Tectonic Hazards EQ1 - Locations at risk from tectonic hazards

Question 1

What is a natural hazard?

Answer 1

A naturally occurring process or event that has the potential to affect people

Question 2

What is the global distribution of earthquakes?

Answer 2

The main earthquake zones are often found in clusters along plate boundaries - 90% of earthquakes are found at plate boundaries

Question 3

What is the global distribution of volcanoes?

Answer 3

Commonly found at the regions where plate margins occur, for example around the Pacific Ring of Fire. However they can occur away from plate boundaries, at ‘hotspot’ regions

Question 4

What is the global distribution of tsunamis?

Answer 4

Most tsunamis are generated along subduction zones, and 90% occur around the Pacific Basin

Question 5

What happens at divergent plate boundaries?

Answer 5

Two plates are moving away from each other (can be continental or oceanic), leading to new crust being created by magma being pushed up from the mantle

Question 6

What happens at convergent plate boundaries? (oceanic-continental)

Answer 6

Where two plates meet, the denser oceanic lithosphere is forced down and under the more buoyant continental lithosphere in the process of subduction. The friction between the plates prevents the subducting oceanic plate from sliding smoothly, so it drags over the overlying plate causing both to fracture and deform.

Question 7

What happens at transform (conservative) plate boundaries?

Answer 7

Two plates are sliding past each other, often at different speeds. This causes a build up of friction as they drag against each other.

Question 8

What is a volcanic hotspot, and how do volcanoes form here?

Answer 8

A volcanic hotspot is an area in the mantle from which heat rises as a hot thermal plume from deep in the Earth. This is able to melt through the lithosphere, rising through the cracks to form active volcanoes on the Earth’s surface. As the tectonic plate moves over the hotspot like a conveyer belt, a series chain of volcanoes forms (e.g. Hawaii)

Question 9

What are the properties of the two different types of crust?

Answer 9

Oceanic crust - thin, composed primarily of basalt but denser
Continental crust - thicker, composed primarily of granite and less dense

Question 10

How do convection currents drive plate movement?

Answer 10

Heat derived from the Earth’s core rises within the mantle to drive rocks upwards. Near the crust, these rocks drag along the tectonic plates, slowly moving them before sinking as they lose heat from the Earth’s core. This creates a convection current cell, heat constantly rising and falling to power plate movement

Question 11

How does slab pull drive plate movement?

Answer 11

The force of a cold, dense oceanic plate plunging into the mantle due to it’s own weight drives the movement of the plate

Question 12

How does ridge push drive plate movement?

Answer 12

Rigid lithosphere slides down the hot, raised asthenosphere below mid ocean ridges, pushing plates away from each other

Question 13

What is sea floor spreading?

Answer 13

The continuous input of magma forming a mid ocean ridge, for example the Mid-Atlantic Ridge

Question 14

What is paleomagnetism?

Answer 14

Paleomagnetism is the record of the Earth’s magnetic field in rocks, sediment and archaeological materials

Question 15

How does paleomagnetism provide evidence for sea floor spreading?

Answer 15

Zones of magma ‘lock in’ the Earth’s magnetic polarity, when it changes every million or so years, so scientists can use this to create a ‘geo-timeline’

Question 16

What is a locked fault?

Answer 16

A fault that is not slipping because the frictional resistance on the fault is greater than the shear stress across it.

Question 17

Why do locked faults often create dangerous tectonic hazards?

Answer 17

When plates are locked together, frictional stress builds. When the stress exceeds a given threshold, a failure occurs along the fault plane that results in a megathrust earthquake

Question 18

What is the Benioff zone?

Answer 18

An area of seismicity corresponding with the slab being thrust downwards in a subduction zone`

Question 19

What are the 3 types of seismic waves?

Answer 19

P waves
S waves
L waves

Question 20

What are the properties of primary waves?

Answer 20

Caused by compression, and spread quickly from the fault at a rate of about 8km/sec

Question 21

What are the properties of secondary waves?

Answer 21

Vibrate at right angles to the direction of travel and cannot travel through liquids. Much more destructive but move more slowly, at around 4km/sec

Question 22

What are the properties of love waves?

Answer 22

Surface waves with vibration occurring in the horizontal plain. They have a high amplitude

Question 23

What is the difference between the hypocentre and epicentre?

Answer 23

Hypocentre = point of rupture where the strain energy of the earthquake stored in the rock is first released. Whereas the epicentre is the point on the earth’s surface vertically above the hypocentre.

Question 24

How is an earthquake generated?

Answer 24

1 - gradual build up of tectonic strain
2 - when pressure exceeds strength of fault, the rock fractures
3 - producing sudden release of energy, creating seismic waves radiating from point of fracture
4 - brittle crust then rebounds either side of fracture (ground shaking)

Question 25

What is soil liquefaction?

Answer 25

The process by which water-saturated material can temporarily lose normal strength and behave like a liquid under the pressure of strong shaking

Question 26

What are landslides?

Answer 26

Secondary hazard from earthquakes where slopes weaken and fail

Question 27

How are tsunamis generated?

Answer 27

Series of waves caused by sea bed displacement, often caused by submarine earthquakes at subduction zones leading to the displacement of a water column.

Question 28

What are some of the primary hazards associated with earthquakes?

Answer 28

Crustal fracturing

Ground shaking

Question 29

What are some of the secondary hazards associated with earthquakes?

Answer 29

Liquefaction

Landslides

Question 30

What are some of the primary hazards associated with volcanic eruptions?

Answer 30

Pyroclastic flows, tephra, lava flows, volcanic gases, ash falls

Question 31

What are some of the secondary hazards associated with volcanic eruptions?

Answer 31

Lahars

Jokulhlaups

Question 32

What are lahars?

Answer 32

Volcanic mudflows generally composed of relatively fine sand and silt material

Question 33

What are jokulhlaups?

Answer 33

A type of catastrophic glacial outburst flood which have a very quick onset and rapid discharge of large volumes of water, ice and debris from a glacial source

Question 34

What is the difference between volcanic eruptions at convergent and divergent plate boundaries?

Answer 34

Convergent = larger magnitude due to subduction of old oceanic lithosphere producing andesitic lava (PHILIPPINES)
Divergent = smaller magnitude due to rising magma being basic in nature (ICELAND)

Question 35

Where is Haiti located?

Answer 35

Haiti is located in the Caribbean, west of the Dominican Republic.

Question 36

What is ash in terms of a volcanic hazard?

Answer 36

Fine particles and dust ejected during an eruption, which can remain airborne as clouds or accumulate on the ground.

Question 37

What is continental crust?

Answer 37

Crust that forms the continents of the lithosphere, on average ~35km thick. It has a higher silica content than oceanic crust, making it have a lower average density of 2.7 g/cm^3

Question 38

What is continental drift?

Answer 38

The movement of tectonic plates, due to varying weights of crust. It was originally thought that convection currents caused the movement of the plates, but now slab pull is thought of as the primary driving force.

Question 39

What are convection currents in terms of tectonics?

Answer 39

The circulation of magma within the mantle (asthenosphere). Magma is heated by radioactive processes in the core and cools at the surface, and so circulates between the two places.

Question 40

What is the epicentre of an earthquake?

Answer 40

The point on the surface, directly above the earthquake’s origin.

Question 40

What is the hypocentre of an earthquake?

Answer 40

The place in the crust where the pressure/seismic energy is released - it is the point of origin of the an earthquake within the crust.

Question 41

What is a hot spot?

Answer 41

Volcanoes found away from the plate boundary, due to a magma plume closer to the surface.

Question 41

What are Love Waves?

Answer 41

Surface earthquake waves with horizontal displacement. They are the last to arrive as they are the slowest, but cause a lot of damage due to wave energy being focused on the surface.

Question 41

What is a mid-ocean ridge?

Answer 41

A constructive plate boundary where plates are pushed apart as new oceanic crust is formed on the sea bed.

Question 42

What is oceanic crust?

Answer 42

Crust that forms at the bottom of oceans. It has a lower silica content than continental crust, making it have a higher average density of 3.0 g/cm^3. On average, it is 7 km thick.

Question 43

What does partial melting mean?

Answer 43

Partial melting occurs due to different minerals in rocks having different melting points, and so certain minerals within a rock will begin to melt first once they have reached their melting point. This leads to a partially melted rock.

Question 44

What is a pyroclastic flow?

Answer 44

A mixture of gases and rock fragments, at high temperatures travelling at rapid speeds. They mostly form at explosive volcanoes.

Question 45

What is slab pull?

Answer 45

Occurs at a convergent plate boundary (subduction zone). Oceanic crust subducts due to being denser than continental crust, and as the subducting slab descends, it pulls remaining oceanic crust down along due to the weight and gravity acting on the subducting slab.

Question 46

What is subduction?

Answer 46

It is a process that occurs due to density differences in crust, usually when oceanic crust is forced below continental crust due to oceanic crust being denser than the continental plate. However, at certain settings, older (denser) oceanic crust can subduct underneath younger (less dense) oceanic crust.

Question 47

What is a tsunami?

Answer 47

It involves a vertical displacement of water (caused by a submarine earthquake or landslide) which creates waves, with large destructive power.

Question 48

What is a volcanic island arc?

Answer 48

A series of volcanoes (often in the shape of an arc) that are formed consecutively, as a tectonic plate moves across a stationary magma plume (e.g. Hawaii)

Question 49

What is the Wadati-Benioff zone?

Answer 49

A region of the subducting plate, most affected by pressure and friction, where most destructive margin earthquakes originate.