Digital ACT Reading Practice Question #1737

The primary focus of the passage is on explaining: Passage Excerpt (Natural Science): While most valley glaciers advance at centimeters per day, surge-type glaciers exhibit dramatic cyclical speedups. During a surge, flow velocity accelerates by two orders of magnitude, advancing dozens of meters daily and fracturing the surface with crevasses. Glaciologists long debated whether climate warming or subglacial dynamics cause surges. Borehole telemetry in Alaska's St. Elias Mountains confirmed the hydraulic hypothesis. During quiescent phases, meltwater drains through interconnected bedrock tunnels. Over decades, internal ice deformation closes these conduits, trapping basal meltwater under immense hydrostatic pressure. This water film decouples the glacier from its bed, reducing basal friction and allowing the ice mass to slide catastrophically down-valley.