How weather changes the topography
What do you think about when you look at mountains? Eternity? It seems that the mountains have always existed, and that they always will. But in the lifetime of the planet, mountains are more like sandpits in a sandbox, left in the rain. Once they appeared quickly, and now, they are slowly melting, turning into barely visible hills. And it’s about the weather again, of course! Today, we’ll talk about how the atmosphere is slowly but surely changing the face of the Earth.
Air wears down stone
The Earth’s crust (the layer from the surface to 40-280 km deep) is divided into lithospheric plates, which are constantly moving against and colliding with each other. At the point where two plates collide, their edges rise. This process has formed mountains throughout Earth’s history.
Map of the lithospheric plates. Today, only those mountains at the boundaries between the plates are growing.
Mountains are growing very fast by geological standards. For example, the Himalayas and Andes are growing at about 1 mm per year, right now. But at some point, the mountains stop growing. For example, the Appalachians and the Urals have not grown since a long time ago.
Mountains that are no longer growing begin to slowly erode: this process is called weathering. One of the main contributors to this process is, as you might have guessed, the weather:
- The wind sharpens the rocks by blowing dust particles into them
- Temperature fluctuations cause rocks to expand in the heat and contract in the cold, destroying their structure
- Flowing water erodes rocks
- Ice can destroy individual rocks, through freezing and thawing, as well as demolishing entire mountain slopes
The bottom line is that the older the mountains are, the lower they become, and the flatter their peaks are.
Photo: katsuma tanaka / Unsplash
“Find 10 Differences.” Above is the Waterbergh Plateau in South Africa, one of the oldest mountain systems in the world. Below is the Himalayas, one of the youngest mountains.
Photo: Bisesh Gurung / Unsplash
By the way, weathering is not only caused by the atmosphere. It also can involve chemical reactions, and even something as simple as plants sprouting in the cracks of rocks, and destroying them with their growing roots.
Sometimes, some part of the rock is harder than the surrounding rocks, and is destroyed more slowly. That’s how remnants appear. These bizarre sculptures often become national symbols, and are honored by the local population or made into national parks.
Manpupuner rock formations, also known as Seven Strong Men, in the Urals. Photo: Kasimys / Wikimedia
Trees, saviors of the soil
Plants can be called children of the weather and the climate. Depending on the temperature and precipitation, they can be steppe grasses, tropical forests, or sparse desert shrubs. But as soon as plants appear, they affect both climate and the relief.
The relief is mainly affected by trees (although perennial grasses are not far behind). They all slow down erosion—the breakdown of soil. The deeper the root system of plants penetrates the soil, the better it binds the soil together and keeps it from eroding.
This is especially important for agriculture. Plowing land deprives it of its natural vegetation cover, which causes the rate of soil destruction to increase hundreds and sometimes thousands of times! For example, because of rain, the soil can literally leak out of the field, and deep potholes, and later, ravines, will appear in what was a completely flat place.
Another common problem is wind. If a strong wind blows across a newly plowed field for a long time, it can blow away the top, most fertile layer of soil. People living near such fields will suffer from dirty air and dust storms, and future crops will suffer a severe decline.
This is why people nearly always try to plant protective trees or forest belts around crop fields.
Ice Age
Every few thousand years, the Earth experiences global warming and cooling events that change the entire face of the planet (we have two articles about the «anatomy» of climate: the first and the second). If it is about 4-6°C colder than it is now for many centuries, the snow that falls in polar and temperate latitudes may not have time to melt over the summer. As a result, it accumulates and compacts, turning into glaciers.
Twenty-five thousand years ago, the areas shaded white on the map were covered with ice year-round. This includes, for example, the whole of Canada(!) and Scandinavia.
But it gets even more interesting when a glacier starts to melt. Like a drying puddle, it slowly retreats from the captured territories until it melts away. The area which the glacier has just left resembles a Martian landscape: everything is dug up, ground down, and covered with rocks, and only streams of water give away the earthly nature.
The edge of the glacier in Greenland. This is roughly what Europe and North America looked like at the end of the Ice Age. Photo: Visit Greenland / Unsplash
The topography left behind after the end of glaciation forms the basis for all future terrain:
- Streams of water flowing from the glacier make their own channels and turn into rivers.
- Lakes form where the flow of melting water was blocked by mountains of rock (also left behind by the glacier). The largest and most famous glacial lakes are the Great Lakes in North America.
- In some places, a glacier may leave deep ditches when it retreats, which later either remain dry or fill with water. For example, troughs and tunnel valleys.
- Often, piles of stones left lying after a glacier retreats are oriented in the direction of its retreat. This is how, for example, the Drumlins and Ozy appeared.
Antarctica and Greenland are now almost entirely covered by glaciers. And there are also glaciers in many mountains, so everyone can touch these incredible landscapes.
In nature, everything is interconnected, and often, behind the big changes are small, imperceptible processes. Millions of years from now, the Earth’s modern appearance may be changed by the very rain from which you can easily hide under an umbrella!
Text: Eugenio Monti, a meteorologist and a climatologist
Cover photo: Gitte Winter / Unsplash
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