Tectonic Processes and Hazards - Structure of the Earth and Plate Margins

Question 1

What is the core of the Earth?

Answer 1

Comprised of iron and nickel and is the size of Mars. Can reach 6200 degrees C

Question 2

What is the mantle?

Answer 2

Mainly silicate rocks, the top layer is solid and beneath is the asthenosphere which is liquid. Extends up to 2900km down and is 5000 degrees. High temperature generates convection currents.

Question 3

What is the crust?

Answer 3

Less than 100km thick. The oceanic crust is known as ‘sima’ and is 6-10km thick, 1200 C maximum. The continental crust is largely granite and up to 70km deep.

Question 4

What separates the crust from the mantle?

Answer 4

The Moho discontinuity.

Question 5

What is Earth’s surface made up of?

Answer 5

7 major rigid lithosphere plates.

Question 6

What is the traditional thinking on the mechanics of plate tectonics?

Answer 6

Rising limbs of convection bring more heat rom Earth’s core to the surface, spreading out either side of the centre ridge and carrying the plates with them. This theory rests on the theory that plates float on the asthenosphere and move with the lithosphere.

Question 7

What is the modern thinking on the mechanics of plate tectonics based on?

Answer 7

Tomography hasn’t identified convection in the asthenosphere as a driver of plate movement. The new theory is based on the fact that fresh magma doesn’t push plates apart but fills an existing gap.

Question 8

What is the modern thinking on the mechanics of plate tectonics?

Answer 8

Molten material wells up at diverging plate margins due to a thinning lithosphere, resulting in partial melting of the upper mantle. Lithosphere rises and forms an ocean ridge above the surrounding sea floor. Fresh rock formed at spreading centres is hot and more buoyant that rock further from the margin. Gravity acts on the older lithosphere so it slides away from the new ridge, so the lithosphere melts as well.

Question 9

What is ridge pull?

Answer 9

The passive process resulting from gravitational sliding.

Question 10

What is slab pull?

Answer 10

Occurs at subduction, where colder, denser portions of plates sink into the mantle and pull the rest of the plates with them. This is key for movement.

Question 11

What evidence supports slab pull?

Answer 11

It is supported by tomography as cold, dense slabs have been identified in Earth’s mantle.

Question 12

What are the 3 types of plate margin?

Answer 12

Typically movement is at 4cm/year.
Diverging - East African Rift Valley
Converging - South American
Conservatives - San Andreas Fault

Question 13

What is an oceanic-oceanic diverging plate margin driven by?

Answer 13

Slab pull, pressure from this leads to diverging of Earths surface and formation of a ridge, such as the Mid Atlantic Ridge.

Question 14

What are the details of the Mid Atlantic Ridge?

Answer 14

10,000km long, created 60 million years ago.
Moves at 2.5cm/year
Creates underwater volcanoes along the margin, which formed islands such as Iceland which is a plateau 200m above sea level, formed from a hotspot plume as as basaltic lava poured out.
Most activity is shallow and low magnitude and high frequency, the highest recorded was a 6.5 in June 2000

Question 15

What is a continental-continental diverging margin?

Answer 15

Where continental plates diverge, the crust features, forming parallel faults.
The East African Rift Valley has East Frida moving NE and North Africa moving North, the rift valley extends 4000km from Mozambique to the Red Sea

Question 16

What is an oceanic-oceanic converging plate margin?

Answer 16

Subduction occurs as one plate is slightly older, cooler or denser. This plate melts at approx 100km below. Melted material rises up through lines of weakness towards surface. Extrusive volcano activity forms a chain of volcanoes above the subduction zone - the Island Arc.

Question 17

What’s an example of an island arc?

Answer 17

The Mariana Islands formed this way with the Pacific Plate subjected to form the Mariana Trench.

Question 18

What is an oceanic-continental converging plate margins?

Answer 18

The oceanic plate is denser so it is subjected by slab pull. Continental plate is not subjected but is uplifted and buckled to form mountains. Rising magma through the split forms volcanoes.

Question 19

Whats an example of an oceanic-continental converging plate margin?

Answer 19

The Nazca/South American margin with the Andres 7000m above sea level. The Peru-Chile trench is 8000m deep at subduction.

Question 20

What is a collision margin?

Answer 20

As both continental plates are buoyant, no subduction occurs. Former ocean sediments are thrusted upward under intense compression, forming complex mountains. Usually no volcanic activity as no crust is destroyed. Earthquakes can occur - Nepal.

Question 21

What is an example of a collision margin?

Answer 21

The Himalayas, the Indo-Austrialian plate has been moving north 6cm/year into the Eurasian plate. Complex mountains has caused wedging on uplift.

Question 22

What is a conservative plate margin?

Answer 22

Where 2 plates move laterally past each other. At a collision margin there is no volcanic activity because no crust is being destroyed. Shallow earthquakes can occur, with a low magnitude as pressure along the margin is easily released.

Question 23

What is an example of a conservative plate margin?

Answer 23

San Andreas Fault - Pacific and North American plate meet.