Ch. 2 - Plate Tectonics

Class: GEOL-101

Notes:

A Scientific Revolution Unfolds

Plate Tectonics:

Plate tectonics is the first theory to provide a comprehensive view of the processes that produced Earth’s major surface features, including the continents and ocean basins.

2.1 From Continental Drift to Plate Tectonics

Summarize the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

Early Perspectives vs. Wegener's Hypothesis

Presentation content:

• Pre-1960s: continents and ocean basins were fixed in place.
• Early Challenge: Alfred Wegener (1915) proposed the continental drift hypothesis.
• Wegener’s Hypothesis:
  • All landmasses were once joined in a supercontinent called Pangaea (“all lands”).
  • Pangaea began breaking apart about 200 million years ago (in the Mesozoic era).
• Reception: Most geologists rejected his hypothesis at the time.
• Later shift: Advances in the 1960s led to the theory of plate tectonics, which explained continental movement and Earth’s major surface features.

Notes from the lecture:

• Alfred Wegener proposed that over geological time (millions of years) continents formed
  • Continents fit very nicely as a one big puzzle
  • He was right, but rejected at that time
    • He didn't have a mechanism to explain how the continents were moving
• Huge technological advances allowed us to understand the ocean and provide evidence
  • This was the mechanism that Wegener didn't have at that time

2.2 Continental Drift: An Idea Before Its Time

List and explain the evidence Wegener presented to support his continental drift hypothesis.

Continental Drift: Supporting Evidence

Presentation content:

• Continental Jigsaw Puzzle
• Fossils Matching Across the Seas
• Rock Types and Geologic Features
• Ancient Climates

Notes from the lecture:

• Some of these organisms could not possible swim from one continent to another

• Similar mountain belts on the coasts

• If you reconfigure the continents, Coal swamps will be set at the equator

• Distribution of Coal

• Plants and Animal fossils

• Distribution of Mountains

The Great Debate – Objections to the continental drift hypothesis:

Presentation content:

1. Wegener’s inability to identify a credible mechanism for continental drift:
   • Incorrectly proposed the gravitational forces of the Moon and Sun were capable of moving the continents.
   • Incorrectly suggested that continents broke through the ocean crust like icebreakers.
2. There was strong opposition to this hypothesis from all areas of the scientific community, and it was rejected.
3. At the time, scientists believed in land bridges for animal migration.
   • Rafting
   • Transoceanic Land Bridges
   • Island Stepping Stones

2.3 The Theory of Plate Tectonics

List the major differences between Earth’s lithosphere and asthenosphere and explain the importance of each in the plate tectonics theory.

Harry Hess “Founding Fathers” of Plate Tectonic

Presentation content:

• Following World War II, oceanographers with new equipment explored the seafloor

  • Oceanic ridge system winds through all of the major oceans
  • Oceanic crust <180 million years old
  • Continental crust billions of years old
  • Thin sediment accumulation in the deep oceans

• By 1968 these developments and others led to the theory of plate tectonics.

Captain of USS Cape Johnson
Use sonar and echo sounder to survey the ocean floor. Identified Sea Floor Spreading Centers.

Notes from the lecture:

• Over time he convinced his superiors to start running Sonar 24/7
• Used data to help map the Ocean floor
• They were able to age some of these rocks
  • Found that oceanic crust is very very old
• He was challenged time and time again
• Mechanism that drives the configuration of our ocean basins and how continents are shifting every year

Rigid Lithosphere Overlies Weak Asthenosphere

Presentation content:

• The lithosphere is Earth’s strong, outer layer.
• The asthenosphere is a hotter, weaker region of the mantle under the lithosphere.
• Because of the differences in physical properties, the lithosphere is effectively detached from the asthenosphere, allowing layers to move separately.

Notes from the lecture:

• As you move into the Asthenosphere, there is a layer were rocks are near their melting temperature

Earth's Major Plates

Presentation content:

• The lithosphere is broken into approximately two dozen smaller sections called lithospheric plates.
• These plates are in constant motion.

Plate Movement:

• Plates move as rigid units relative to each other.
• Most deformations occur along plate boundaries.
• Types of plate boundaries:
  • Divergent - plates move apart
  • Convergent - plates move together
  • Transform - plates grind past each other

Notes from the lecture:

• Divergent: Plates getting ripped apart
  • As magma rises, it expands and cools
  • This is what creates these large underwater mountains
• Convergent: Plates sub-ducting under another plate
  • Number of major volcanoes in these regions
  • Magma travels to the surface trough the volcanoes
• Transform: The boundary between convergent plates
  • Example: San Andreas

2.4 Divergent Plate Boundaries and Seafloor Spreading

Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

Divergent Plate Boundaries & Seafloor Spreading

Presentation content:

• New oceanic crust forms as two plates move apart.
• Most divergent boundaries are along oceanic ridges (Earth’s longest topographic feature, >70,000 km).
• Key Features of Ridge System:
  1. Rift valley along ridge crest
  2. Volcanism creates new seafloor
  3. Thin sediment cover
  4. Youngest crust at the ridge, older crust farther away
• Spreading Rates:
  • Average ~5 cm/year
  • Mid-Atlantic Ridge ~2 cm/year
  • East Pacific Rise ~15 cm/year

Notes from the lecture:

• Volcanism creates the new sea floor

Continental Rifting

Presentation content:

• A continental rift, an elongated depression, will develop where continental crust sinks.
• Eventually the depression lengthens and deepens, forming a narrow sea, and then a new ocean basin.
• Example: East African Rift

Notes from the lecture:

• You can see the initial states of the rifting
• A little valley is created, it becomes thinner and thinner, until it becomes an ocean

2.5 Convergent Plate Boundaries and Subduction

Compare and contrast the three types of convergent plate boundaries and name a location where each type can be found.

Convergent Plate Boundaries:

Presentation content:
Two plates move toward each other and leading edge of one slides beneath the other

• Where lithosphere descends (subducts) into the mantle: subduction zones
• Deep-ocean trenches are the surface manifestations produced at subduction zones (e.g., Peru-Chili Trench, Mariana Trench, Tonga Trench)
  • Trench depth depends on the slab angle.

1. North West US, -> Volcanoes
2. Convergence between oceanic crusts, generates a volcano
3. Continent-continent collision, collide and they both uplift

Notes from the lecture:

• If it is very old oceanic crust, it will probably subduct down, if it is young it can subduct at a shallow angle
• Nasca Plate

Oceanic–oceanic convergence

Presentation content:

• When two oceanic slabs converge, one descends beneath the other.
• As with oceanic–continental convergence, partial melting initiates volcanic activity.
• If the volcanoes emerge as islands, a volcanic island arc forms (e.g., Aleutian Islands)

Notes from the lecture:

• This is because of the angle of Subduction
• Shallow trench = very large earthquakes
• There is a density difference
• Deeper subduction angle generates a deeper oceanic trench
• An older oceanic crust tends to be more dense = deeper trench (greater angle)
• A younger oceanic crust tends to collide/subduct = shallow trench

Note for Quiz 2: Lithosphere is composed of the crust and the uppermost part of the mantle

Continental–continental convergence

Presentation content:

• Continued subduction can bring two continents together.
• Less dense, buoyant continental lithosphere resists subduction.
• Continental collision produces mountain belts of deformed rocks (e.g., Himalayas, Alps, Appalachians)

Notes from the lecture:

• Continents are very buoyant
• Two continents are coming together, the rate of movement is very high (very quickly)
• Ocean crust located between the two continents is basically subducting away
• On collision, both continent parts are uplifted
• This is the case with the Himalayans

Oceanic-continental convergence

Presentation content:

• The denser oceanic slab sinks into the mantle beneath the buoyant continental block.
• At a depth of ~100 kilometers, melting is triggered when water from the sub-ducting plate mixes with the hot rocks of the asthenosphere.
• Forms a continental volcanic arc on land (e.g., Andes and Cascade Range).

Notes from the lecture:

• Oceanic crust sub-ducting underneath the continent
• Sediments are being recycled into the earth, incorporated as magma

2.6 Transform Plate Boundaries

Describe the relative motion along a transform fault boundary and locate several examples of transform faults on a plate boundary map.

Transform Fault

Presentation content:

• Plates slide horizontally past one another, without production or destruction of lithosphere.
• Most occur on the seafloor joining two spreading center. Known as fracture zones.

Most transform faults offset segments of a spreading center, producing a plate margin that exhibits a zigzag

• Transform faults can move oceanic ridges toward subduction zones
• A few transform faults cut through continental crust (e.g., The San Andreas Fault)
• No crust created or destroyed
• Typically offset mid-ocean ridges

Juan de Fuca Plate

San Andreas Fault

Notes from the lecture:

• No crust being created or destroyed
• In this case you have what is named a fracture zone
  • You have divergent plate boundaries
  • There are basically few areas were we have sliding horizontally in opposite direction
• In Juan de Fuca Plate, we have a fracture zone
• The San Andreas Fault is a very well known transform fault
  • Northern part is moving in eastern direction
  • Southern part is moving in western direction
• A major concern because they do a lot of damage to infrastructure

2.7 How Do Plates and Plate Boundaries Change

Explain why plates such as the African and Antarctic plates are increasing in size, while the Pacific plate is decreasing in size.

How do plates, and plate boundaries change

Presentation content:

• Although Earth’s total surface area does not change, the size and shape of individual plates are constantly changing.
  • Plate boundaries migrate
  • Plate boundaries are created and destroyed
• Today: African and Antarctic Plates growing, while Pacific Plate is Shrinking
• Breakup of Pangaea:
  • Formation of the Atlantic Ocean
  • India collided with Asia to form the Himalayas

Notes from the lecture:

• A lot of the Oceanic crust has been destroyed
• The oldest oceanic crust now is like 200 M years old
• Plate boundaries are dynamic, they change over time, can be created or destroyed
• Continents that exists today, actually are an amalgamation of plates that have collided to each other
• It has happen over billions of years

2.8 Testing the Plate Tectonics Model

List and explain the evidence used to support the plate tectonics theory.

Evidence from Ocean Drilling

Presentation content:

• Hundreds of holes were drilled through layers of sediments that blanket the ocean floor and the basaltic crust
• Sediments increase in age with distance from the ridge crest
• Sediments are almost absent on the ridge crest and thickest furthest from the spreading center
• Oceanic Crust <180 Ma, why?
  Pattern of distribution is expected with seafloor spreading hypothesis being correct

Notes from the lecture:

• A lot of our evidence for this is actually through Ocean Drilling.
• If you look at the sediment thickness, as you get closer to the ridge, the oceanic sediment thickness thins.
  • Thicker aways from the ridge
  • Thinner as you get closer to the ridge
• Another line of evidence is using Uranium Lead to calculate the age of the rocks in the oceanic crust
• Why is the Oceanic crust only around 180 M olds
  • Because a lot of it gets sub-ducted away over time

Evidence from Hot Spots and Mantle Plumes

Presentation content:

• A mantle plume is a cylindrically shaped upwelling of hot rock.
• The surface expression of a mantle plume is an area of volcanism called a hot spot.
• As a plate moves over a hot spot, a chain of volcanoes, known as a hot-spot track, forms.
• The age of each volcano indicates how much time has elapsed since it was over the mantle plume.
• Example: Hawaiian Island chain

Notes from the lecture:

• There is a series of volcanic islands
  • Linear trend of volcanic islands
  • They get older as you go up
• Plate is moving, magma is rising, and is forming these volcanic islands
• The orientation is actually changing, there is some rotational component to it
• Why is Hawaii a much bigger island than the others?
  • Explanation 1: Rate of volcanism is increasing
  • Explanation 2: Rate of movement is increasing

Evidence of Seafloor Spreading from Paleomagnetism

Presentation content:

• Basaltic rocks contain magnetite, an iron-rich mineral influenced by Earth’s magnetic field.
• When the basalt cools the iron-rich minerals become magnetized and align with the existing magnetic field.
• The magnetite is then “frozen” in position and, like a compass needle, indicates the position of the north pole at the time of rock solidification.
• This is referred to as paleomagnetism.

Notes from the lecture:

• Other tool for understanding plate tectonics is paleomagnetism
• Oceanic crust cools, crystallizes, and iron is locked into position, they are now oriented towards the magnetic (geographic) north pole.
• Assumptions about where the magnetic pole was, and the tectonic plate were

Apparent Polar Wandering

Presentation content:

• The apparent movement of the magnetic poles indicates that the continents have moved.
• It also indicates North America and Europe were joined in the Mesozoic.

Notes from the lecture:

• If we take the present day orientation of iron crystals and do not assume that plates have moved, you would just see random movement
• If we assumed that the plates did move, we find that there is a path that is similar, a trajectory we can follow
• Magnetic north pole is reversing, shifting, and the continents react to this, moving

Magnetic Reversals as Evidence for Seafloor Spreading

Presentation content:

• Over periods of hundreds of thousands of years, Earth’s magnetic field reverses polarity.
• During a magnetic reversal, the north pole becomes the south pole, and vice versa.
  • Rocks that exhibit the same magnetism as the present magnetic field exhibit normal polarity.
  • Rocks that exhibit the opposite magnetism exhibit reverse polarity.
• Once this concept was confirmed, researchers established a timescale for these occurrences, called the magnetic time scale.

• Magnetometer across the seafloor can see strips
• This way you can see how fast the spreading rates are and see the orientation of crystals

Notes from the lecture:

• First magnetic reversal was about 1.2 mya
• Idea: 100-1M years the pole magnetically reverses
• Crystals actually flip, re-orient
  • They are also oriented based on the movement
• Purely based on the orientation of Iron crystals

2.9 How is Plate Motion Measured?

Describe two methods researchers use to measure relative plate motion.

Geologic Measurement of Plate Motion

Presentation content:

• Dates of ocean floor from hundreds of locations gathered by ocean-drilling ships
• Combined with paleomagnetism data to make maps of the age of the ocean floor

Notes from the lecture:

• A way to backtrack the last 100 M years
• Reconstruct paleogeographic configuration
• Very important but very difficult tool to use

Measuring Plate Motion from Space

Presentation content:

• Global Positioning System (GPS) data are collected at numerous sites over years
• Measure plate motions to the millimeter

Notes from the lecture:

• Triangulate positions with satellites
• Measure motion, velocity, acceleration, rotation, etc..

2.10 What Drives Plate Motion

Describe plate–mantle convection and explain two of the primary driving forces of plate motion.

Convection

Presentation content:

• Convection in the mantle is the ultimate driver of plate tectonics.
• Forces That Drive Plate Motion:
  • The subduction of cold, dense oceanic lithosphere is a slab-pull force.
  • Elevated lithosphere at oceanic ridges will slide down due to gravity, causing the ridge-push force.

Models of Plate–Mantle Convection

Presentation content:

• The slab-pull and ridge-push forces of plate tectonics are part of the same system as mantle convection.
• What is not known is the exact structure of this convective flow.

Whole-Mantle Convection (Plume Model)

• Cold lithosphere sinks to the core-mantle boundary and stirs the entire mantle

End of Chapter 2 - Concept Check

2.1 From Continental Drift to Plate Tectonics

1. Briefly describe the view held by most geologists prior to the 1960s regarding the ocean basins and continents.
2. Name the early-twentieth-century hypothesis that was at first rejected by geologists and the more comprehensive theory that later replaced it.

2.2 Continental Drift: An Idea Before Its Time

1. What was the first line of evidence that led early investigators to suspect that the continents were once connected?
2. Explain why the discovery of the fossil remains of Mesosaurus in both South America and Africa, but nowhere else, supports the continental drift hypothesis.
3. Early in the twentieth century, what was the prevailing view of how land animals apparently migrated across vast expanses of open ocean?
4. Describe two aspects of Wegener’s continental drift hypothesis that were objectionable to most Earth scientists.

2.3 The Theory of Plate Tectonics

1. What new findings about the ocean floor did oceanographers discover after World War II?
2. Compare and contrast Earth’s lithosphere and asthenosphere.
3. List the three types of plate boundaries and describe the relative motion along each.

2.4 Divergent Plate Boundaries and Seafloor Spreading

1. Sketch or describe how two plates move in relation to each other along divergent plate boundaries.
2. What is the average rate of seafloor spreading in modern oceans?
3. List four features that characterize the oceanic ridge system.

2.5 Convergent Plate Boundaries and Subduction

1. Why does oceanic lithosphere subduct, while continental lithosphere does not?
2. What characteristic of a slab of oceanic lithosphere explains the formation of a deep oceanic trench as opposed to one that is less deep?
3. What distinguishes a continental volcanic arc from a volcanic island arc

2.6 Transform Plate Boundaries

1. Sketch or describe how two plates move in relationship to each other along a transform plate boundary.
2. List two characteristics that differentiate transform faults from the two other types of plate boundaries.

2.7 How Do Plates and Plate Boundaries Change?

1. Name two plates that are growing in size. Name a plate that is shrinking in size.
2. What new ocean basin was created by the breakup of Pangaea?

2.8 Testing the Plate Tectonics Model

1. What is the age of the oldest sediments recovered using deep-ocean drilling? How do the ages of these sediments compare to the ages of the oldest continental rocks?
2. What did the study of preserved magnetism in ancient lava flows tell researchers about the geographic locations of North America and Europe about 180 million years ago?
3. Assuming that hot spots remain fixed, in what direction was the Pacific plate moving while the Hawaiian Islands were forming?
4. Describe how magnetic reversals provide evidence of the seafloor-spreading hypothesis.

2.9 How is Plate Motion Measured

1. What does the orientation of transform faults indicate about plate motion?
2. Based on what you see in Figure 2.33, which three plates appear to exhibit the highest rates of motion?

2.10 What Drives Plate Motions?

1. Which of these forces—slab pull or ridge push—contributes more to plate motion?
2. Briefly describe the whole-mantle convection (plume) model.