risk of tectonic hazards 1

earthquake distribution

• The majority of earthquakes (about 95%) occur close to or at a plate boundary

• Many occur around the 'Ring of Fire' surrounding the Pacific Ocean

• The most powerful earthquakes are usually associated with convergent or conservative plate boundaries

volcano distribution

• at or near to plate boundaries

• Many (about 75%) occur around the 'Ring of Fire' surrounding the Pacific Ocean

• Volcanoes occur at convergent and divergent plate boundaries

• They can also be found at hot spots in the middle of plates like Hawaii in the central Pacific

Tsunami distribution

• Over 70% of tsunamis occur around the Pacific Ocean

  • 15% Mediterranean Sea, 9% Caribbean Sea and Atlantic Ocean, and 6% Indian Ocean

• They are caused by tectonic activity 

• Most occur due to activity at convergent boundaries

divergent plate boandary

Plates moving apart

• The Mid-Atlantic Ridge is an example of a constructive plate boundary

• Both volcanic eruptions and earthquakes can occur at this type of plate boundary

Convergent

Plates moving together

• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate

• This forms deep ocean trenches in the subduction zone

• the subduction is known as the benioff zone

Conservative plate boundary

plates moving past each other or in the same direction at different speeds

• They become stuck and pressure builds, the plates eventually 'snap' past each other

• The friction causes earthquakes but not volcanoes

• Land is neither made or destroyed

intra plate earthquake

• These can occur anywhere

• The cause is not fully understood but is thought to be:

  • Tectonic stresses causing ancient fault lines to reactivate

  • The plates are moving over a spherical surface and this causes zones of weakness

• The New Madrid earthquake in 1812 and the 2011 Virginia earthquake are examples of intra-plate earthquakes

Hotspot volcanoes

• These occur over stationary magma plumes (columns of rising magma) in the asthenosphere

• The tectonic plate moves over the plume leading to the formation of a chain of volcanic islands (Hawaii)

• The oldest island is the one furthest away from the plume

development of plate tectonic theory

The theory of convection currents

When magma is heated, it becomes less dense and rises, while cooler, denser fluid sinks, creating a cyclical flow known as a convection current.

• The heat from radioactive decay in the core moves upwards into the mantle

• It creates convection currents, which push up into the spreading mid-ocean ridges, forcing them further apart called the ridge push

paleomagneticism

A record of changes in the magnetic field of the earth, this is shown in the orientation of the magnetic field in the rocks

sea floor spreading

• Palaeomagnetism provides evidence that the sea floor has gradually moved apart at a mid-ocean ridge

• Lava cools and solidifies with the minerals lining up with the magnetic field

subduction

• Convection currents in the mantle drag the overlying lithosphere towards each other

• A subduction zone is formed when two plates meet

  • The heavier, denser plate subducts under the lighter, less dense plate

slab pull

• As oceanic crust cools, it becomes denser and thicker, and gravity forces the lithosphere down into the subduction zone 

• As it sinks, it drags or pulls the plate with it

primary p waves

• Body wave

• Fastest

• Reach the surface first

• Travel through liquids and solids

• Cause backwards and forwards shaking

• Least damaging

secondary s waves

• Body wave

• Slower than P waves

• Only travel through solids

• Cause a sideways motion

• More damaging

love l waves

• Surface wave

• Slowest

• Cause a side to side motion

• Larger and energy is focussed on the surface

• Most damaging

primary hazards

A direct result of the earthquake

• Ground shaking

• Crustal fracturing (when the movement causes the Earth's crust to crack)

secondary hazards

A result of primary hazards

• Landslides and avalanches - the movement of the Earth may trigger the collapse of material down steep slopes

• Liquefaction - when the shaking causes particles in the ground to move further apart causing them to act like a liquid rather than a solid

• Flooding - caused by tsunami

Primary hazards of volacanic hazards

Pyroclastic flow - a mix of dense, hot, rock, ash and gases

Lava flow - most move slowly enough that they are not a risk to human life but can reach over 1000^oC

Ash falls - can travel many km, causing injuries, damage, deaths and disruption to transport

Gas eruptions - gases trapped in the magma are released during an eruption, they may form gas clouds which are hazardous to health

Secondary Hazards of volcanos

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

Jökulhlaups - floods caused by a sudden release of water and rocks when glacial ice is melted by the eruption

causes of tsunamis

• earthquake under the sea bed

• Landslides which may be due to earthquakes and volcanic eruptions displacing the water

• Underwater volcanic eruptions

• Rarely they can be caused by a meteor strike