Mountains have played a central role in human culture since times immemorial. Yet it’s only recently that we’ve started to understand how mountains form and develop; to this day, these magnificent landforms still hold many secrets. There are several ways to analyze and classify mountains depending on what field of science you come from, but here, we’ll have a look at the most common classification and then go into a bit more detail.
The types of mountains
Generally, mountains are split into: fold mountains, block mountains, dome mountains, and volcanic mountains. Plateau mountains, uplifted passive margins, and hotspot mountains are also sometimes considered.
- Fold mountains — the most common type, formed when two or more tectonic plates collide.
- Block mountains (or fault-block) — formed through geological processes which push some rocks up and others down.
- Dome mountains — formed as a result of hot magma pushing beneath the crust.
- Volcanic mountains — also known by a simpler name: volcanoes.
- Other types of mountains sometimes brought into classifications are plateau mountains, uplifted passive margins, and hotspot mountains.
These are the not only the most common, but also the biggest types of mountains (on Earth, at least). Fold mountain chains can spread over thousands of kilometers — we’re talking about the Himalayas, the Alps, the Rockies, the Andes, all the big boys. They’re also relatively young mountains (which is another reason they’re so tall, as they haven’t been thoroughly eroded), but that’s “young” in geological terms — still a few good tens of millions of years.
In order to understand how fold mountains form and develop, we have to dip our fingers into some tectonics. The Earth’s litosphere is split into rigid plates which move independently to each other. There are seven major plates, and several smaller ones across the world. When two plates collide, all sorts of things can start happening. For instance, if one is denser than the other (oceanic plates are typically denser due to the rocks they are made of), a process a subduction will start — the heavier one will slowly glide beneath the other one. But if they have relatively similar densities, then they will start to crumple up and drive movement upwards. Basically, the tectonic plates are pushed, but since neither can slide beneath each other, they just build up geological folds. To get a better idea of how this looks like, try to push two pieces of papers towards each other. Some parts will go up, and those are the mountains.
This process is called orogeny (giving birth to mountains) and it generally takes millions of years for it to complete. Many of today’s fold mountains are still developing as the tectonic process unfolds. The process doesn’t only happen on tectonic edges, sometimes the mountain-generating fold process can take place well inside a tectonic plate.
Block mountains (or fault-block)
Whereas the previous category was all about folds, this one is all about faults; geological faults, that is.
Let’s go back to the previous idea for a moment. Let’s say that under pressure, some parts of a tectonic plate is starting to fold. As the pressure grows and grows, at one point the rock can simply break. Faults are those breaks, they’re the planar fractures or discontinuities in volumes of rock. They can vary tremendously in size from a few centimeters to mountain-sized.
Basically, when big blocks of rock are broken through faulting, some of them can be pushed up or down, and thus block mountains can result. Higher blocks are called horsts and troughs are called grabens. Their size can also be impressive, though they’re generally not as big as the fold mountains because the process which generates them takes place on a smaller scale and involves less pressure. Still, the Sierra Nevada mountains, which are a good example of block mountains, feature a block 650 km long and 80 km wide. Another good example is the Rhine Valley and the Vosges mountain in Europe. Rift valleys can also generate block mountains, as is the case in the Eastern African Rift, for example.
It can be quite difficult to identify a block mountain without knowing its underlying geology but generally, they tend to have a steep side and a slowly sloping side.
Everyone knows something about volcanoes, though we rarely think about them as mountains (and truth be told, they aren’t always mountains).
Volcanic mountains are created when magma from deep under the surface starts to rise up. At one point, it erupts in the form of lava, and then cools down, solidifies, and starts to pile on, building a mountain. Mount Fuji in Japan and Mount Rainier are typical examples of volcanic mountains — with Mount Rainier being one of the most dangerous volcanoes in the world. However, it’s not necessary for the volcano to be active.
Several types of volcanoes can generate mountains, with Stratovolcanoes typically being the biggest ones. Despite Mount Everest being the tallest mountain above sea level, Mauna Kea is actually much taller than Everest — at over 10,000 meters. However, much of it is submerged, with only 4205 meters rising above sea level.
Dome mountains are also the result of magmatic activity, though they are not volcanic in nature.
Sometimes, lots of magma can accumulate beneath the ground and start to swell the surface up. Sometimes, it never reaches the surface but still forms a dome. As that magma cools down and solidifies, it is often tougher than other surrounding rocks can be exposed after millions of years of erosion. The mountain is this dome — a former accumulation of magma which cooled down and was exposed by erosion.
Round Mountain is a relatively recent dome mountain. It represents a volcanic feature of the Canadian Northern Cordilleran Volcanic Province that formed in the past 1.6 million. Black Dome Mountain is another popular example, also in Canada.
Other types of mountains
As we mentioned above, there’s no strict classification of all mountains, so other types are sometimes mentioned.
Basically, plateau mountains aren’t formed by something going up — they’re formed by something going down. Imagine a plateau, for instance. Let’s say it has a river on it. Year after year, that river carves a part of the plateau, bit by bit. After some time, there might only be a bit of the original plateau left un-eroded, and that part basically becomes a mountain. This generally takes a very long time even by geological standards and can go up to billions of years. Some geologists group all these mountains along with dome mountains into a broader category called erosional mountains.
Uplifted passive margins
There’s no geological model to fully explain how uplifted passive margins formed, but we do see them in the world. The Scandinavian Mountains, Eastern Greenland, the Brazilian Highlands or Australia’s Great Dividing Range are such examples, owing their existence to some uplifting mechanism.
Although once thought to be identical to volcanic mountains, new research has shed some light on this belief. Hotspots are volcanic regions thought to be fed by a part of the underlying mantle which is significantly hotter than its surroundings. However, even though that hot area is fixed, the plates move around it — causing it to leave a hotspot trail of mountains.
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