Summary: What is dynamic topography? Well, it depends on who you ask. Dynamic topography is similar to other terms, like uplift, that have been used in so many different ways that you really have to look at the document you’re reading to understand what the author is talking about. This term has been applied to places around the world, like the Colorado Plateau in the United States, South Africa, the Aegean, and East Asia, which makes it even more complicated to tease out its meaning. Most broadly, dynamic topography refers to a change in the elevation of the surface of the earth in response to something going on in the mantle. This “something” can include both the flow of the mantle, as well as differences in mantle temperature or density. For the purposes of this podcast, I will use a more strict definition: Dynamic topography is the change in the elevation of the surface of the earth in response to the upward or downward flow of the mantle. How much higher or lower can dynamic topography make the earth’s surface? Well, that’s a matter of debate. Earlier studies have suggested that several kilometers, or over 6000 feet of modern elevations can be explained by things going on in the mantle. More recent work instead suggests that dynamic topography creates changes of at most a three hundred meters, or a thousand feet. A good example of a place where this process is thought to be active is Yellowstone. As Dick Gibson discussed in the December 19th, 2014 episode, Yellowstone is thought to be a hot spot. That is, an area of the earth where hot material moves from deep within the mantle to the base of the crust, causing significant volcanism at the surface of the earth. Other well-known hot spots are located in Hawaii, and Iceland. So how can a hot spot like Yellowstone cause dynamic topography? Well, you’ve probably seen a similar process at play the last time you played in a pool or a lake. Think of the surface of the pool like the surface of the earth. If you start moving your hands up and down under water, the surface of the pool starts to move up and down. If you ever tried to shoot a water gun upwards underwater when you were a kid, you probably remember it pushing up the surface of the water, and being disappointed that it didn’t shoot out at your friend or sibling. As an adult, you could try holding a hose upwards in a pool. Again, it probably won’t shoot out, but will gently push upwards on the surface of the pool. Dynamic topography concept. © Commonwealth of Australia (Geoscience Australia) 2017, used under Creative Commons Attribution 4.0 International Licence The principle for a hot spot creating dynamic topography is the same. The flow of the mantle pushes upwards, warping the crust and increasing the elevation of the earth’s surface above the hot spot. Near Yellowstone, this results in an area of high elevation which lies next to the Snake River plain. But Dynamic Topography doesn’t just cause increases in elevation, it can also pull the earth’s surface downward. In North America, dynamic topography is thought to have been in part responsible for the creation of the Cretaceous interior seaway. As a reminder, the Cretaceous interior seaway was a shallow sea that covered parts of western North America, in middle to late Cretaceous time, about 100 to 79 million years ago. Its size varied, but at its greatest extent the seaway stretched through Texas and Wyoming in the US, and Alberta and to the Northwest Territories in Canada. It was widest near the US-Canadian border, where it stretched from Montana to western Minnesota. Low elevations in western North America that allowed the ocean to flood in and form this shallow sea may have been caused by downwards flow in the mantle. This downwards flow was likely caused by oceanic crust that was subducted at the western margin of North America. That is, oceanic crust that went underneath the North American plate and into the mantle. Because this crust was par
