Chapter 2 Plate Tectonics:

A Scientific Revolution Unfolds

Continental Drift: An Idea Before its Time Alfred Wegener

• First proposed his continental drift hypothesis in 1915 before the theory of Plate Tectonics

Continental drift hypothesis • Supercontinent called Pangaea began

breaking apart about 200 million years ago • Continents "drifted" to present positions

Continental Drift: An Idea Before its Time, cont’d

Evidence used in support of continental drift hypothesis Fit of the continents Fossil evidence Rock type and structural similarities Paleoclimatic evidence

The Great Debate

Objections to the continental drift hypothesis Lack of a mechanism for moving continents Wegener incorrectly suggested that

continents broke through the ocean crust, much like ice breakers cut through ice

Strong opposition to the hypothesis from all areas of the scientific community

Continental Drift and Paleomagnetism Renewed interest in continental drift

initially came from rock magnetism Magnetized minerals in rocks

• Declinations show the direction to Earth’s magnetic poles when rocks formed

• Inclinations provide a means of determining the rocks latitude of origin and how much it moved north or south since it was created.

Continental Drift and Paleomagnetism Polar wandering

• The apparent movement of the magnetic poles indicates that the continents have moved

• Indicates Europe was much closer to the equator when coal-producing swamps existed

Continental Drift and Paleomagnetism, cont’d Polar wandering

• Curves for North America and Europe have similar paths but are separated by about 24 of longitude

• Differences between the paths can be reconciled if the continents are placed next to one another

A Scientific Revolution Begins During the 1950s and 1960s

technological strides permitted extensive mapping of the ocean floor

Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s

• Oceanic crust created and spread apart at shallow mid-oceanic ridges

• Oceanic crust destroyed at deep trenches

A Scientific Revolution Begins, cont’d Geomagnetic reversals

Earth's magnetic field periodically reverses polarity – the north magnetic pole becomes the south magnetic pole, and vice versa. This flips the remnant magnetic inclination of the rocks 180 degrees.

Dates of geomagnetic reversals determined from lava flows.

A Scientific Revolution Begins, cont’d Geomagnetic reversals

Geomagnetic reversals are recorded in the ocean crust as alternating linear stripes of high and low magnetic anomalies (highs for normal polarity and lows for reversed polarity).

These linear anomalies parallel the mid-ocean ridges and are symmetrical with respect to them.

In 1963 Vine and Matthews tied the discovery of these “magnetic stripes” in the ocean crust near mid-ocean ridges to Hess’s concept of seafloor spreading.

A Scientific Revolution Begins, cont’d Earthquakes and Subduction Zones

Wadati and Benioff mapped out the depths and locations of earthquakes along trenches and beneath arcuate trends of volcanoes that parallel them.

They found shallower earthquakes closer to the trench, and progressively deeper earthquakes towards the volcanic arcs and beyond them.

Postulated that the oceanic crust and upper-most mantle is sinking or subducting down into the rest of the mantle along these zones and is being destroyed.

Plate Tectonics: The New Paradigm Earth’s major tectonic plates

• Earth is divided into seven major lithospheric plates and several other minor plates that are relatively rigid.

• Plates are composed of the oceanic and/or continental crust and the uppermost mantle.

• Several plates include an entire continent plus a large area of seafloor.

Plate Tectonics: The New Paradigm, cont;d Earth’s major tectonic plates cont.

• Plates are in constant slow motion with respect to one another and are continually changing in shape and size.

• Earthquakes, deformation and volcanic eruptions occur where these plates meet.

• The interior of plates generally have fewer earthquakes, less deformation and less volcanic activity compared to the margins.

Plate Tectonics: The New Paradigm, cont’d Plate boundaries

• Interactions between individual plates occur along their boundaries

Types of plate boundaries • Divergent plate boundaries (constructive

margins formed by extension) • Convergent plate boundaries (destructive

margins formed by compression)• Transform fault boundaries (conservative

margins formed by side-to-side shear)

Divergent Plate Boundaries Most are located along the crests of oceanic

ridges Oceanic ridges and seafloor spreading

• The seafloor is elevated along well-developed divergent plate boundaries, forming oceanic ridges

Geologic features include• crustal and lithospheric thinning• rift zones formed by extensional faulting• volcanic activity• shallow earthquakes

Divergent Plate Boundaries, cont’d

Continental rifting Splits landmasses into two or more

smaller segments along a continental rift

Examples include the East African rift valleys and the Rhine Valley in northern Europe

Produced by extensional forces acting on lithospheric plates

Convergent Plate Boundaries Older portions of oceanic plates are

returned to the mantle or subducted in these destructive plate margins

• Surface expression of the descending plate is an ocean trench

• Oceanic plate moves downward along subduction zones

• Geologic features include• volcanic arcs• shallow-deep earthquakes• mountain belts

Types of Convergent Plate Boundaries Oceanic-continental convergence

• Denser oceanic slab sinks into the asthenosphere

• Along the descending plate partial melting of mantle rock generates magma

• Resulting volcanic mountain chain is called a continental volcanic arc (Andes and Cascades)

Types of Convergent Plate Boundaries, cont’d Oceanic-oceanic convergence

• When two oceanic slabs converge, one descends beneath the other

• Often forms volcanoes on the ocean floor• If the volcanoes emerge as islands, a

volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

Types of Convergent Plate Boundaries, cont’d Continental-continental convergence

• Continued subduction can bring two continents together

• Less dense, buoyant continental lithosphere does not subduct

• Resulting collision between two continental blocks produces mountains (Himalayas, Alps, Appalachians)

Transform Fault Boundaries Plates slide past one another and no new lithosphere is

created or destroyed Most join two segments of a mid-ocean ridge along

breaks in the oceanic crust known as fracture zones A few (e.g. the San Andreas fault) cut through

continental crust Geologic Features include

strike slip faults lack of volcanic activity shallow earthquakes inactive fracture zones which extend outward from oceanic transform

faults (active fracture zones) and separate lithosphere of different age and density

Testing the Plate Tectonics Model

Hot spots and mantle plumes• Caused by rising plumes of heat or hot mantle

material• Volcanoes and calderas can form over them

(Hawaiian Island chain, Yellowstone caldera)• Mantle plumes

• Long-lived structures that are fixed in one subsurface location while the plates drift over them

• Some originate at great depth, perhaps at the core-mantle boundary

Measuring Plate Motion Paleomagnetism and plate motions

• Paleomagnetism stored in rocks on the ocean floor provides a method for determining plate motions

• Both the direction and rate of seafloor spreading can be established

Measuring Plate Motion, cont’d

Measuring plate velocities from a fixed reference frame in space

• Accomplished by establishing exact locations on opposite sides of a plate boundary and measuring relative motions

• Two methods are used• Very Long Baseline Interferometry (VLBI)• Global Positioning System (GPS)

What Drives Plate Motions

Many, but not all researchers agree that convective flow in the mantle is the basic driving force of plate tectonics

• Theory I: Causes elevation and density differences due to temperature variations, which create gravitational forces

• Theory II: Causes frictional drag on the plates above the convecting mantle

End of Chapter 2