Geophysical systems practice questions.

Iceland sits astride a boundary where two plates move apart, and it also lies above a mantle plume. Which combination of features would you expect?

• Basaltic fissure eruptions with shallow-focus earthquakes
• Andesitic stratovolcanoes with deep-focus earthquakes
• Explosive rhyolitic eruptions with no seismic activity
• Frequent deep earthquakes with no volcanic activity at all

Answer: Basaltic fissure eruptions with shallow-focus earthquakes

At a divergent boundary the crust is thin and the magma is low-silica basalt, which is fluid, so eruptions are effusive and often along fissures, and the faulting is shallow. Deep foci and andesite require a subducting slab, which is absent here.

The Hawaiian islands form a chain that increases in age away from the currently active volcano. What does this pattern demonstrate?

• A plate moving over a relatively stationary magma source
• A plate boundary migrating steadily across the Pacific
• A subduction zone that has changed its dip through time
• Repeated collision between two oceanic plates

Answer: A plate moving over a relatively stationary magma source

A hotspot supplies magma from deep in the mantle at a roughly fixed location while the plate slides over it, so each volcano is carried away and goes extinct. The chain therefore records plate motion, and it shows that volcanism does not require a plate boundary.

Which property of magma most directly determines whether an eruption is explosive or effusive?

• Silica content, which controls viscosity and gas escape
• The depth of the magma chamber below the summit
• The total volume of magma stored beneath the volcano
• The age of the crust through which the magma rises

Answer: Silica content, which controls viscosity and gas escape

High-silica magma is viscous, so dissolved gas cannot escape and pressure builds until the magma fragments; low-silica basalt lets gas bubble out and flows freely. Volume and depth affect the size and timing of an eruption, not its style.

A slope of saturated clay-rich soil fails after a week of heavy rain and moves downhill as a coherent block rotating about a curved surface. Which mass movement is this?

• A rotational slump
• A rockfall
• Soil creep
• A pyroclastic flow

Answer: A rotational slump

The curved failure surface and the back-tilted block are diagnostic of slumping, which typically happens in weak, water-saturated material. Rockfall is free movement of fragments, creep is imperceptibly slow, and a pyroclastic flow is volcanic rather than gravitational.

A student writes: 'The Himalayas lie on an active margin where the Indian and Eurasian plates converge, so the range should contain many active volcanoes.' Identify the single error in this reasoning.

• No dense oceanic lithosphere is being consumed there, so little magma is generated.
• The Indian and Eurasian plates are in fact moving apart from one another.
• Converging plates never produce volcanic activity anywhere on the Earth's surface.
• The Himalayas sit on a conservative margin, where crust is neither made nor destroyed.

Answer: No dense oceanic lithosphere is being consumed there, so little magma is generated.

Magma at a convergent margin comes from water released by a descending oceanic slab, which lowers the melting point of the mantle wedge above it. Where two continental plates meet, both are too buoyant to descend far, so the crust thickens and shortens but almost no melt is produced.

On a grassy hillside the fence posts lean downslope, tree trunks curve upward near their base and small terracettes cross the slope. There is no scar at the top and no debris lobe at the foot, and survey pegs move only a few millimetres each year. Which mass movement is described?

• Rotational slump
• Soil creep
• Rockfall
• Debris flow

Answer: Soil creep

Creep is continuous and imperceptibly slow, driven by repeated wetting, drying and freezing that lift particles perpendicular to the slope and drop them vertically; the absence of a rupture surface, a scar or deposited debris rules out the three discrete failures.