Tectonic hazards and processes - 1 Locations at risk from tectonic hazards

Question 1

What can tectonic hazards be classified as?

Answer 1

Tectonic hazards can be classified as either seismic or volcanic.

Question 2

What are seismic hazards?

Answer 2

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

Question 3

What are volcanic hazards?

Answer 3

Associated with eruption events.

Question 4

What percentage of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean?

Answer 4

About 70 per cent of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean.

Question 5

Which boundaries are the most powerful earthquakes associated with?

Answer 5

The most powerful earthquakes are associated with convergent or conservative boundaries, although rare intra-plate earthquakes can occur.

Question 6

What are intra-plate earthquakes?

Answer 6

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

Question 7

What does the distribution of earthquakes reveal?

Answer 7

The distribution of earthquakes reveals the following pattern of tectonic activity:

• The oceanic fracture zone (OFZ) - a belt of activity through the oceans along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California.
• The continental fracture zone (CFZ) - 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.
• Scattered earthquakes in continental interiors. A small minority of earthquakes can also occur along old fault lines and the hazard is associated with the reactivation of this weakness, for example, the Church Stretton Fault in Shropshire.

Question 8

When are earthquakes a common hazard?

Answer 8

Earthquakes are a common hazard and can develop into a major disaster, especially when they are both high magnitude and occur in a densely populated area.

Question 9

What is the violence of a volcanic eruption determined by?

Answer 9

The violence of a volcanic eruption is determined by the amount of dissolved gases in the magma and how easily the gases can escape.

Question 10

How many active volcanoes are there throughout the world?

Answer 10

There are about 500 active volcanoes throughout the world and, on average, around 50 of them erupt each year.

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 are the three types of plate boundary?

Answer 12

• Divergent
• Convergent
• Conservative

Question 13

What happens at divergent (constructive) margins?

Answer 13

Divergent (constructive) margins, 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 14

What happens at convergent margins?

Answer 14

Convergent (where plates move together): these are actively deforming collision locations with plate material melting in the mantle, causing frequent earthquakes and volcanoes.

Question 15

What happens at conservative (oblique-slip, sliding or transform) margins?

Answer 15

Conservative (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 plate margins do not result in volcanic activity, there are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude.

Question 16

d

Answer 16

d

Question 17

d

Answer 17

d

Question 18

d

Answer 18

d

Question 19

What type of earthquakes are seen at divergent ‘spreading ridges’?

Answer 19

The earthquakes seen at these boundaries tend to be frequent, small and typically a low hazard risk because of their geographical position (that is, the ocean) and they do not typically trigger tsunamis.

Question 20

What is happening along the San Andreas Fault in California?

Answer 20

The Pacific Plate (moving north) creates a zone of friction against the North American Plate (moving north at a different speed).

Question 21

What is the name of the plate boundary that generates some of the largest and most damaging earthquakes?

Answer 21

The plate boundaries that generate some of the largest and most damaging earthquakes are those where two plates are moving towards each other (convergent).

Question 22

What are active subduction zones characterised by?

Answer 22

Active subduction zones are characterised by magmatic activity, a mountain belt with thick continental crust, a narrow continental shelf and active seismicity.

Question 23

Where are passive continental margins found?

Answer 23

Passive continental margins are found along the remaining coastlines. Because there is no collision or subduction taking place, tectonic activity is minimal here.

Question 24

What is plate tectonics?

Answer 24

A theory developed more than 60 years ago to explain the large-scale movements of the lithosphere (the outermost layer of the Earth). It was based around the evidence from seafloor spreading and ocean topography, marine magnetic anomalies, palaeomagnetism and geomagnetic field reversals. Knowledge of Earth’s interior and outer structure is essential for understanding plate tectonics.

Question 25

What are the names of the different tectonic settings where volcanoes are found?

Answer 25

1 Destructive
2 Divergent
3 Hotspot volcanoes

Question 26

When does a destructive plate boundary occur?

Answer 26

These occur at locations where two plates are moving together.

Question 27

What is formed when two plates at a destructive plate boundary are moving together?

Answer 27

Here they form either a subduction zone or a continental collision, depending on the type of plates.

Question 28

What happens at destructive plate boundaries?

Answer 28

When a dense oceanic plate collides with a less-dense continental plate, the oceanic plate is typically thrust underneath because of the greater buoyancy of the continental lithosphere, forming a subduction zone. Surface volcanism (volcanoes at the ocean floor or the Earth’s surface) typically appears above the magma that forms directly above down-thrust plates. During collisions between two continental plates, however, large mountain ranges are formed, such as the Himalayas.

Question 29

What is the lithosphere?

Answer 29

The surface layer of the Earth is a rigid outer shell composed of the crust and upper mantle. It is on average 100 km deep. The lithosphere is always moving, but very slowly, fuelled by rising heat from the mantle which creates convection currents. The distinction between lithosphere and asthenosphere is one of physical strength rather than a difference in physical composition. The lithosphere is broken into huge sections, which are the tectonic plates.

Question 30

What type of volcanic eruption occurs at destructive plate boundaries?

Answer 30

These volcanic eruptions tend to be more infrequent but more destructive.

Question 31

What do divergent boundaries create?

Answer 31

Divergent boundaries create rift volcanoes where plates diverge from one another at the site of a thermally buoyant mid-ocean ridge.

Question 32

What type of volcanic eruption occurs at destructive plate boundaries?

Answer 32

These are generally less explosive and more effusive, especially when they occur under deep water in the ocean floor, for example the Mid-Atlantic Ridge.

Question 33

Where are hotspot volcanoes found?

Answer 33

Hotspot volcanoes are found in the middle of tectonic plates and are thought to be fed by underlying mantle plumes that are unusually hot compared with the surrounding mantle.

Question 34

What is another word for the presence of a hotspot?

Answer 34

Anomalous volcanism

Question 35

What is a volcanic hotspot?

Answer 35

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.

Question 36

What happens as the tectonic plate moves over the stationary hotspot?

Answer 36

As the tectonic plate moves over the stationary hotspot, the volcanos are rafted away and new ones form in their places. As oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands, atolls and seamounts.

Question 37

What happens as oceanic volcanoes move away from the hotspot?

Answer 37

As oceanic volcanoes move away from the hotspot, they cool and subside, producing older islands, atolls and seamounts. Over long periods of time, this can also create chains of volcanoes, such as the Hawaiian islands.

Question 38

What are the two different types of crust?

Answer 38

Thin oceanic crust

Thicker continental crust

Question 39

What types of rock make up the thin oceanic crust?

Answer 39

Thin oceanic crust, which underlies the ocean basins, is composed primarily of basalt

Question 40

What types of rock make up the thicker continental crust?

Answer 40

Thicker continental crust, which underlies the continents, is composed primarily of granite.

Question 41

What does the low density of the thick continental crust allow?

Answer 41

The low density of the thick continental crust allows it to ‘float’ high on the much higher density mantle below.

Question 42

What does it mean by the fact that the Earth’s mantle has a geothermal gradient?

Answer 42

The highest temperatures occur where the mantle material is in contact with the heat-producing core so there is a steady increase of temperature with depth. Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot and plastic (but not molten). Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes. However, rocks in the lower mantle are plastic and flow when subjected to forces instead of breaking.

Question 43

What is the difference in characteristics between rocks in the upper mantle compared to rocks in the lower mantle?

Answer 43

Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle and hot and plastic (but not molten). Rocks in the upper mantle are brittle enough to break under stress and produce earthquakes. However, rocks in the lower mantle are plastic and flow when subjected to forces instead of breaking.

Question 44

Where does sea floor spreading occur?

Answer 44

Sea floor spreading occurs at divergent 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 45

What can palaeomagnetism be used to date?

Answer 45

A technique involving the reconstruction of paleomagnetic reversals (called palaeomagnetism) can be used to date the age of new tectonic crust.

Question 46

What is palaeomagnetism?

Answer 46

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

Question 47

What is the Benioff Zone?

Answer 47

The Benioff Zone is 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. This theoretical framework is, therefore, an imp