Question

Columnar basalt is a striking geological formation consisting of hexagonal columns of rock, typically formed during the cooling of thick lava flows. When basaltic lava erupts onto the Earth's surface, it begins to cool from both the top, where it meets the atmosphere, and the bottom, where it contacts the cooler underlying ground. As the lava cools, it contracts. This contraction creates immense internal tensile stress within the cooling rock. If the cooling is sufficiently slow and uniform, the stress cannot be accommodated by simple cracking, and the lava instead fractures along a network of joints that form a geometric pattern, usually hexagonal. Because the cooling occurs perpendicular to the cooling surfaces, these fractures propagate vertically downward from the top and upward from the bottom, meeting in the middle to create the long, solid columns. However, if the lava cools too rapidly—such as when it comes into direct contact with water—the rapid drop in temperature prevents the slow, orderly contraction required for geometric fracturing. Instead, this rapid quenching causes chaotic, irregular cracks, resulting in blocky or glassy basalt structures rather than neat columns.

According to the passage, the vertical direction of the fractures that form columnar basalt is directly caused by which of the following?