Locations at risk from tectonic hazards

Question 1

The global distribution of earthquakes

Answer 1

• the main earthquake zones are found along plate boundaries
  -about 70% of all earthquakes are found in the ring of fire in the pacific ocean
  -most powerful earthquakes are on convergent plate boundaries
• ## rare intra-plate earthquakes can occur due to things such as old fault lines

Question 2

the global distribution of volcanoes

Answer 2

• around 500 active volcanoes around the world
• around 50 erupt each year but the strength of a volcano is determined by how much gas is dissolved in the magma and how easily that gas can escape

Question 3

distribution of divergent boundaries

Answer 3

• most clearly displayed at mid-ocean ridges
• at these locations there are many large numbers of shallow focus low mag earthquakes.
• most are under the sea

Question 4

divergent boundaries and earthquakes

Answer 4

• new oceanic crust which is thinner and denser than the continental is created
• earthquakes here are frequent, small and typically low hazard risk due to them being normally in the ocean and they do not typically trigger tsunamis

Question 5

conservative boundaries and earthquakes

Answer 5

• more risk
• san andreas fault is a good example- in california
• the pacific plate moving north is creating friction against the north american plate thats moving north at a different speed

Question 6

convergent boundaries and earthquakes

Answer 6

• typically when one plate subducts under the other strain builds over time in the subduction zone and the friction between the two rock masses is overcome releasing energy.

Question 7

convergent boundaries and volcanoes

Answer 7

• when a subduction zone is created by the oceanic plate being thrust underneath the continental due to the continentals buoyancy.
• volcanoes typically appear above the magma that forms directly above down-thrust plates
• create composite volcanoes which are more violent- andesitic magma which is very runny and viscious

Question 8

divergent boundaries and volcanoes

Answer 8

• create rift volcanoes where plates diverge from one and other
• shield volcanoes which are less violent- basaltic magma which is thicker and less viscious

Question 9

hotspot volcanoes

Answer 9

• found in the middle of tectonic plates
• due to mantle plumes - area in the mantle from which heat rises as a hot thermal plume from deep in the earth
• high heat and low pressure at the base of the lithosphere enable melting of the rock.
• magma rises and escapes through cracks in the earths crust and erupts to form a volcano
• as the tectonic plates move over the stationary mantle plume the volcanoes move away and a new one forms in its place
• the old volcanoes cool and subside creating islands
• example of chain volcanoes can be seen in the hawaiian islands

Question 10

explain plate tectonic theory

Answer 10

a theory explained more than 60 years ago to explain large scale movements of the lithosphere
-based around evidence such as sea-floor spreading, ocean topography and paleomagnetism

Question 11

what is paleomagnetism

Answer 11

• results from the iron in a zone of magma lining up with the earths polarity at the time of cooling
• scientists can then use this to determine historic periods of large-scale tectonic activity through the reconstruction of relative plate motions

Question 12

what are convection currents

Answer 12

• thought to be a possible driving force for the movement of plate tectonics
• The heat from the core is transferred to the mantle. Liquid rock, close to the core, is heated and rises. When it reaches the crust it is forced sideways as often it can not pass through the crust. The friction between the convection current and the crust causes the tectonic plate to move. The liquid rock then sinks back towards the core as it cools. The process then repeats.

Question 13

what is sea floor spreading

Answer 13

-the formation of fresh areas of oceanic crust which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side.

Question 14

what is a benioff zone

Answer 14

• an area of seismicity corresponding with the slab being thrust downwards in a subduction zone
• the different speeds and movements of rock at this point produce numerous earthquakes
• it is the site of intermediate/deep-focused earthquakes

Question 15

what is a locked fault

Answer 15

• a fault that is not slipping because the frictional resistance on the fault is greater than the stress across the fault- its stuck
• these faults can store massive amounts of energy for long periods of time and is eventually released in a large magnitude earthquake
• the 2004 indian ocean tsunami was the result of a mega-thrust locked fault

Question 16

whats the hypocentre and the epicentre

Answer 16

hypocentre- the point within the earth where an earthquake rupture starts- usually any depth between the earths surface and 700km
epicentre- the point on the earths surface directly above the hypocentre

Question 17

primary waves

Answer 17

• vibrations caused by compression like a shunt through a line of connected train carriages
• they spread very quickly

Question 18

secondary waves

Answer 18

• move slower

- they vibrate at right angles to the direction of travel and cannot travel through liquids like p waves can

Question 19

love waves

Answer 19

• surface waves with the vibration occurring in the horizontal plain
• high amplitude

Question 20

which waves are the most destructive

Answer 20

• secondary and love waves are more destructive than p waves as they have a higher amplitude

Question 21

what is soil liquefaction

Answer 21

• process by which water saturated material can temporarily lose normal strength and behave like a liquid under the pressure of string shaking
• it can cause buildings to settle, tilt and eventually collapse in the worse case events

Question 22

landslides

Answer 22

• where slopes weaken and fail- another earthquake secondary hazard
• landslides can travel several miles from their source and get bigger along the way
• over the last 40 years around 70 percent of all deaths caused by earthquakes globally have been due to landslides
• for example- in the 2008 sichuan earthquake they accounted for a third of the deaths

Question 23

tsunamis

Answer 23

• tsunamis are a series of waves caused by sea bed displacement
• the global distribution- 90% of all events occur within the pacific basin associated with the activity at the plate margins
• most are generated at subduction zones

Question 24

what is a pyroclastic flow

Answer 24

• result from frothing of molten magma in the vent of the volcano
• the bubbles burst and eject hot gases and pyroclastic material which contains glass shards, pumice and ash
• a primary volcanic hazard

Question 25

what is tephra

Answer 25

• when a volcano erupts and ejects material such as rock fragments into the atmosphere
• tephra can cause building roofs to collapse as well as start fires on the ground
• dust can reduce visibility and air travel
• primary volcanic hazard

Question 26

what are volcanic gases

Answer 26

• the mix normally includes water vapour, sulphur dioxide, hydrogen and carbon monoxide
• most deaths have been associated with carbon monoxide as it is colourless and odourless
• primary volcanic hazard

Question 27

what is a lahar

Answer 27

• volcanic mud flows composed of fine sand and silt material
• heavy rainfall as a trigger as old tephra deposits on steep slopes
• secondary hazard

Question 28

what is a jokulhlaup

Answer 28

• a glacial outburst flood
• can cause widespread land form modification through erosion and deposition
• these floods occur very suddenly with rapid discharge of large volumes of water, ice and debris from a glacial source
• volcanoes can erupt underneath to burst the glacier