Earth & Enviro Year 12 Term 1 Study

What is a geological hazard?

A natural process originating within the Earth (e.g., earthquakes, volcanic eruptions) that can harm humans, ecosystems, or infrastructure.

What causes earthquakes?

Stress builds along faults → rocks deform → sudden release of elastic potential energy → seismic waves.

What are P-waves and S-waves?

P-waves are compressional, travel through solids and liquids, and are fastest; S-waves are shear waves, travel only through solids, and are slower.

How are earthquake epicentres located?

By comparing P-wave and S-wave arrival time differences from at least three seismic stations to triangulate the epicentre.

What are shallow, intermediate, and deep earthquakes associated with?

Shallow: crustal faults & volcanoes; Intermediate & deep: subducting slabs in convergent plate boundaries.

What is a subduction zone?

A convergent boundary where denser oceanic lithosphere descends beneath another plate, creating trenches, volcanic arcs, and deep quakes.

What is partial melting?

The process where the subducting slab releases water, lowering the melting point of overlying mantle, generating silica-rich magma.

Why are convergent boundary volcanoes explosive?

They produce silica-rich, viscous magma that traps gases, causing pressure buildup and violent eruptions.

What are lahars?

Volcanic mudflows formed when ash mixes with rainwater or rivers, travelling rapidly and causing destructive flooding.

What hazards come from ash eruptions?

Roof collapse, respiratory issues, contaminated water, engine failure, reduced visibility, crop damage.

What is a lava dome?

A mound that forms when viscous silica rich magma slowly extrudes and cools, often collapsing into pyroclastic flows.

What is tsunami generation during subduction?

Vertical seafloor displacement during megathrust earthquakes, pushing water and generating long-wavelength waves.

What were the atmospheric effects of the 1991 eruption?

20 Mt of SO2 formed global aerosols resulting in roughly 0.5C cooling for 1-2 years.

What biosphere impacts come from volcanic eruptions?

Ash burial, soil sterilisation, crop destruction, wildlife loss, contaminated water, long-term ecosystem recovery.

How do volcanic gases affect the atmosphere?

SO2 forms sulfate aerosols, reflects sunlight, results in cooling. CO2 increases greenhouse effect; ash reduces visibility.

How do eruptions affect climate?

SO2, global cooling for 1-3 years. Stratospheric aerosols, short term stratospheric warming.

What atmospheric layers are impacted by large eruptions?

Troposphere (local ash, storms) and stratosphere (SO2 aerosol spread leads to climate effects).

What secondary hazards follow eruptions?

Lahars, landslides, flooding, contaminated waterways, food shortages.

What does seismic monitoring detect in volcano prediction?

Increased earthquake frequency, shallower foci, harmonic tremor indicating magma movement.

What does ground deformation monitoring measure?

Swelling/inflation of volcano using GPS, tiltmeters, and EDM which indicates rising magma.

How do scientists use volcanic gas monitoring?

Rising SO2 indicates magma decompression; falling SO2 may signal vent blockage, dangerous pressure buildup.

Hazard vs Disaster

A hazard is a natural event; it becomes a disaster when it significantly impacts people, infrastructure, or systems.

What is the purpose of hazard mapping?

To identify zones of ashfall, lahars, lava flow, and pyroclastic flow risk for evacuation planning.

What caused magma formation beneath Mt Pinatubo?

Water released from the subducting Eurasian Plate lowered the melting point of the mantle wedge, partial melting, producing andesitic magma.

Why was Pinatubo's eruption highly explosive?

Silica-rich viscous magma trapped gases, so pressure built until sudden explosive release.

How did earthquake foci change before the eruption?

From deeper (5-8km) to shallower (0-3km) as magma rose - indicating imminent eruption.

What were the main hazards from Pinatubo?

Ashfall, pyroclastic flows, SO₂ emissions, roof collapse, aviation disruption, and years of lahars.

What prediction technologies successfully forecasted the eruption?

Seismic monitoring, gas measurements, ground deformation (EDM), and satellite observations.

Why was the evacuation so successful?

Early warnings from USGS + PHIVOLCS, clear precursors (shallow quakes, rising SO2, deformation), around 60,000 people safely relocated.

What biosphere effects occurred after the eruption?

Forest burial, destroyed crops, contaminated water, displaced communities, and long-term soil recovery issues.