Week nine

Question 1

How Does Continental Drift Relate to Plate Tectonics?

Answer 1

Wegener saw the continents as a jigsaw puzzle that fit together
into a prior single supercontinent, Pangea, surrounded by the sea
Panthalassa.

Question 2

Earth is made up of 3 main layers

Answer 2

-Core
- Mantle
- Crust

Question 3

The Earth’s crust

Answer 3

Continental Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old

Oceanic Crust
- thin (~7 km)
- dense (sinks under
continental crust)
- young

Question 4

Plate Tectonics

Answer 4

Ø The Earth’s crust is divided into 12 major plates which
are moved in various directions.
Ø This plate motion causes them to collide, pull apart, or
scrape against each other.
Ø Each type of interaction causes a characteristic set of
Ø Earth structures or “tectonic” features.
Ø The word, tectonic, refers to the deformation of the crust
as a consequence of plate interaction

Question 5

What are tectonic plates made of?

Answer 5

Plates are made of rigid lithosphere

Question 6

What lies beneath the tectonic plates?

Answer 6

The asthenosphere

Question 7

Plate movement

Answer 7

Plates of the lithosphere are moved around by the underlying hot mantle convection cells

Question 8

Plate tectonic theory

Answer 8

– Strong lithospheric plates move atop the weaker, plastic
asthenosphere.

– Deformation occurs at or near the edges of plates where
they interact with other plates.

– The interiors of the plates are relatively undeformed.

Question 9

3 types of plate boundary (q card for visual)

Answer 9

• Divergent <–>
• Convergent
• Transform

Question 10

Divergent Plate boundaries: Mid-Ocean Ridges

Answer 10

Shows the creation of new oceanic lithosphere from upwelling mafic magmas as well as a widening of the ocean basin

Question 11

Convergent Boundaries (types)

Answer 11

There are three types of convergent plate boundaries
– Continent-oceanic crust collision
– Ocean-ocean collision
– Continent-continent collision

Question 12

Oceanic-continental boundary

Answer 12

Much of the western boundary of South America is this type of boundary. Denser oceanic lithosphere flexes under the less dense, much older, continental crust.

Question 12

Oceanic-continental boundary

Answer 12

Much of the western boundary of South America is this type of boundary. Denser oceanic lithosphere flexes under the less dense, much older, continental crust.

Question 13

Subduction Zones

Answer 13

Ø Oceanic lithosphere
subducts underneath the
continental lithosphere
Ø Oceanic lithosphere heats
and dehydrates as it
subsides
Ø The melt rises forming
volcanism
E.g. The Andes

Question 14

Cascadia Subduction Zone (CSZ)

Answer 14

Ø from northern Vancouver Island to northern California.
Ø It is a subduction zone fault that separates the Juan
de Fuca and North America plates.
Ø At depths shallower than 30 km, CSZ is locked by friction
Ø strain slowly builds up as the subduction forces act, until the fault’s frictional
strength is exceeded
Ø As the rocks slip past each other along the fault, a mega-thrust
earthquake can be caused.

Question 15

Oceanic-oceanic boudary

Answer 15

Ø When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
Ø The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.
Ø The worlds deepest parts of the ocean are found along trenches.
– E.g. The Mariana Trench is 11 km deep (east
pacific)

Question 16

What is the evidence that subduction occurs at convergent plate boundaries?

Answer 16

Earthquakes under the mountains continental side (edge of south American plate) of a subduction zone reveal the depth to the subducting Nazca plate when plotted

Question 17

Volcanoes are formed by

Answer 17

Subduction

Rifting

Hotspots

Question 18

Volcanoes are formed by

Answer 18

Subduction

Rifting

Hotspots

Question 19

What does the mantle-plume hypothesis explain that plate tectonics cannot explain? Hint : hots form where

Answer 19

1. Hots spots form where narrow columns (plumes) of unusually hot mantle convectively rise from the core-mantle boundary
2. Plume locations are stationary in the mantle

Question 20

What does the mantle-plume hypothesis explain that plate tectonics cannot explain? hint:hot spots

Answer 20

Hot spots leave tracks on moving plates. These trails of volcanic seamounts in the Pacific fit well in the fixed spot mantle-plume modek

Other data indicate that plumes move slowly, if at all

Question 21

1 How to classify mass movements?
Distinguishing the materials in motion

Answer 21

– Rocks
– “Debris” is coarse-grained; 20–80 percent >2 mm in size
– “Earth” is fine-grained; 80 percent or more are <2 mm in size

Question 22

Distinguishing the speed of motion

Answer 22

• slow (creep)
• fast (avalanche)

Question 23

Three factors make up the criteria for the classification scheme or mass movements

Answer 23

• Nature of mixture of solids (rock, debris, or earth materials)
• Type of motion (fall, slide, or flow)
• Velocity of motion (avalanche, flow or creep)

Question 24

How to classify mass movements?

Answer 24

Flow occurs on different scales. Creep is slow and only detected by dislocation or bending of features at the surface, whereas debris avalanches are very rapid flows of rock, regolith, vegetation, and/or sometimes ice.

Ø Rock falls are common where the rock is highly jointed and on a steep slope, such as New Hampshire’s “Old Man of the Mountain,” which collapsed in May 2003.

Question 25

Talus Slopes

Answer 25

The loose material that piles up at the base of steep slopes is called talus.
The lack of vegetation or soil on talus indicates that material is actively
Accumulating

Question 26

Planar slides

Answer 26

• move downslope in contact with a surface of rupture, typically along bedding planes, foliation, or joint planes oriented parallel to the slope.

– If the rupture surface curves, then the slide mass rotates as it moves downslope, causing rock layers and surface features to tilt. The curved rotational slide surfaces are scoop shaped, and they usually form in regolith or poorly consolidated or weak rock where bedding or joints do not influence failure. This is called a slump.

Question 27

Flows

Answer 27

Flows are the continuous movement of rock, regolith, or both that behave like a high-viscosity liquid. Figure 6.7a illustrates the “liquid” appearance of moving flows.

Question 28

Slow flows - creep

Answer 28

Are detected only by dislocation or bending of features at the surface like these sedimentary layers and trees.

Question 29

2 What forces affect slope stability?
Experiments to Find Out

Answer 29

Under the conditions below, a brick on a board will slide if tilted far enough.
What does this mean?

1) Increasing slope favors motion or slope instability. Next, we may sand the board and find it moves more easily.

2) Smoother surfaces favor motion and slope instability, and if we wet the board, it moves easier still.
3) The presence of water favors motion and slope instability.

This simple set of experiments suggests that slope, surface roughness, and water are
factors affecting the balance between driving and resisting forces.

Question 30

Gravity force

Answer 30

For motion to happen, the gravity force parallel to the surface (called the driving force) must be greater than the resisting strength.

Question 31

Friction

Answer 31

Friction is the force that opposes motion between two objects that are touching one another. The friction increases as the roughness of the surfaces increases. Friction also varies with slope angle because on lower slopes the weight of fragments pulls them down in stronger contact with underlying material.

Question 32

Cohesion

Answer 32

the other component of resisting strength, is the attraction of
particles at the atomic level. Materials such as loose sand and gravel have very little cohesion between particles, but clay particles, with charged surfaces, have high
cohesion.

Question 33

What factors determine slope stability?

Answer 33

Angle of repose: the maximum angle of a stable slope as determined by the friction, cohesion, and particle shape

eg: adding small amounts of water to sand initially increases cohesion between particles, but too much water leads to failure

Question 34

The role of vegetation

Answer 34

Vegetation is a very important factor in slope stability. Roots penetrate and bind together regolith and absorb water from precipitation.

Question 35

Thickness of Regolith

Answer 35

These figures illustrate the role of vegetation plus climate in determining the
thickness of regolith on hillslopes. Here in a temperate mid-latitude forest, we
have dense tree cover and thick regolith. As a result, slopes tend to be fairly
stable. Mass movement occurs, but generally it is slow-velocity types such as
creep, slumps, and flows.

Question 36

Arid climate

Answer 36

In arid regions with low vegetation abundance and slow rates of weathering, surface-water removes most regolith particles almost as quickly as they loosen from bedroom. Mass movements are typically rock falls and slides