Mountains are primarily considered a constructive force due to the process of plate tectonics, particularly in the first scenario you've mentioned: the plates collide and go up causing the build-up of land.
When two tectonic plates converge, the collision can result in the uplift of the Earth's crust, forming mountain ranges. This process is known as orogeny, and it leads to the accumulation of material that contributes to the elevation of the land. The other scenarios you mentioned—plates spreading apart or subducting—also involve constructive forces to some degree, but they are more associated with different geological processes:
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Plates spreading apart: This process typically occurs at mid-ocean ridges, where new oceanic crust is created as magma rises to the surface. This can also lead to the formation of new land but is more characteristic of oceanic features rather than continental mountains.
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Plates colliding and subducting: While this can lead to mountain formation through volcanic activity, the subduction process also involves one plate being pushed down into the mantle, which can lead to destruction of land rather than direct build-up.
In summary, the primary constructive force that leads to mountain formation is the upward thrust of the Earth's crust when tectonic plates collide.
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