Interlayered Coal Seams and mudstones from Alaska Range

Mountain Building

Mountains are magnificent and inspiring features. Just thinking of mountains conjures up beautiful images of places like the Rockies, the Andes, Hawaii and the Alps. But not all mountains are the same. Though they all take a very long time to form, mountains are created in different ways depending on where they are on Earth. Some mountains are created by land pushing together, while others are formed over hotspots on Earth. Let's take a closer look at the mountain building process to better understand how these incredible structures come to be Plate Boundary Mountains.

The most common type of mountain is a fold mountain. Fold mountains form when continental tectonic plates are pushed together, like the Himalayas in South Asia. Tectonic plates make up the Earth's crust (both the continents and the ocean floor) and are like puzzle pieces floating around on the mantle below. They move around very slowly, but sometimes push into each other along their edges.
When this happens, the intense pressure of the edges pushing together forces the plate material upward since there's nowhere else for it to go. This would be like you taking two pieces of bread and sliding them across a surface into each other from opposite directions. As the edges of the bread are compressed together, they rise upward, forming a sort of bread mountain.

Sometimes oceanic plates run into continental plates, and when this happens we also get mountain formation, but it's a bit different because the oceanic crust is much denser than the continental crust. Because of this, the oceanic crust sinks down below the continental crust, a process known as subduction. The crust material gets heated and then rises to the surface as magma, pushing upward and forming a volcano. As volcanic eruptions spew their material out, the material builds up around the ground, forming the mountain itself. This is why we find many of the world's volcanoes along oceanic-continental boundaries, like the area known as the Ring of Fire.

Oceanic plates may also collide with other oceanic plates, and when this happens, the older one is subducted under the younger one because the older one is denser. This type of plate compression results in a volcanic island arc. As the name implies, these are mountainous islands made of volcanoes. Japan is one such set of mountainous islands along oceanic plate boundaries in the Pacific Ocean.

Not all volcanoes and mountains are along plate boundaries. Some mountains form when magma pushes upward on earth's crust from underneath. Like with fold mountains, the land has nowhere to go but up, so upward as a mountain it goes! If the magma makes it through to the surface, this creates a volcano. But sometimes the magma never reaches the surface, in which case we get a dome mountain. Both the Black Hills in South Dakota and the Adirondacks in New York are examples of dome mountains.
Other mountains are formed in the middle of ocean plates, and a great example of this is Hawaii. The islands of Hawaii were formed over a hotspot on Earth. This is a place where magma pushes upward through the crust as the surface moves over it. It's like a conveyor belt of mountain building.

Imagine you had a large flame under a moving belt, and on the belt, you had objects like pieces of chocolate. Each piece would be heated (and probably pretty melted!) as it moved over the flame, but it would cool and harden as it moved away from it. Hotspots work the same way. They build volcanic mountains as the land moves over it, forming a long chain of islands. So, the northwest islands of Hawaii are actually much smaller and older than the ones nearer to the hotspot.

We can also get mountain formation due to large-scale erosion. These erosional mountains, sometimes called plateau mountains, are large areas of land that are already higher than the surrounding ground, but don't start out in the shape of a typical mountain. Over many, many years, wind and water wear down parts of the land, leaving behind large mountain peaks. Pike's Peak in the Rockies is an example of a mountain created by erosional forces.

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