Plate Boundaries

Question 1

What are the different types of plate boundary?

Answer 1

Divergent/Constructive
Convergent/Destructive
Conservative/Transform

Question 2

What kinds of crust can be at a divergent plate boundary?

Answer 2

oceanic-oceanic
continental-continental

Question 3

What kinds of crust can be at a convergent plate boundary?

Answer 3

oceanic-continental
continental-continental
oceanic-oceanic

Question 4

What are the two largest tectonic plates?

Answer 4

N. American plate
Eurasian plate

Question 5

How many major tectonic plates are there?

Answer 5

7

Question 6

What is the rate of spreading of a divergent plate boundary in
a) Atlantic
b) Pacific
c) Iceland

Answer 6

a) 1-3cm/year
b) 6-10cm/year
c) 2cm/year

Question 7

What causes the creation of oceanic crust?

Answer 7

fractures in the crust allowing magma to rise to the surface and cool

Question 8

How long is the Mid-Atlantic ridge?

Answer 8

15,000km

Question 9

What is rifting?

Answer 9

The process by which land thins and drops (sinks) due to diverging plates

Question 10

What are the features composing the oceanic crust/divergent plate boundaries?

Answer 10

Pillow lavas
Vertical sheeted dykes
Gabbro

(top down)

Question 11

What are the characteristics of pillow lavas?

Answer 11

form where magma is pushed out into seawater
- extrusive
- fine grained
- cooled fast

Question 12

What are the characteristics of vertical sheeted dykes?

Answer 12

intrude into cracks resulting from spreading
- intrusive
- medium grained
- cooled slowish/medium speed

Question 13

What are the characteristics of gabbro?

Answer 13

formed when magma cools more slowly in the lower part of the new crust
- intrusive
- coarse grained
- cooled slow

Question 14

What is a divergent plate boundary?

Answer 14

spreading boundaries where new oceanic crust is created from magma derived from partial melting of the mantle caused by decompression as hot mantle rock from depth is moved towards the surface

Question 15

How thick is the oceanic crust produced by divergence? Why?

Answer 15

6km

60km triangular zone of partial melting near the ridge crest, 10% volume is magma

Question 16

Where are most divergent plate boundaries located?
What type of crust is created?

Answer 16

ocean ridges

oceanic crust (mafic igneous rock e.g. basalt/gabbro)

Question 17

What factors influence divergence?

Answer 17

Convection currents
Heat

Question 18

How does
a) convection currents
b) heat
influence divergence?

Answer 18

a) causes movement of plates along with slab push/ridge pull

b) partial melting and expansion contribute to the expansion of the crust forming the ridge
- magma can emerge out = further growth of oceanic crust

Question 19

What happens at diverging continental plates?

Answer 19

rifting may occur
- it is likely that as many as 20 mantle plumes (many still exist) were responsible for the initiation of the riftinf of Pangea along what is now the Mid-Atlantic Ridge

1. spreading starts within a continental area with up-warping or doming related to an underlying mantle plume/series of mantle plumes
2. the buoyancy of the mantle plume creates a dome within the crust causing it to fracture in a radial pattern with 3 arms spaced approx. 120 degrees apart
3. when a series of plumes exist underneath a large continent the resulting rifts may align and lead of formation of a rift valley e.g. Great Rift Valley in Eastern Africa
4. this valley may develop into a linear sea e.g. Red Sea and eventually into an ocean e.g. Atlantic

Question 20

What happens at an oceanic-oceanic convergence?

Answer 20

1. usually the older, colder and denser plate is subducted beneath the younger, hotter plate, commonly with an ocean trench along the boundary
2. the subducted plate heats and releases water which mixes with the overlying mantle, lowering the melting point and forming magma (FLUX MELTING)
3. magma is lighter than surrounding mantle so rises through it and overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands called an island arc
4. mature arc develops into chain of relatively large islands as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands e.g. Japan, Indonesia

Question 21

Give an example of oceanic-oceanic convergence

Answer 21

subduction of Indian plate south of Indonesia under the Eurasian plate

Question 22

Where do Earthquakes occur in oceanic-oceanic convergence?
Where do the largest ones occur?

Answer 22

Benioff zone
- close to the boundary between the subducting crust and overriding crust
- largest ones occur near the surface where the subducting plate is still cold and strong

Question 23

What happens at oceanic-continental convergence?

Answer 23

1. oceanic plate pushed beneath continental plate
2. sediment that has accumulated on the continental slope is thrust up into an accretionary wedge and compression leads to thrusting within the continental plate
3. mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock
4. resulting magma ascends through the crust, producing a mountain chain with many volcanoes

Question 24

Give an example of oceanic-continental convergence

Answer 24

subduction of the Nazca plate under the South American plate, which has created the Andes

Question 25

What happens at a continental-continental convergence?

Answer 25

1. continent/large island that has been moved along with subducting oceanic crustcollides with another continent 2. colliding continental material will not be subducted because it is too light (composed largely of light, silica-rich rocks) but the root of the oceanic plate will eventualy break off and sink into the mantle 3. large amount of deformation of pre-existing continental rocks and creation of mountain from that rock, from any sediment that had accumulated along the shores of both mases and commonly also from some ocean crust and upper mantle material

Question 26

Give an example of continental-continental convergence

Answer 26

India plate with the Eurasian plate creating the Himalayas