Chapter 1: Risks of Tectonic Hazards

EQ1, Why are some locations more at risk from tectonic hazards?

Summarise the Ring of Fire

70% of all earthquakes are found here

75% of volcanoes are found here

Intra plate earthquakes

Earthquakes that do not happen at plate boundaries

Tectonic stresses cause ancient fault lines to reactivate

Plates are moving over a spherical surface causing zones of weakness

Summarise tsunamis

Mainly occur at subduction zones

Created when plates suddenly shift displacing large volume of water which creates a wave

Can also be created during a volcanic eruption under the sea- magma displaces water creating a wave

As wave gets closer to shore, wave increases in height

Tsunami waves have much longer wavelength than regular sea waves

Typically a series of waves called a wave train

What are the four types of plate boundaries

Constructive

Destructive

Collision

Conservative

Summarise constructive

Also called divergent

Plates moving apart

Magma rises through the middle forming shield volcanoes

Volcanic activity and earthquakes

E.g. Mid Atlantic Ridge

Summarise Destructive

Also called convergent

Plates moving towards each other

Oceanic and continental plates meet

Oceanic plate subducts below lighter less dense continental plate

Forms deep ocean trenches in the subduction zone

Earthquakes and volcanoes

E.g. boundary between the Nazca Plate and South American Plate

Benioff Zone

Narrow area where earthquakes tend to occur in subduction

Summarise collision plate margins

Two continental plates moving towards each other

Similar density- neither plate can sink

Plates are crushed, crumpled and forced upwards

Create collision fold mountains

E,g, the Himalayas

No subduction meaning no volcanic activity

Earthquakes are the main hazard here

Summarise conservative plate margins

Two plates slowly moving past each other either in same or opposite directions

Can get stuck and pressure builds

Can suddenly shift causing an earthquake

Friction causes earthquakes but not volcanoes

Land is neigther made or destroyed

Summarise hotspot volcanoes

Occur over stationary magma plumes

Tectonic plate moves over the plume leading to the formation of a chain of volcanoes

Define magma plumes

Columns of rising magma in the asthenosphere

Define Moho

The boundary between the crust and the mantle

What evidence is there of plate tectonic movements?

Crust samples gathered and tested using radio metric dating

Palaeomagnetism

Summarise palaeomagnetism

Provides evidence that the sea floor has gradually moved apart

Lava cools and solidifies with the mineral sand magnetic rock lining up with the magnetic field

Direction of minerals on either side is a mirror image

Slab pull

Occurs at a subduction zone where two plates meets

Heavier denser plate is subducted

As oceanic crust cools it becomes denser and thicker

Gravity forces the lithosphere down into the subduction zone

As the plate sinks it drags the rest of the plate with it

Ridge Push

Occurs at constructive boundaries

As plates move apart, magma rises

Magma cools to form new plate material

As it cools, it becomes denser and slides down away from the ridge

Causes plates to move away from each other

Rhyolite lava

High silica content

High viscosity

Difficult for gas to escape

Explosive eruptions

Basalt magma

Low silica content

Low viscosity

Easy for gas to escape

Gentle eruptions

What are the three types of seismic waves

Primary, secondary and love waves

Summarise primary waves

Body wave

Fastest waves- reach the surface first

Travel through solids and liquids

Least damaging

Longitudinal waves

Summarise secondary waves

Body wave

Slower than P waves

Only travel through solids

More damaging

Transverse waves

Love waves

Surface wave

Slowest

Cause side to side motion

Larger and energy focused on the surface

Most damaging

What is the difference between a primary and secondary hazard?

Primary- direct result of the earthquake

Secondary- result of the primary hazard

What are the primary hazards of earthquakes?

The ground shaking + crustal fracturing- when the movements causes the Earth’s crust to crack

What are the secondary hazards of earthquakes?

Landslides and avalanches

Liquefaction

Tsunamis

Flooding- caused by tsunami

What are the primary hazards of volcanoes?

Pyroclastic flow

Lava flow

Ash falls

Gas eruptions

What are the secondary hazards of volcanoes?

Lahars

Jokulhaups

Define crustal fracturing

When the movement caused by an earthquake causes the Earth’s crust to crack.

What is pyroclastic flow?

A mix of hot, dense, rock, ash and gases released from a volcano during volcanic eruptions.

What are lahars?

A mixture of rocks, mud and water which flow down the volcano. They are fast flowing and destroy everything in their path.

What are jökulhaups?

Floods caused by a sudden release of water and rocks when glacial ice is melted by the eruption.

Describe the ways in which a tsunami can be formed

Tectonic movement- causes an earthquake, sudden shift in tectonic plat causes sea bed to jolt and displace large amounts of water which is forced upwards creating a wave.

Landslides- may be due to an earthquake, causes a displacement of water, creating a wave.

Underwater volcanic eruptions- lava displaces the water

Where do tsunamis mainly occur/ where are they typically located?

Usually occur close to plate boundaries and are most common in the area surrounding the Pacific Ocean Ring of Fire

Summarise the 2004 Boxing Day tsunami

Occurred early in the morning

Killed over 220,000 people

650,000 were seriously injured

Triggered by a 9.1 magnitude earthquake

Left 2 million homeless.