Continental drift: An idea
before its time

• Alfred Wegener
• First proposed his continental drift hypothesis in 1915
• Published The Origin of Continents and Oceans
• Continental drift hypothesis
• Supercontinent called Pangaea began breaking apart about 200 million years ago

Continental drift: An idea before its time

• Continental drift hypothesis
• Continents "drifted" to present positions
• 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
• Inability to provide a mechanism capable of moving continents across the globe
• Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice

Wegener’s matching of mountain ranges on different continents

Paleoclimatic evidence for Continental Drift

The great debate

• Continental drift and the scientific method
• Wegner’s hypothesis was correct in principle, but contained incorrect details
• For any scientific viewpoint to gain wide acceptance, supporting evidence from all realms of science must be found
• A few scientists considered Wegner’s ideas plausible and continued the search

Continental drift and paleomagnetism

• Initial impetus for the renewed interest in continental drift came from rock magnetism
• Magnetized minerals in rocks
• Show the direction to Earth’s magnetic poles
• Provide a means of determining their latitude of origin
• Polar wandering
• The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved
• Shows that Europe was much closer to the equator when coal-producing swamps existed

• Polar wandering
• 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

Apparent polar-wandering paths for Eurasia and North America

The 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 (not Howard Hesseman!) in the early 1960s

• Geomagnetic reversals
• Earth's magnetic field periodically reverses polarity — the north magnetic pole becomes the south magnetic pole, and vice versa
• Dates when the polarity of Earth’s magnetism changed were determined from lava flows

• Geomagnetic reversals
• Geomagnetic reversals are recorded in the ocean crust
• In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading

Paleomagnetic reversals recorded by basalt at mid-ocean ridges

• Geomagnetic reversal
• Paleomagnetism (evidence of past magnetism recorded in the rocks) was the most convincing evidence set forth to support the concepts of continental drift and seafloor spreading

Plate tectonics: The new paradigm

• Much more encompassing theory than continental drift
• The composite of a variety of ideas that explain the observed motion of Earth’s lithosphere through the mechanisms of subduction and seafloor spreading

• Earth’s major plates
• Associated with Earth's strong, rigid outer layer
• Known as the lithosphere
• Consists of uppermost mantle and overlying crust
• Overlies a weaker region in the mantle called the asthenosphere

• Earth’s major plates
• Seven major lithospheric plates
• Plates are in motion and continually changing in shape and size
• Largest plate is the Pacific plate
• Several plates include an entire continent plus a large area of seafloor

• Earth’s major plates
• Plates move relative to each other at a very slow but continuous rate
• Average about 5 centimeters (2 inches) per year
• Cooler, denser slabs of oceanic lithosphere descend into the mantle

• Plate boundaries
• Each plate is bounded by a combination of the three types of boundaries
• New plate boundaries can be created in response to changes in the forces acting on these rigid slabs

Divergent plate boundaries

• Most are located along the crests of oceanic ridges and can be thought of as constructive plate margins
• Oceanic ridges and seafloor spreading
• Along well-developed divergent plate boundaries, the seafloor is elevated forming oceanic ridges

Divergent plate boundaries

• Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge system
• Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by spreading rates

Divergent boundaries are located mainly along oceanic ridges

Divergent boundaries

• Spreading rates and ridge topography
• Topographic differences are controlled by spreading rates
• At slow spreading rates (1-5 centimeters per year), a prominent rift valley develops along the ridge crest that is wide (30 to 50 km) and deep (1500-3000 meters)
• At intermediate spreading rates (5-9 centimeters per year), rift valleys that develop are shallow with subdued topography

• Spreading rates and ridge topography
• Topographic differences are controlled by spreading rates
• At spreading rates greater than 9 centimeters per year no median rift valley develops and these areas are usually narrow and extensively faulted
• Continental rifts
• Splits landmasses into two or more smaller segments

Divergent boundaries

• Continental rifts
• Examples include the East African rifts valleys and the Rhine Valley in northern Europe
• Produced by extensional forces acting on the lithospheric plates
• Not all rift valleys develop into full-fledged spreading centers

The East African rift — a divergent boundary on land

Convergent plate boundaries

• Older portions of oceanic plates are returned to the mantle in these destructive plate margins
• Surface expression of the descending plate is an ocean trench
• Called subduction zones
• Average angle at which oceanic lithosphere descends into the mantle is about 45°

Convergent plate boundaries

• Although all have the same basic characteristics, they are highly variable features
• Types of convergent boundaries
• Oceanic-continental convergence
• Denser oceanic slab sinks into the asthenosphere

Convergent plate boundaries

• Types of convergent boundaries
• Oceanic-continental convergence
• As the plate descends, partial melting of mantle rock generates magmas having a basaltic or, occasionally andesitic composition
• Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)

Convergent plate boundaries

• Types of convergent boundaries
• 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)

Convergent plate boundaries

• Types of convergent boundaries
• Continental-continental convergence
• Continued subduction can bring two continents together
• Less dense, buoyant continental lithosphere does not subduct
• Result is a collision between two continental blocks
• Process produces mountains (Himalayas, Alps, Appalachians)

The collision of India and Asia produced the Himalayas

Transform fault boundaries

• The third type of plate boundary
• Plates slide past one another and no new lithosphere is created or destroyed
• Transform faults
• Most join two segments of a mid-ocean ridge as parts of prominent linear breaks in the oceanic crust known as fracture zones

Transform fault boundaries

• Transform faults
• A few (the San Andreas fault and the Alpine fault of New Zealand) cut through continental crust

Testing the plate tectonics model

• Plate tectonics and earthquakes
• Plate tectonics model accounts for the global distribution of earthquakes
• Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory
• Deep-focus earthquakes are closely associated with subduction zones
• The pattern of earthquakes along a trench provides a method for tracking the plate's descent

Deep-focus earthquakes occur along convergent boundaries

Earthquake foci in the vicinity of the Japan trench

Testing the plate tectonics model

• Evidence from ocean drilling
• Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment
• Age of deepest sediments
• Thickness of ocean-floor sediments verifies seafloor spreading

Testing the plate tectonics model

• Hot spots
• Caused by rising plumes of mantle material
• Volcanoes can form over them (Hawaiian Island chain)
• Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary

The Hawaiian Islands have formed over a stationary hot spot

The driving mechanism

• No one driving mechanism accounts for all major facets of plate tectonics
• Researchers agree that convective flow in the rocky 2,900 kilometer-thick mantle is the basic driving force of plate tectonics
• Several mechanisms generate forces that contribute to plate motion
• Slab-pull
• Ridge-push

The driving mechanism

• Models of plate-mantle convection
• Any model describing mantle convection must explain why basalts that erupt along the oceanic ridge
• Models
• Layering at 660 kilometers
• Whole-mantle convection
• Deep-layer model

Importance of plate tectonics

• Theory provides a unified explanation of Earth’s major surface processes
• Within the framework of plate tectonics, geologists have found explanations for the geologic distribution of earthquakes, volcanoes, and mountains
• Plate tectonics also provides explanations for past distributions of plants and animals