The most common type of landform created when two tectonic plates separate is a rift valley. Rift valleys are formed when two plates move away from each other, forming a deep trough. The sides of the trough often rise up forming walls hundreds to thousands of feet high. The trough created by the two plates separating can run for hundreds to thousands of miles. As the two plates continue to move away from each other, it can eventually create a new sea or ocean.
When two oceanic plates separate, a mid-ocean ridge can form. These are long ridges that form underwater and can be thousands of miles long. The ridge forms as molten material from the asthenosphere rises and cools, creating new lithospheric material. This new lithospheric material slowly pushes the two tectonic plates apart, forming the ridge.
In some cases, when two continental plates separate, a new mountain range can form. For example, when the African and Eurasian plates separated millions of years ago, they created the Alps mountain range. This happened when a fault line developed between the two plates, pushing them apart and forming mountains as they moved.
Another landform that can be formed when two tectonic plates separate is a volcanic arc. This happens when one plate dips beneath the other, which leads to melt in the mantle rising to the surface and forming volcanoes along the edge of the plate. The volcanoes form an arc shape because they are located along the circumference of the plate.
Finally, two tectonic plates can also form what is known as an oceanic trench when they separate. An oceanic trench is a deep depression in an ocean floor that is created when one plate slides beneath another plate. The result is a deep, long and narrow trench in the seafloor that can be thousands of miles long.