Volcanoes and earthquakes

CCEA examiners expect detailed knowledge of how volcanoes and earthquakes are formed, their key features and distribution, and a named case study of both a volcanic event and a seismic (earthquake) event.

Volcanoes

Types of volcano

Shield volcanoes (constructive boundaries):

• Wide, gently sloping sides. Built from many layers of thin, runny (low viscosity) basaltic lava.
• Eruptions are relatively gentle, lava flows slowly.
• Example: Mauna Loa, Hawaii (hotspot); Icelandic volcanoes on the Mid-Atlantic Ridge.

Composite volcanoes (stratovolcanoes) (destructive boundaries):

• Steep, cone-shaped. Built from alternating layers of lava and ash.
• Silica-rich, viscous magma → explosive eruptions (magma traps gases; pressure builds until explosive release).
• Can produce pyroclastic flows (superheated gas and ash moving at 700 km/h).
• Examples: Mount Fuji (Japan), Popocatépetl (Mexico), Montserrat (Caribbean).

Volcanic features

• Vent / crater: the opening through which lava and gases escape.
• Magma chamber: underground reservoir of magma.
• Secondary vents: subsidiary outlets on the sides of the volcano.
• Lava flows: streams of molten rock.
• Pyroclastic flows: fast-moving clouds of hot gas, ash and rock fragments.
• Lahars: volcanic mudflows — melted snow/ice mixing with volcanic debris.

Volcanic case study: Mount Pinatubo, Philippines (1991)

One of the 20th century's largest eruptions. Pinatubo is a composite volcano on a destructive boundary (Pacific Plate subducting beneath the Philippine Plate).

• Warning signs: small earthquakes and steam explosions weeks before eruption.
• Effects: 800+ killed directly; pyroclastic flows buried entire towns; lahars destroyed 100,000 homes; eruption column reached 40 km into the stratosphere; ash cloud circled the globe, temporarily lowering global temperatures by ~0.5°C; 58,000 evacuated.
• Context: nearby Clark Air Base (US) was evacuated, saving many American lives.

Earthquakes

How earthquakes form

• The focus (or hypocentre) is the point underground where the earthquake originates.
• The epicentre is the point on the Earth's surface directly above the focus.
• Energy radiates outward as seismic waves (P-waves travel through solids and liquids; S-waves travel only through solids).
• Earthquake magnitude measured on the Richter scale (logarithmic — a magnitude 7 is ten times more powerful than a magnitude 6) or more recently the Moment Magnitude Scale (Mw).
• Aftershocks: smaller earthquakes following the main event.

Distribution of earthquakes

Earthquakes are concentrated at plate boundaries:

• Most severe at destructive and conservative boundaries.
• Deep-focus earthquakes only at destructive (subduction) boundaries.

Seismic case study: Nepal earthquake, April 2015 (Mw 7.8)

Nepal lies at the collision boundary between the Indian Plate and the Eurasian Plate.

• Focus: shallow (15 km depth) — shallow focus earthquakes cause the most surface damage.
• Effects: 8,900+ killed; 3.5 million displaced; 600,000 buildings destroyed; avalanche triggered on Everest killed 21 climbers; Kathmandu severely damaged; UNESCO World Heritage Sites destroyed (Durbar Square, Bhaktapur).
• Why so devastating: poor-quality building construction (unreinforced brick and stone), mountainous terrain making rescue difficult, limited emergency services in a low-income country.
• Response: international aid (India, China, US, UK); temporary shelters for winter; long-term reconstruction continues.

Alternative seismic case study: Haiti, 2010 (Mw 7.0)

Conservative boundary (Caribbean Plate + North American Plate). 220,000+ killed — one of the deadliest in modern history. Poor construction + extreme poverty + inadequate emergency services.