Topography and Tectonics
Why are some mountains very high--such as those in the Himalayas--while others, like the Appalachians, are much lower? Mountains and all areas of high topography need to be maintained by some sort of dynamic process. If they are not, they will eventually erode and become flat plains. The existence of regions of high topography tells us that some sort of tectonic process must be operating, even if we cannot observe it directly.

If we take a map of the distribution of earthquakes and overlay it on global topography, we see that all of the world's large mountain ranges are seismically active. This is true for mountains on continents as well as those in the ocean. Thus the processes responsible for mountain-building thus appear related to the processes that generate earthquakes. The same stresses that break rocks and cause earthquakes also lift rocks high up above Earth's surface.

In the Himalayan Mountains of Asia two of the Earth's tectonic plates are converging. The Indian plate is moving northward and colliding with the Asian plate. Because both India and Asia are made of continental crust, which is thick and low-density, the plates buckle and bend, creating a thick and deformed region of continent. The surface expression of this collision are the high peaks of the Himalayas. The collision is accompanied by many earthquakes. Additionally, the high topography above the surface is mirrored in the subsurface by a "crustal root," a region of thick crust extending down into the mantle, just as the high mountains extend upward into the atmosphere.

Left:Location of Himalayan cross-section drawn in red on top of topography. Right: Cross-sectional view of the Himalayas.

 

Other types of tectonic activity can be inferred by looking at topography. For example, around the Pacific rim it is common to see deep sea trenches adjacent to chains of volcanoes--either as strings of islands as in Japan and the Phillipines, or along the edges of continents, as in Central and South America. These features mark subduction zones, a second type of convergent plate boundary. Here thin and dense oceanic plates descend into the Earth's mantle. Where the plate bends down there is a topographic low, or trench. In the mantle the plates melt and rise to the surface again, forming the chains of volcanoes.

Divergent plate boundaries are also marked by distinctive topographic features. The mid-ocean ridges occur where two oceanic plates move away from each other. Mid-ocean ridges are topographic highs on the sea floor that are formed as hot, buoyant lithosphere rises to the surface and separates, pulling two newly-formed plates in opposite directions. High-resolution topographic data shows that many mid-ocean ridges have an axial rift valley--a deep canyon that runs along the crest of the ridge. In places where continental plates separate there are also rift valleys. The East African Rift Zone is a good example of the topographic expression of diverging continental plates.

Transform boundaries, where two plates move laterally past each other, also have a clear topographic expression. Because the landscape is different on either side of the boundary (or "seascape," if the boundary is on the sea floor), there is a topographic mismatch that marks major transform faults.

Photo courtesy of NASA
The San Andreas fault from space and in digital topography.

Hot spots are another example of a tectonic feature that can often be identified by topography alone. The Hawaiian Islands sit above a hot spot, a stable plume of rising mantle that erupts as volcanic islands on the overriding plate. As the plate moves above the hot spot a chain of volcanoes forms. As the volcanoes move away from the hot spot they erode and subside, becoming smaller until they disappear below sea level. In the case of the Hawaiian hot spot, the volcanic chain stretches 5000 miles, all the way to Alaska. If we compare these volcanoes to the subduction zone volcanoes we find that we can distinguish different types of volcanoes based on topography alone. Basaltic, or shield volcanoes, such as those in Hawaii are very broad, with gently-sloping slides. The stratovolcanoes that occur above subduction zones are formed of more viscous andesitic lavas and have a steeper and more conical shape.

Click on image to view Hawaii in 3D

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