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

Question 1

What is Crustal Fracturing?

Answer 1

Occurs when the earth’s crust causes rock to break and
fracture under stress and strain caused by seismic stresses

Question 2

What is the Benioff Zone?

Answer 2

A zone of seismicity where earthquakes are produced by the interaction of a down going oceanic crustal plate in a subduction zone.
The deepest earthquakes recorded occur in this zone.

Question 3

What is ‘Slab Pull’?

Answer 3

Slab Pull involves old oceanic crust being submerged into the
mantle. This pulling action drags the rest of the plate with it.
As a crustal plate moves further from an oceanic ridge, it cools and becomes increasingly dense. Eventually the dense oceanic slab of crust becomes heavier than the asthenosphere below it so it descends under its own weight. The weight of this sinking, cooling plate causes a major pulling action, which causes the rest of the plate to be pulled downwards as well.

This is thought to be a primary mechanism for plate movement.

Question 4

Name the primary hazards of volcanoes

Answer 4

• Lava flows
• Pyroclastic flows
• Tephra and ash flows
• Volcanic gases

Question 5

What are the characteristics of P (primary) waves?

Answer 5

• Fastest and first to reach the surface
• Travels through solids and liquids (travel forwards and backwards)
• Only damaging in the most powerful earthquakes

Question 6

True or false?
Earthquakes can occur in the middle of plates

Answer 6

True -
They are called ‘intra-plate’
The causes of this are not fully understood but it is assumed that plates have
pre-existing weaknesses which become reactivated, forming seismic waves

Question 7

What are the characteristics of L (love) waves?

Answer 7

The slowest and last waves to arrive
- Cause the most damage (ground shakes side to side)
- Larger and focus all energy on the surface

Question 8

What are the 3 main types of seismic waves?

Answer 8

• P - Primary (or pressure)
• S - Secondary (or shear)
• L waves - Love

Question 9

Identify secondary hazards of earthquakes

Answer 9

• Liquefaction
• Landslides
• Tsunamis

Question 10

What are the characteristics of S (secondary) waves?

Answer 10

• Slower than P waves (by 60%)
• Only travel through solids (move in a sideways motion)
• Do more damage than P waves

Question 11

Are Lahars and Jökulhlaups Primary or Secondary Hazards of volcanic events?

Answer 11

Secondary

Question 12

What are Lahars?

Answer 12

Lahars are a combination of rock, mud and water which travel quickly down volcanoes. These can occur when an eruption coincides with heavy rainfall.

Question 13

What are Jökulhlaups?

Answer 13

An Icelandic word - Snow and ice in glaciers melt after an eruption which causes sudden outburst floods which are dangerous .

Question 14

What is a natural hazard?

Answer 14

A perceived natural/geophysical event that has the potential to threaten both life and property.
With regards to earthquakes this is due to the movement of tectonic plates

Question 15

What is a natural disaster?

Answer 15

The realisation of a hazard, when it causes significant impact on a vulnerable population

Question 16

What is the lithosphere?

Answer 16

Ridged outer part of Earth (crust and upper mantle), broken up into a number of plates, about 100km thick

Question 17

What is the asthenosphere?

Answer 17

The part of the mantle, below the lithosphere, where rock is semi-molten (it is ductile - think like play dough!)

Question 18

How are volcanic hotspots created?

Answer 18

A mantle plume is an upwelling of abnormally hot rock within the Earth’s mantle causing a hot spot on the surface (the lithosphere melts and allows magma to escape to the surface). The plume stays where it is as the crust slowly migrates over it leading to the formation of island arcs.

Question 19

Can you explain the distribution of tectonic hazards?

Answer 19

An overview:
- Most hazards occur next to plate boundaries.
- Highest magnitude earthquakes appear to be on destructive and conservative boundaries.
- Concentration of Earthquakes around the Pacific (- Ring of Fire), Alpide belt and Mid-Atlantic Ridge.
- Most of the worlds active volcanoes are located around the Pacific Ring of Fire.
- Deepest earthquakes tend to occur at convergent zones where subduction is occurring (in Benioff Zone).
- Some hazards occur away from plate boundaries e.g. Hawaii (volcanic hotspots) (intra-plate)
- Low magnitude and shallow earthquakes occur near divergent boundaries.

Question 20

Theory of Plate Tectonics:
Who hypothesised continents had once formed one land mass named Pangaea, before breaking apart and drifting to their current location? He called his theory ‘Continental Drift’.

Answer 20

Alfred Wegener, in 1912

Question 21

Theory of Plate Tectonics:
What evidence did Alfred Wegener have?

Answer 21

1. Continents fit like puzzle pieces
2. Freshwater animal fossils on separate sides of the ocean
3. Plant fossil findings in Antarctica
4. Similar rock layers on separate continents
5. Glacial deposits in now tropical/desert areas

Question 22

Theory of Plate Tectonics:
In the 1960s Harry Hess proposed a new concept following on from Alfred Wegner.

What was his theory?

Answer 22

Seafloor spreading - a geologic process in which tectonic plates split apart from each other, creating ocean basins.

Question 23

Theory of Plate Tectonics:
What is seafloor spreading?

Answer 23

Basaltic magma from the mantle rises to create new ocean floor at mid-ocean ridges (MORs).
On each side of the ridge the ocean floor moves away from the ridge. As the oceans became wider the continents drifted apart.
Sea floor spreading provides the mechanism that allows the continents to move.

Question 24

What is Palaeomagnetism?

Answer 24

Resulting from some minerals cooling aligned with the magnetic polarity of planet earth.
The magnetic fields of Earth vary with time and sometimes flip completely. These changes can then be seen in the rocks as you move away from plate boundaries.

Question 25

What is the difference between magnitude and intensity?

Answer 25

Magnitude measures energy released at the source of the earthquake.
Determined from measurements on seismographs.
Intensity measures the strength of shaking produced by the earthquake/explosivity of a volcano at a certain location.
Determined from effects on people, human structures, and the natural environment.

Question 26

How can volcanoes be measured?

Answer 26

Via the Volcanic Explosivity Index (VEI).
It gives us a way to measure the relative explosiveness of volcanic eruptions. It measures how much volcanic material is ejected, the height of the material thrown into the atmosphere, and how long the eruptions last.
The scale is logarithmic, or based on 10; therefore, an increase of “1” on the scale indicates an eruption 10 times more powerful than the number before it.

Question 27

Have earthquake or volcanoes caused more deaths over recorded history?

Answer 27

Earthquakes (by quite a mile!)
Pyroclastic Flows account for 50% of this

Question 28

True or false? Volcanoes have an influence on the Earth’s climate?

Answer 28

True. Volcanoes can have a major impact on the Earth’s climate. Supervolcanoes have previously had a huge impact and will into the future.

Question 29

Which would create more risk: A shallow or deep Benioff zone? Why?

Answer 29

Shallow focus earthquakes. Deeper have more area to dissipate their energy.

Question 30

Give three factors that might cause a volcano to be more explosive (greater magnitude).

Answer 30

Gases, pressure, viscosity/type of magma

Question 31

Can you identify the layers in the structure of the Earth?

Answer 31

Question 32

There are three main types of plate boundary. Can you identify the name of this plate boundary?

Answer 32

Divergent

Question 33

There are three main types of plate boundary. Can you identify the name of this plate boundary?

Answer 33

Transform

Question 34

There are three main types of plate boundary. Can you identify the name of this plate boundary?

Answer 34

Convergent