LSAT 151, Section 1, RC Passage 4, Plate Tectonics

RC Passage 4 Explanation

PASSAGE TEXT: According to the generally accepted theory of plate tectonics, the earth's crust consists …

Paragraph Summaries

1. Standard plate tectonics explains most earthquakes as the result of plates colliding and one plate being forced under the other. But some areas have intense subduction and very few earthquakes, which the standard theory didn’t explain well.
2. A new proposal says the key is how the plates collide. Plates moving toward each other tend to create shallow subduction, more contact, more friction, and more earthquakes; plates moving in the same direction tend to create steep subduction, less contact, less friction, and fewer earthquakes.
3. This theory also suggests that some supposedly safe regions may still be earthquake risks. Even low-subduction areas could be dangerous if the subduction occurs in a way that creates lots of friction.

Analysis

This passage is basically one big mechanism.

The old expectation was simple: more subduction = more earthquakes. But the weird fact is that some high-subduction zones are quiet. So the new theory adds another variable: the angle of descent.

The angle depends on how the plates are moving relative to each other. When plates move toward each other, the subducted plate descends shallowly. Shallow descent means a big contact surface between the plates, like two sheets of sandpaper rubbing together. More contact, more friction, more earthquakes.

When plates move in the same general direction, one plate is basically overtaking the other. The faster plate’s leading edge gets pushed under. Because that collision zone moves quickly relative to the mantle, the mantle resists it and forces the plate down steeply. Steep descent means less contact, less friction, fewer earthquakes.

So don’t read the quiet zones as places without collisions. They still have collisions and subduction. They’re quiet because the geometry of the collision reduces friction.

This also explains the warning at the end. A region with low subduction isn’t automatically safe. If the plate is descending at a shallow angle, even a smaller amount of subduction can create enough plate-on-plate contact to be dangerous.

The questions revolve around this chain:

same direction → overtaking → steep descent → less contact → less friction → fewer earthquakes

opposite directions → shallow descent → more contact → more friction → more earthquakes

A lot of trap answers focus only on “amount of subduction.” The passage is saying amount matters less than people thought. The type of collision and resulting contact surface can matter a lot.