IGCSE Geography / Hazardous environments

Volcanoes and hotspots revision notes

Edexcel International GCSE Geography (4GE1), chapter 3: Hazardous environments, specification point 3.1c. Free lesson.

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Volcanoes form where magma reaches the surface at constructive and destructive plate boundaries, and above hotspots within plates. Their eruption style and shape depend on the lava, while eruptions can create several hazards.

Why volcanoes form at two types of boundary

At a constructive boundary, plates move apart and magma rises from the mantle into the gap. At a destructive boundary, plates move together and the oceanic plate sinks. Rock melts deep down, and the less dense magma rises. Lava is magma once it reaches the surface. Constructive boundaries: runny lava with little gas lets gas escape, so eruptions are gentle and frequent. Lava flows a long way before it sets, building a wide shield volcano with gentle slopes. Destructive boundaries: thick, sticky lava contains more gas. It traps the gas until it blasts out, causing explosive…

Why some plate boundaries have no volcanoes

Volcanoes need magma. At a conservative boundary, plates slide past each other, so magma is not made. At a collision boundary, neither continental plate sinks, so no rock melts to make magma. Volcanoes form at constructive and destructive boundaries, and at hotspots. There are no volcanoes at conservative or collision boundaries.

How a hotspot leaves a chain of volcanoes

A hotspot is a place where volcanoes form in the middle of a plate, far from a plate boundary. A mantle plume, a column of very hot rock rising from deep in the mantle, melts through the plate above it. The hotspot stays fixed while the plate moves. Magma rises and forms an active volcano directly above the hotspot, often building an island. As the plate slides on, it carries the volcano away from its magma supply. The old volcano becomes extinct, while a new one forms above the hotspot. Over time, this makes a chain. Hawaii is a chain of volcanic islands in the middle of the Pacific…

Parts of a composite volcano and its hazards

A composite volcano has a magma chamber, which stores molten rock under the volcano. The vent is the pipe the magma rises through. The crater is the hollow at the top where the vent opens. Ash cloud: tiny pieces of rock blasted into the air, which fall as ash over a wide area. Lava flow: molten rock running downhill and destroying what it reaches. Pyroclastic flow: very hot gas, ash and rock rushing down the slope too fast to outrun. Lahar: a mudflow made when ash mixes with heavy rain or melted snow and pours down valleys. Volcanic gases: some are poisonous,…

Why composite volcanoes have steeper sides

Composite volcanoes form at destructive boundaries, where thick lava sets near the vent and builds steep sides. Shield volcanoes form at constructive boundaries, where runny lava flows much farther before setting, so it builds gentle slopes.