Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

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It is well known that beneath Yellowstone caldera lies a large magma reservoir composed of mostly crystallized silicic magma called rhyolite. We can even estimate what the magma reservoir may look like from seismology. But did you know that Yellowstone's magma reservoir produces a "shadow" that we can observe on the Earth's surface?

At first it may seem odd to think about a volcanic system such as Yellowstone's having a shadow. How could something that is underground even create a shadow? Instead of resulting from blocking rays of sunlight, this magmatic "shadow" results from Yellowstone's magma reservoir blocking deeply sourced, mafic magma (called basalt, which is very similar to the type of magma that erupts in Hawai'i) from reaching the Earth's surface. The presence of this shadow in fact was some of the first evidence that Yellowstone hosts a large magma reservoir.

Before we can understand how this shadow forms and what it looks like, we need to know a few things about different types of magma at Yellowstone. Yellowstone erupts two different types of magma, rhyolite and basalt. Rhyolite magmas are the most famous at Yellowstone, as these can be very explosive. In fact, it was a large eruption of rhyolite that produced present-day Yellowstone caldera. Subsequent, less explosive eruptions of rhyolite have since filled Yellowstone caldera with large lava flows. This rhyolite is derived from a magma reservoir located in the shallow crust, only 5 to 17 km (about 3 to 10 mi) below the Earth's surface, and that provides the heat to fuel Yellowstone's vast hydrothermal system of hot springs, mudpots, and geysers. While rhyolite eruptions are the most dramatic events at Yellowstone, there is another equally important magma type that erupts at Yellowstone, but in a less dramatic fashion.

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This other magma type is called basalt. While rhyolites at Yellowstone are formed in the shallow crust, basalt magmas hail from much deeper parts of the Earth. Basalt magmas are commonly generated in the upper part of the Earth's mantle (below the crust) at depths greater than 40 km (about 25 mi) and are injected into the Earth's crust where they provide the heat necessary to generate Yellowstone's famous rhyolites. Sometimes these basalts are erupted at Yellowstone, but only in very specific places. For example, NO basalts are present inside Yellowstone caldera! Instead, basalts associated with Yellowstone caldera are located exclusively around the caldera margins. Most notably, these basalts can be found southwest of Yellowstone caldera, as well as north of the caldera in the region between Mammoth Hot Springs and Norris Geyser Basin. A particularly nice place to view one of these basalts is at Sheepeater Cliffs, just south of Mammoth Hot Springs.

So how does any of this relate to Yellowstone having a shadow? The "shadow" at Yellowstone relates to the distribution of basalt and rhyolite magmas. In particular, Yellowstone's "shadow" refers to the fact that there is a complete lack of basalt within Yellowstone caldera (where only rhyolite is found), but an abundance of basalt surrounding the caldera. The reason for this shadow is that hot rhyolite magma is less dense that basalt magma. Just as your shadow results from you blocking the sun, the magmatic shadow at Yellowstone results from the low-density rhyolite magma reservoir in the shallow crust blocking deep-sourced basalts from reaching the surface. Instead, these basalts are either trapped under the rhyolite magma reservoir or erupt along the outside of this magma reservoir. So, the basalt-free region in Yellowstone caldera shows the shadow of Yellowstone's shallow rhyolite magma reservoir. It is through seemingly simple observations like this that early geologists at Yellowstone and other volcanic systems recognized the presence of large magma bodies in the shallow crust!

Over time, as the shallow rhyolite body cools and solidifies, it will become possible for the deeper basalt to punch through and erupt within the caldera; this is what has happened in the older calderas that predate the current Yellowstone magmatic system. But the lack of any basalt in Yellowstone caldera—the existence of a magmatic "shadow"—is good evidence that the rhyolite magma chamber is still at least partially molten.