In my last post I covered the basics of mantle dynamics and now I should be just about ready to dive in to the actual content of this series of posts: lithospheric delamination. I’ll start by just explaining the concept. In my next post in this series I’ll talk about type-localities in the western U.S. and some basics on the methods I use to study the process. I’ll finish the series by explaining specifics of my research and where I hope to go with it.
In geodynamics, delamination refers to a process in which something that was formerly attached to the lithosphere becomes unstable and falls off, sinking deep into the mantle. This can happen if the material is substantially more dense than the material below it. We call this a buoyant instability.
A nice way to visualize what’s happening, and an analogy that I’ll return to in a moment, is to imagine a piece of wood floating in water with a heavy chunk of metal glued to its bottom. The wood in this case represents the buoyant crust and the metal represents the cold upper mantle that’s stuck to its bottom. The water it’s floating on represents the rest of the mantle. Delamination would be the process of the glue that holds the two together coming undone and allowing the metal to sink away.
So, why would we care? This is a process that happens tens or hundreds of kilometers below the surface of the Earth so how does it affect anything else on Earth?
First of all, in Earth sciences even the processes which seem most esoteric and intangible usually have massive consequences to other processes and systems which directly affect life on Earth. Secondly, it’s just interesting, okay? It shouldn’t need justification to be interesting.
In this case however there is a direct, tangible consequence that’s fairly interesting in its own right. Let’s go back to the floating board being weighed down by the chunk of metal analogy for a moment. Basic physics tells us that the buoyant force pushing the board up equals the weight of the displaced water and that the system will find equilibrium when the weight of the floating stuff equals this buoyant force. Since the metal is dense, when the two are attached the whole thing will float relatively low (assuming it’s not so heavy that it just sinks all together). When the metal comes loose and sinks away, the board is much less dense and will float higher in the water. It’s similar to a boat weighed down with cargo floating super low until the cargo is unloaded.
So, what does this look like in geophysical terms? If there’s heavy material attached to the bottom of the crust, it’s being weighed down and will float relatively low, but if the heavy material falls off the crust will float up higher. This is usually expressed on the surface as (relatively) rapid uplift. In other words, it can build mountains.
We suspect that this has happened in the southern Sierra Nevada (adding to an already impressive mountain range), the Wallowa Mountains in eastern Oregon (building a mountain range entirely from scratch), and around the edges of the Colorado Plateau (responsible for its dish-like topographic expression).
I think I’ll cut this post short here and talk about the different forms of delamination in my next post. As illustrations, I’ll give specifics on the three type-localities I just mentioned: the southern Sierra Nevada, Wallowas and Colorado Plateau.