Hazardous Earth

Question 1

What is the definition of a hazard?

Answer 1

A threat (natural or human) that has the potential to cause loss of life, injury, property damage, socio-economic disruption or environmental degradation.

Question 2

What is the structure of the Earth?

Answer 2

• Crust
• Mantle
• Core

or

• Lithosphere
• Asthenosphere
• Outer core
• Inner core

Question 3

What is the lithosphere?

Answer 3

A cool, brittle outer shell, comprising crust and uppermost, rigid part of mantle. Broken into lithospheric plates (tectonic plates).

Question 4

What is the asthenosphere?

Answer 4

Solid mantle rock, but hotter and under greater pressure so more ductile (can deform and flow). Plays an important role in plate tectonics, allowing the rigid plates to slide over it.

Question 5

What is mantle convention?

Answer 5

• A mechanism by which the Earth cools over time through the formation and movement of plates at the surface.
• Previously thought that plates were moved by convection currents in the mantle, however geologists now believe plate movement and mantle convection are mainly driven by cooling from above.

Question 6

What is the role of convection currents?

Answer 6

• At subduction zones, slabs of cold oceanic lithosphere become dense enough to sink into the mantle (downwelling).
• This causes the lithosphere to be stretched and thinned elsewhere, reducing pressure on the underlying mantle rock and causing it to upwell in response.
• Some upwelling is thought to occur in thin column-like mantle plumes rising from the core-mantle boundary.

Question 7

What is contintental drift?

Answer 7

Theory that rather than being fixed as geologists thought, continents were actually moving, and were previously joined into a single super-continent.

Question 8

What is the evidence for continental drift?

Answer 8

• Jigsaw fit
• Geological fit
• Fossil records
• Ancient glaciations

Question 9

What is sea-floor spreading?

Answer 9

The idea that the ocean floor is continuously recycled, sinking into the mantle in some areas and replaced by new lithosphere elsware.

Question 10

What is the evidence for sea-floor spreading?

Answer 10

Paleomagnetism:
- Iron-rich minerals in lava align with Earth’s magnetic field, acting like tiny compasses frozen in time once the lava hardens.
- Shows that every few hundred thousand years, Earth’s magnetic field reverses its polarity.
- Studies found that new ocean floor is produced by volcanic activity at MORs, then spreads away on both sides as plates diverge.
Age of Sea-floor Rocks
- Gradually increases with distance each side of an MOR

The modern theory, building on continental drift and sea-floor spreading, is called plate tectonics.

Question 11

What are the different types of plate boundary?

Answer 11

• Convergent (towards each other)
• Conservative (alongside)
• Divergent (away from each other)

Question 12

How does the composition of the magma affect volcanic landscapes?

Answer 12

• Those with a silicic composition are much more viscous than mafic (low silica) magmas

Question 13

What is different about explosive and effusive eruptions?

Answer 13

Explosive: Long periods with no activity, steep-sided stratovolcanoes, usually convergent, lava, gases, dust, ash, lava bombs

Effusive: Regular eruptions, can last for many months or years, shield volcanoes with gentle slopes, lava, gases

Question 14

Why do eruptions not at plate boundaries occur?

Answer 14

• Mantle plumes reach base of lithosphere and cause partial melting (hot spot). Plume remains stationary whilst plate moves over it, leading to a chain of volcanoes which become extinct when transported away.

Question 15

What is the VEI index

Answer 15

Volcanic explosivity index - measures the magnitude of volcanic eruptions, allowing them to be compared. Each increase in value represents a 10x increase in explosivity.