Earthquakes & Volcanoes
Focus vs epicenter, P-waves and S-waves, locating an earthquake, and the three volcano types.
Earthquakes
An earthquake releases energy when rock under stress suddenly slips along a fault. Two terms you must keep straight:
- Focus — the point underground where the rock first breaks.
- Epicenter — the point on the surface directly above the focus.
Seismic waves: P and S
- P-waves (primary) — fastest, arrive first. They are compressional and can travel through solids and liquids.
- S-waves (secondary) — slower, arrive second. They are shear waves and travel through solids only — which is how we know the outer core is liquid (S-waves can't cross it).
Because P-waves outrun S-waves, the gap between their arrival times grows with distance. The bigger the P–S time gap, the farther the station is from the epicenter.
Locating the epicenter
One station tells you the distance to the epicenter (from the P–S gap) but not the direction — so you draw a circle of that radius around the station. With three stations, three circles overlap at a single point: that intersection is the epicenter. This is called triangulation.
Volcano types
- Shield volcano — broad and gently sloping, built from runny (low-silica) basaltic lava. Quiet eruptions (Hawaii).
- Composite (strato) volcano — tall and steep, built from alternating layers of lava and ash. Thick, sticky lava → explosive eruptions (Mount St. Helens).
- Cinder cone — small, steep cone of loose volcanic fragments.
Most volcanoes and earthquakes cluster along plate boundaries — especially the Ring of Fire around the Pacific, a chain of subduction zones.