Week nine Flashcards
How Does Continental Drift Relate to Plate Tectonics?
Wegener saw the continents as a jigsaw puzzle that fit together
into a prior single supercontinent, Pangea, surrounded by the sea
Panthalassa.
Earth is made up of 3 main layers
-Core
- Mantle
- Crust
The Earth’s crust
Continental Crust
- thick (10-70km)
- buoyant (less dense
than oceanic crust)
- mostly old
Oceanic Crust
- thin (~7 km)
- dense (sinks under
continental crust)
- young
Plate Tectonics
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
What are tectonic plates made of?
Plates are made of rigid lithosphere
What lies beneath the tectonic plates?
The asthenosphere
Plate movement
Plates of the lithosphere are moved around by the underlying hot mantle convection cells
Plate tectonic theory
– 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.
3 types of plate boundary (q card for visual)
- Divergent <–>
- Convergent
- Transform
Divergent Plate boundaries: Mid-Ocean Ridges
Shows the creation of new oceanic lithosphere from upwelling mafic magmas as well as a widening of the ocean basin
Convergent Boundaries (types)
There are three types of convergent plate boundaries
– Continent-oceanic crust collision
– Ocean-ocean collision
– Continent-continent collision
Oceanic-continental boundary
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.
Oceanic-continental boundary
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.
Subduction Zones
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
Cascadia Subduction Zone (CSZ)
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.
Oceanic-oceanic boudary
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)
What is the evidence that subduction occurs at convergent plate boundaries?
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
Volcanoes are formed by
Subduction
Rifting
Hotspots
Volcanoes are formed by
Subduction
Rifting
Hotspots
What does the mantle-plume hypothesis explain that plate tectonics cannot explain? Hint : hots form where
- Hots spots form where narrow columns (plumes) of unusually hot mantle convectively rise from the core-mantle boundary
- Plume locations are stationary in the mantle
What does the mantle-plume hypothesis explain that plate tectonics cannot explain? hint:hot spots
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
1 How to classify mass movements?
Distinguishing the materials in motion
– 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
Distinguishing the speed of motion
- slow (creep)
- fast (avalanche)
Three factors make up the criteria for the classification scheme or mass movements
- Nature of mixture of solids (rock, debris, or earth materials)
- Type of motion (fall, slide, or flow)
- Velocity of motion (avalanche, flow or creep)
How to classify mass movements?
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.
Talus Slopes
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
Planar slides
- 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.
Flows
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.
Slow flows - creep
Are detected only by dislocation or bending of features at the surface like these sedimentary layers and trees.
2 What forces affect slope stability?
Experiments to Find Out
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.
Gravity force
For motion to happen, the gravity force parallel to the surface (called the driving force) must be greater than the resisting strength.
Friction
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.
Cohesion
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.
What factors determine slope stability?
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
The role of vegetation
Vegetation is a very important factor in slope stability. Roots penetrate and bind together regolith and absorb water from precipitation.
Thickness of Regolith
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.
Arid climate
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
