Credit:
Bennington, et al.
While they collectively contain a lot of molten basaltic material (between 4,000 and 6,500 cubic kilometers of it), it’s not very concentrated. Instead, this is mostly relatively small volumes of molten material traveling through cracks and faults in solid rock. This keeps the concentration of molten material below that needed to enable eruptions.
After the two streams of basaltic material merge, they form a reservoir that includes a significant amount of melted crustal material—meaning rhyolitic. The amount of rhyolitic material here is, at most, under 500 cubic kilometers, so it could fuel a major eruption, albeit a small one by historic Yellowstone standards. But again, the fraction of melted material in this volume of rock is relatively low and not considered likely to enable eruptions.
From there to the surface, there are several distinct features. Relative to the hotspot, the North American plate above is moving to the west, which has historically meant that the site of eruptions has moved from west to east across the continent. Accordingly, there is a pool off to the west of the bulk of near-surface molten material that no longer seems to be connected to the rest of the system. It’s small, at only about 100 cubic kilometers of material, and is too diffused to enable a large eruption.
Future risks?
There’s a similar near-surface blob of molten material that may not currently be connected to the rest of the molten material to the south of that. It’s even smaller, likely less than 50 cubic kilometers of material. But it sits just below a large blob of molten basalt, so it is likely to be receiving a fair amount of heat input. This site seems to have also fueled the most recent large eruption in the caldera. So, while it can’t fuel a large eruption today, it’s not possible to rule the site out for the future.
