Tectonic Plate Boundaries

Question 1

What are Tectonic Plates?

Answer 1

Upper, rigid parts of the Earth: lithosphere (heat transfer is by conduction)
Plates drift/ride on moving rock: asthenosphere (heat transfer is by convection)

Question 2

How does earth keep balanced (stay the same size)?

Answer 2

some plates are destroyed

others are formed/enlarged

Question 3

What are the three types of plate boundary?

Answer 3

divergent (oceanic ridge)
transform (transform fault)
convergent (trench, subduction zone)

Question 4

What happens at divergent plate boundaries?

Answer 4

Fractures in lithosphere, plates move apart (extension)

magma rises, forming new crust

Question 5

What are some examples of divergent plate boundaries?

Answer 5

MORs

continental rifts e.g. East African rift

Question 6

What are the characteristics of divergent plate boundaries?

Answer 6

High heat flow
Rift valleys
Active mafic (basaltic) volcanism: fissure eruptions form ocean crust
Shallow, small earthquakes

Question 7

What are some examples of convergent plate boundaries?

Answer 7

ocean-ocean subduction
ocean-continent subduction
continent-continent collision

Question 8

At divergent boundaries, plates move away from each other - so why do topographic ridges form?

Answer 8

material is hottest at the ridge

rocks expand when hot -> less dense -> more buoyant

Question 9

Why do divergent plate boundaries host only small, shallow earthquakes?

Answer 9

crust cools by conduction from ocean, only surface part is brittle enough to fracture in an earthquake

Question 10

at a convergent plate boundary, which plate subducts? why?

Answer 10

denser plate subducted

older plate subducted in the case of two oceanic plates

Question 11

What are the characteristics of subduction zones?

Answer 11

(1) Deep sea trench: depression in seafloor where oceanic plate bends and subduct e.g. Mariana Trench, 11 km deep (ocean-ocean subduction)
(2) Volcanic Arc: arc-shaped chain of volcanic islands (or continental volcanoes) e.g. cascade volcanic arc, Mt St Helens

Question 12

What are the characteristics of convergent boundary earthquakes?

Answer 12

shallow to deep (0-660km deep)

small to very large (up to mag.9‐9.5 at subduction zones)

Question 13

What happens at continental collision zones?

Answer 13

mountain building (orogeny) e.g. Mount Everest 8,848m

Question 14

What are the “steps” that offset mid-ocean ridges?

Answer 14

Transform Boundaries

Question 15

What happens at transform boundaries? What are the characteristics of transform boundaries?

Answer 15

Plates slide past each other

Shallow earthquakes, may be large (M8 but not M9)
Major vertical fractures and scarps
Features are offset horizontally
Usually no volcanic activity – no lithosphere created/destroyed

e.g. San Andreas fault

Question 16

How can we measure plate motion?

Answer 16

(direction, speed relative to each other)
DIRECTION:
MOR orientation - perpendicular to plate motion
Transform fault orientation - parallel to plate motion

SPEED:
Mag. stripe width, age

Question 17

Can we directly measure plate motion?

Answer 17

Relatively speaking..
GPS satellites (& antennas) -> DIRECTION and SPEED

Question 18

How do plates move relative to the Earth’s interior?(“absolute” motion)

Answer 18

Plumes of hot mantle material rise from great depth and cause volcanism at the surface (e.g., Hawaii)

Hot spots (mantle plumes): remain stationary as plates drift over them and leave a track of volcanic islands showing how the plate has moved

Question 19

What is the distance between the youngest and the oldest Hawaiian islands?

Answer 19

around 500km

Question 20

What is the difference in age between the youngest and the oldest Hawaiian islands?

Answer 20

around 5 million years

Question 21

How fast is the Pacific Plate moving (cm/yr)?What direction?

Answer 21

Big Island -> Kauai: ~500 km
Age difference of volcanoes: ~5 million yrs

plate is moving north west at around 10cm/yr

Question 22

Which plates are moving faster?

Answer 22

Pacific, India-Australia, Philippine Sea plates

Question 23

Which plates are moving slower?

Answer 23

Antarctic, Africa, N. America, S. America

Question 24

What are the driving mechanisms for plate motion?

Answer 24

Convection in Earth’s interior -> Mantle drag

Gravity; Physical properties of plates (cooling, density change)

• > Slab pull
• > Ridge push