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

Question 1

Where are the main earthquake zones found?

Answer 1

(Often in clusters) along plate boundaries.

Question 2

Where are about 70% of all earthquakes found?

Answer 2

The ‘Ring of Fire’ in the Pacific Ocean.

Question 3

What are the most powerful earthquakes associated with?

Answer 3

Convergent or conservative boundaries, although rare intra-plate earthquakes can occur.

Question 4

What are intra-plate earthquakes?

Answer 4

These occur in the middle or interior of tectonic plates and are much rarer than boundary earthquakes.

Question 5

What is the continental fracture zone (CFZ)?

Answer 5

A belt of activity following the mountain ranges from Spain, via the Alps, to the Middle East, the Himalayas to the East Indies and then circumscribing the Pacific.

Question 6

Where can a small minority of earthquakes occur?

Answer 6

Along old fault lines and the hazard is associated with the reactivation of this weakness, for example the Church Stretton Fault in Shropshire.

Question 7

What type of hazards are earthquakes and what can they cause?

Answer 7

Primary hazards (ground movement and ground shaking) but also cause secondary hazards such as landslides and tsunamis.

Question 8

What is the violence of a volcanic eruption dependant on?

Answer 8

The amount of dissolved gases in the magma and how easily the gases can escape.

Question 9

How many active volcanoes are there in the world and, on average, how many erupt each year?

Answer 9

About 500

Around 50 of them erupt each year

Question 10

What are seismic hazards?

Answer 10

Generated when rocks within 700km of the Earth’s surface come under such stress that they break and become displaced.

Question 11

What is a volcano?

Answer 11

A landform that develops around a weakness in the Earth’s crust from which molten magma, volcanic rock, and gases are ejected or extruded.

Question 12

What is a divergent (constructive) plate boundary?

Answer 12

Most clearly displayed at mid-ocean ridges. At these locations there are large numbers of shallow focus and generally low magnitude earthquake events. Most are submarine (under the sea).

Question 13

What is a convergent plate boundary?

Answer 13

Where plates move together. These are actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.

Question 14

What is a conservative plate boundary?

Answer 14

(oblique-slip, sliding or transform) margins, where one plate slides against another. Here, the relative movement is horizontal and classified as either sinistral (to the left) or dextral (to the right). Lithosphere is neither created nor subducted, and while conservative margins do not result in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude.

Question 15

What are the three ways plates can move?

Answer 15

Pull away from each other
Slide past each other
Crunch into each other

Question 16

What happens when plates move away from each other and where does this happen?

Answer 16

The places where they move away from each other are the divergent ‘spreading ridges’ in the oceans. New oceanic crust, which is thinner and denser than the continental crust, is created. The earthquakes seen at these boundaries tend to be frequent, small and typically a low hazard risk because of their geographical position (the ocean) and they don’t usually trigger tsunamis.

Question 17

Name a location where plates are sliding past each other and what is happening there.

Answer 17

The San Andreas Fault in California, where the Pacific Plate (moving north) creates a zone of friction against the North American Plate (moving north at a different speed). This presents more risk.

Question 18

Describe what happens when plates move towards each other (convergent).

Answer 18

These are the plate boundaries that generate some of the largest and most damaging earthquakes. Typically when this happens, one plate starts sliding under the other. As the strain builds up over time in the subduction zone, the friction between the two masses of rock is overcome, releasing energy. This will produce both earquakes - such as the tsunami generating ones off Japan in 2011 and Aceh in Indonesia in 2004 - and volcanoes, the magma of which are fed by the melting of the subducting plate. The subduction zones at the edge of the Pacific Plate are the reason for the Ring of Fire that is a feature of this ocean.

Question 19

What is the distribution of volcanoes controlled by?

Answer 19

The global geometry of plate tectonics.

Question 20

What is a destructive plate boundary?

Answer 20

Occur at locations where two plates are moving together. Here they form either a subduction zone or a continental collision, depending on the type of plates. When a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoancy of the continental lithosphere, forming a subduction zone. Large mountain ranges are formed, such as the Himalayers. These volcanic eruptions tend to be less frequent but more destructive.

Question 21

What is a divergent plate boundary?

Answer 21

Create rift volcanoes where plates diverge from one another at the site of a thermally buoyant mid-ocean ridge. These are generally less explosive and more effusive, especially when they occur under water deep in the ocean floor, for example the Mid-Atlantic Ridge. Here there is basaltic magma, which has low viscosity.

Question 22

What are hotspot volcanoes?

Answer 22

Found in the middle of tectonic plates and are thought to be fed by underlying mantle plumes that are unusuakky hot compared with the surrounding mantle. The presence of a hotspot is inferred by anomolous volcanism (not at a plate boundary), such as the Hawaiin volcanoes within the Pacific Plate. A volcanic hotspot is an area in the mantle from which heat rises as a hot thermal plume from deep in the Earth. High heat and lower pressure at the base of the lithosphere enable melting of the rock. This molten material, magma, rises through cracks and erupts to form active volcanoes on the Earth’s surface. As the tectonic plate moves over the stationary hotspot, the volcanoes are rafted away and new ones form in their place. As oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands, atolls and seamounts.

Question 23

What are the two different types of crust?

Answer 23

Thin oceanic crust, which underlines the ocean basins, is composed primarily of basalt.
Thicker continental crust, which underlines the continents, is composed primarily of granite. Its low density allows it to ‘float’ high on the much higher density mantle below.

Question 24

What does the Earth’s mantle have?

Answer 24

Temperature gradient (geothermal gradient).

Question 25

Where do the highest temperatures occur in the mantle?

Answer 25

Where the mantle material is in contact wit the heat-producing core so there is a steady increase of temperature with depth.

Question 26

Describe the rocks in the upper mantle.

Answer 26

Cool and brittle. Brittle enough to break under stress and produce earthquakes.

Question 27

Describe the rocks in the lower mantle.

Answer 27

Hot and plastic (but not molten). Flow when subjected to forces instead of breaking.

Question 28

How do convection currents move tectonic plates?

Answer 28

Heat which is derived from the Earth’s core (radioactive decay) rises within the mantle to drive convection currents, which in turn move the tectonic plates. These convection currents operate as cells.

Question 29

How does sea floor spreading occur?

Answer 29

It occurs at diveregent boundaries under the oceans. This is a continuous input of magma forming a mid-ocean ridge, for example the Mid-Atlantic Ridge. On land a rift valley forms.

Question 30

What is paleomagnetism?

Answer 30

Paleomagnetism results from the zone of magma ‘locking in’ or ‘striking’ the Earth’s magnetic polarity when it cools. Scientists can this tool to etermine historic periods of tectonic activity throught reconstruction of relative plate motions. They create a geo-timeline.

Question 31

What happens during tectonic plate movement?

Answer 31

There is likely to be a combined force of convection and gravity driving tectonic plate movement. Gravity in particular causes the denser oceanic crust to be pulled down at the site of subduction. At constructive margins (ie spreading ridges), magma is simply ‘gap filling’, rather than the main driver pushing the plates in opposite directions away from each other.

Question 32

What is the Benioff Zone?

Answer 32

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. This theoretical framework is therefore an important factor in determining earthquake magnitude, since it determines the position and depth of the hypocentre.

Question 33

What are subduction zones?

Answer 33

Broad areas where two plates are moving together, often with the thinner, more dense oceanic plate descending beneath a continental plate. The contact between the plates is sometimes called a thrust or megathrust fault.

Question 34

What is a locked fault?

Answer 34

A fault that is notslipping because the frictional resistance on the fault is greater than the sheer stress across the fault, that is, it is stuck. Such faults may store strain for etended periods that is eventually released in a large magnitude earthquake when the frictional resistance is eventually overcome.

Question 35

Give an example of a hazard that was the result of a megathrust locked fault.

Answer 35

The 2004 Indian Ocean tsunami was the result of a megathrust locked fault (subducting Indian Plate) with strain building up at around 20mm per year. It generated huge seismic waves and the devestating tsunami.