Plate Tectonics

• The surface of the Earth is divided into a small number of rigid plates in constant motion
• Movements of the plates is driven by convection currents in the mantel and by slab pull

Lithospere

• Relatively cold
• Consists of the crust and upper mantel
• Rigid

Asthenosphere

• Weaker and ductile
• Underlies the Lithosphere
• Low seismic velocities
• 1-2% partially molten

Divergent (Constructive)

• Plates move apart to create new lithosphere
• Magma is brought up (partially melted upper mantle)
• Magma crystallises as basalt and pillow lavas
• Dolerite dykes and gabbro beneath surface
• Forms mid-ocean ridges rising from the abyssal plain
• Marked by lines of volcanoes
• Forms transform faults perpendicular to the ridge
• Shallow earthquakes

Convergent (detrsuctive)

• Forms an ocean trench when one plate is subducted
• Low angle thrust faults and reverse faults
• Felsic, andesitic magma
• Ocean-ocean ~ island arcs and inclined benioff zone
• Continent-continent ~Granite batholiths, mountain building and shallow earthquakes
• Ocean-continent (e.g. Nazca and S. America) ~ Andesitic volcanoes and inclined benioff zone
• Contact metamorphism around batholiths
• Regional metamorphism - orogeny and fold mountains

Conservative

• Transform fault
• No magma
• Shallow earthquakes
• E.g. San Andreas fault, San Francisco

• Most often occur at plate boundaries
• Shallow - <70km depth
• Intermediate - 70-300km depth
• Deep - >300km depth
• Deeper than 670km depth, partial melting occurs so there is no friction to cause earthquakes
• Focus: the source of the waves
• Epicentre: the place on land directly above the focus
• P-waves: longitudianal waves which travel through the earth. They travel faster than S and L waves and can travel through liquids and solids
• S-waves: transverse waves which can't travel through liquids
• L-waves: Only travel on the surface so they give no evidence for the structure of the Earth. They cause the most destruction because they have the highest amplitude

The Richter Scale

• Measures magnitude
• Based on the amplitude of waves recorded on a seismometer

Modified Mechalli Scale

• Uses Roman numerals
• Estimates the intensity of the earthquake
• Based on structual damage caused by the earthquake
• Generally decreases the further away from the epicentre

Hazard Maps: places most likely to suffer

Changes in ground levels: area around focus swells and tilts. Shown by tilt metres and lasers

Gas measurements: radon moves up through cracks in rocks. Detected in water wells

Magnetism and animals: Disturbed behaviour due the a change in the Earth's magnetic field (changed migration patterns)

• Soil loses strength and behaves like quicksand
• Tsunami as a result of water displacement
• Landslides
• Liquefaction of the ground - soft sand and silt separate and water rises
• Vibrations in bedrock
• Movement of the ground
• Buildings collapse/infrastructure damage
• Fires as a result of gas pipes breaking
• Waterpipes break and the water becomes contaminated spreading disease

• Design light and flexible buildings
• Planning - forcast number and magnitude of earthquakes and plan where buildings should be located
• Ground/Base Isolation - Rest buildings on large rollers, rubber pads, springs or sliders
• Strengthen framework of buildings - diagonal bracing
• Design tall buildings to be able to sway
• Flexible pipes to avoid broken gas mains

Stratovolcano

• Cone shaped with a wide base and a narrow top
• Andesitic lava - partial melting of the mantle
• Explosive - thick lava (low viscosity) - gets trapped - build up of gas and pressure - released at once
• Found at convergent plate boundaries (e.g. Caribbean and S.America - Montserrat)

Shield Volcano

• Low viscosity lava
• Low lying
• Very wide base spread over a large area
• Frequent but gentle eruptions
• Found at divergent plate boundaries

• Lahar: fast moving slurries of rock, mud and water that flow down river valleys burrying people and destroying buildings
• Ash: Small fragments blasted into the air which can collect of roofs making them collapse. Can also lead to death by choking
• Lava flow: Slow molten flows which bury and burn everything. Deaths are uncommon as you can move out of the way easily
• Landslide: Downhill movements of rock which burry and destroy buildings
• Pyroclastic flow: High speed avalanches of hot rock, gas and ash. Burns, buries and suffocates everything. Leads to lots of deaths. Can reach speeds of 700km/h and temperatures of 1000 degrees C

Patterns of activity: frequency of eruptions

Ground deformation: tilt metres and GPRS picks out swelling suggesting rising magma

Gas Emissions: magma nears surface and pressure decreases. Gas escapes and sulfur dioxide increases

Groundwater Changes: water levels rise

Seismicity: earthquakes indicate a volcanic eruption

• Have an evacuation kit
• Own a car for a quick escape
• Have exclusion zones in active areas which are dangerous
• Have emergency services on standby
• Monitor the volcano and issue warnings (prediction methods)
• Have an effective evacuation plan

Generated by:

• Explosive volcanic eruptions
• Submarine caldera collapse
• Displacement of water and the ocean floor during an earthquake

• Can reach heights of 100m
• Drawback: water on the beach gets sucked back towards the source of the tsunami
• Looks like a normal wave out to sea because it has a very long wavelength
• Shallow land acts as a break. The front of the wave is slowed down and the back of the wave catches up to form a tall wave with a shorter wavelength.

• Keep a development-free zone closer to the water
• Set buildings at right angles to the coast and on concrete piles
• Have a raised coastal road
• Have a quick communication system with earthquake monitors who can predict when and where an earthquake might hit