Earth Science Practice Test: Earthquakes and Volcanoes

The focus (hypocenter) is the point underground where the rock first breaks. The epicenter is the spot on the surface directly above it.

P waves (primary waves) are compressional waves and travel fastest, so they reach a station first. S waves come next, and surface waves such as Love waves arrive last.

The S-wave shadow zone shows that a large liquid layer blocks them, and that layer is the outer core. The inner core is solid, and the mantle is solid rock that only flows very slowly.

Each station gives only its distance from the epicenter, which forms a circle on the map. Two circles cross at two points; a third circle picks out the one point where all three meet.

Distance d satisfies d/3.5 − d/6 = 50, so d × (1/3.5 − 1/6) = 50 and d = 420 km. Multiplying 50 s by one wave speed (300 or 175 km) ignores that both waves are moving.

Two steps means 32 × 32, which is about 1,000 times more energy. The figure of 100 times describes the ground-shaking amplitude (10 per step), not the energy.

Shield volcanoes form from runny basaltic lava that spreads far before cooling, making wide, gently sloped mountains. Composite volcanoes, like Mount St. Helens, are steep and built from layers of ash and lava.

Silica-rich magma is very viscous, so dissolved gases cannot escape easily. Pressure builds until the gas bursts out, blasting the magma into ash. Runny, low-silica magma lets gas escape gently.

Plates around the Pacific sink beneath neighboring plates, and melting above them feeds chains of volcanoes. Hawaii is a hot spot, but it is the exception, not the main cause of the ring.

When a fault under the ocean moves vertically, it shoves the whole column of water above it, sending out long waves. Wind makes ordinary waves, and the Moon causes tides, not tsunamis.