the youngest mountain on Earth? That title goes to the Himalayas in Asia. It’s thought that this range formed about 40 million years ago. Although that’s not exactly what we’d call young!

If you really want to learn more about mountains, it helps to know the features that make up these land masses. The highest point of a mountain is called the peak or summit. And the bottom of the mountain where the ground begins is known as the base. There’s also the snow line. That’s where ice and snow begin to appear on a mountain. Then there’s the gorge, which is a narrow valley between mountains. And while we’re talking mountains, it’s probably worth mentioning that the name for a long group of mountains is called a range. Some mountain ranges can stretch great distances. For instance, the Rocky Mountains extend all the way from Canada to Mexico. 

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No fewer than ten of the highest mountains in the world are in the Himalayas. Highest of all is Everest, which lies on the border of Nepal and China. At a height of 8848m (29,028ft), it is almost 2000m (6562ft) higher than the highest mountain outside the Himalayas — Aconcagua in Argentina, South America.

The mountain’s height was first determined in 1856. The Great Trigonometric Survey of British India pegged the mountain, known to them as Peak XV, at 29,002 feet (8,840 meters). But those surveyors were at a disadvantage because Nepal would not grant them entry due to concerns that the country would be invaded or annexed. The current accepted elevation was determined by an Indian survey in 1955 and backed up by a 1975 Chinese measurement.

In 1865, Andrew Waugh, the British Surveyor General of India, suggested that the mountain be named after his predecessor in the job, Sir George Everest. The Tibetans had referred to the mountain as “Chomolungma,” or Holy Mother, for centuries, but Waugh did not know this because Nepal and Tibet were closed to outsiders.

Mount Everest attracts experienced mountaineers as well as less-seasoned climbers who typically enlist guides known as sherpas. Climbing more than 8,000 feet is no easy feat. Altitude sickness, weather and wind are the major roadblocks to making the summit for most climbers. More than 5,000 people have climbed Everest and 219 have died trying. About 77 percent of those ascents have been accomplished since 2000. In 2007, a record number of 633 ascents were recorded.

Mount Everest has two main climbing routes, the southeast ridge from Nepal and the north ridge from Tibet. Today, the southeast ridge route, which is technically easier, is more frequently used.

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There are four basic kinds of mountains:

1. Fold Mountains (Folded Mountains)
2. Fault-block Mountains (Block Mountains)
3. Dome Mountains
4. Volcanic Mountains

These different types of mountain names not only distinguish the physical characteristics of the mountains, but also how they were formed.

Fold Mountains

Fold mountains are the most common type of mountain. The world’s largest mountain ranges are fold mountains. These ranges were formed over millions of years.

Fold mountains are formed when two plates collide head on, and their edges crumbled, much the same way as a piece of paper folds when pushed together.

The upward folds are known as anticlines, and the downward folds are synclines.

Examples of fold mountains include:

• Himalayan Mountains in Asia
• the Alps in Europe
• the Andes in South America
• the Rockies in North America
• the Urals in Russia

The Himalayan Mountains were formed when India crashed into Asia and pushed up the tallest mountain range on the continents.

In South America, the Andes Mountains were formed by the collision of the South American continental plate and the oceanic
Pacific plate.

 Fault-block Mountains

These mountains form when faults or cracks in the earth’s crust force some materials or blocks of rock up and others down.

Instead of the earth folding over, the earth’s crust fractures (pulls apart). 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.

Examples of fault-block mountains include:

• the Sierra Nevada mountains in North America
• the Harz Mountains in Germany

 Dome Mountains

Dome mountains are the result of a great amount of melted rock (magma) pushing its way up under the earth crust. Without actually erupting onto the surface, the magma pushes up overlaying rock layers. At some point, the magma cools and forms hardened rock. The uplifted area created by rising magma is called a dome because of looking like the top half of a sphere (ball). The rock layers over the hardened magma are warped upward to form the dome. But the rock layers of the surrounding area remain flat.

As the dome is higher than its surroundings, erosion by wind and rain occurs from the top. 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.

Examples of volcanic mountains include:

• Mount St. Helens in North America
• Mount Pinatubo in the Philippines
• Mount Kea and Mount Loa in Hawaii

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Avalanches are huge masses of snow that suddenly crash down a mountainside. They are caused by a combination of heavy snow and a sudden rise in temperature. Avalanches can be up to 1 km (0.6 miles) across and generate winds of up to 300km/h (185mph). Their effects on towns and local populations can be devastating.

Avalanches can be caused by many things. Some of them are natural. For example, new snow or rain can cause built up snow to loosen and fall down the side of a mountain. Earthquakes and the movement of animals have also been known to cause avalanches.

Artificial triggers can also cause avalanches. For example, snowmobiles, skiers, gunshots, and explosives have all been known to cause avalanches. Avalanches usually occur during the winter and spring, when snowfall is greatest. As they are dangerous to any living beings in their path, avalanches have destroyed forests, roads, railroads and even entire towns.

Warning signs exist that allow experts to predict — and often prevent — avalanches from occurring. When over a foot of fresh snow falls, experts know to be on the lookout for avalanches. Explosives can be used in places with massive snow buildups to trigger smaller avalanches that don’t pose a danger to persons or property.

When deadly avalanches do occur, the moving snow can quickly reach over 80 miles per hour. Skiers caught in such avalanches can be buried under dozens of feet of snow. While it’s possible to dig out of such avalanches, not all are able to escape.

If you get tossed about by an avalanche and find yourself buried under many feet of snow, you might not have a true sense of which way is up and which way is down. Some avalanche victims have tried to dig their way out, only to find that they were upside down and digging themselves farther under the snow rather than to the top!

Experts suggest that people caught in an avalanche try to “swim” to the top of the moving snow to stay close to the surface. Once the avalanche stops, do your best to dig around you to create a space for air, so you can breathe easier. Then, do your best to figure out which way is up and dig in that direction to reach the surface and signal rescuers.

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