Digital ACT Reading Practice Question #1751

The simile comparing the collapsing roof rock to a "massive piston" emphasizes that the collapse: Passage Excerpt (Natural Science): Unlike volcanic eruptions where lava flows from a central vent, the formation of a giant caldera represents a catastrophic structural failure of Earth's upper crust. Calderas form above shallow magma reservoirs containing hundreds of cubic kilometers of silica-rich rhyolitic magma. As this viscous magma ascends, expanding gases drive plinian eruptions through ring fractures surrounding the chamber roof. As subterranean pressure drops precipitously, the roof rock loses buoyant support. When the weight of overlying overburden exceeds diminished magma pressure, the entire ceiling founders downward along ring faults like a massive piston into the chamber void. The resulting depression, often spanning tens of kilometers in diameter, leaves a dramatic volcanic basin such as Crater Lake or Yellowstone.