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Please correct me if I have said something wrong, as it might be the answer to my question. Also, these are all the things that I've learned through self-studying, so I might have been mislead.

Anyways,

The plate tectonics theory states that the Earth's lithosphere have been and is still (slowly) moving since about 3.4 billion years ago. It is proved by many evidences, most notably global distributions or locations of volcanoes, earthquake epicenters, and mountain belts. When superimposed, these locations somewhat forms the plate boundaries of a proposed supercontinent pangea.

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Photo credits: https://www.learner.org/wp-content/interactive/dynamicearth/tectonicsmap/index.html

From my understanding, volcanoes, earthquake epicenters, and mountain belts are all the products of colliding plates.

If this is so, and if it is true that the plates moved somewhat away from each other (since it was once a supercontinent, then it is now scattered), then why is it that there are volcanoes, earthquake epicenters, and mountain belts--products of collision--in areas where a collision didn't happen?

Like, the plates moved away right? And collision, from my understanding, is the crashing of two plates, and collision ≠ moving away. So why is it that they formed despite no plates collided to each other?

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I think there are two aspects to consider.

  • Before moving away when a supercontinent breaks up, plates have to collide to form this supercontinent in the first place. This process, called an orogeny, builds mountains. And mountains take a long, long time to erode. For example, the Variscan orogeny happened when Laurussia and Gondwana collided to form Pangaea. This formed many mountains that still exist today, in Europe (French Massif Central, Bohemian Massif...) or in North America (Appalachian). Breaking up the supercontinent doesn't mean that you erase the mountains that were formed when the supercontinent formed. So even if plates move away, you keep old mountains.
  • When plates move away from a broken supercontinent, they can eventually meet and collide. For example, when Pangaea broke up, Asia and India both moved away, but at some point Asia began to move North and collided with Asia, forming the Himalayas. So even if plates move away, they can collide and form new mountains.

Supercontinents actually follow a cycle of formation and breaking up, and both phases can lead to mountain building.

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Volcanoes form mainly in plate boundaries as island arcs or volcanic arcs, but they can form too in hotspots. These are hot points in the mantle that cross the crust producing volcanoes. An example are Canary Islands, that are inside the African Plate.

Deep earthquake epicenters form too close to plate boundaries, but shallow earthquake epicenters can form far away from the plate boundary. This does not mean the origin of the earthquake is not a plate pushing another, it is just that the stress force vector can produce superficial faults to move far away from the boundary. An example is where I live, in Pamplona, Spain. From time to time we have 3-4 magnitude eartquakes because there is a fault close to our city, the Jaca-Pamplona fault. The force that cause the earthquakes is the African plate pushing the Iberian Peninsula, but the African plate is more than a thousand kilometers away.

About mountains, Jean-Marie Prival answered you correctly. There are mountains that were formed in past collisions and have not been eroded completely. There are also mountains as Australia's mountains whose origin is different from a plate pushing the adjacent.

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