V Earlier Theorists Attributed This to Now Sunken Bridges

Plate Tectonics

Birth of a Theory

v In 1911 astronomer, meteorologist, climatologist Alfred Wegener (1880-1930) found earlier studies noting the same fossils on opposite sides of the Atlantic.

v Earlier theorists attributed this to now sunken bridges

v Wegener noted the fit of South America and Africa and became fixed on one continent

v His search for other evidence resulted in The Origin of the Continents and Oceans in 1915, with expanded versions published in 1920, 1922, and 1929

v He postulated a single continent Pangaea (Greek for “all the earth”)

v Wegener was not the first to suggest the idea but the first to include extensive evidence. American F.B. Taylor included little evidence with his 1910 work and was ignored.

Wegener’s Arguments

v Fit of continents

v Appalachian and Caledonian mountains

Fit of Continents

Matching Mountain Ranges

v Fossil evidence

v Mesosaurus, Glossopteris, Cynognathus

v Paleoclimatic evidence

v Widespread glaciation

Glacial Evidence

v Wegener’s ideas were largely unaccepted

v Argued that continents were driven through solid oceanic crust like icebreakers

v Centrifugal or tidal forces

v No acceptable mechanism

Paleomagnetism

v Iron-rich rocks (basalt) acquire a record of earth’s magnetic field upon cooling below the Curie Point (580° C)

v Results in two types of information

v Location of deposition

v Time of deposition

Temporal Paleomagnetism

v Earth’s magnetic field reverses polarity at irregular intervals

v Polarities last 105 to 106 years

v Rocks acquire current field at cooling

v Polarities correlative and can be used for dating

Oceanic Paleomagnetism

v Work on mid-oceanic ridges shows strong and weak magnetic anomalies later determined to be normal and reversed magnetic events

v Events form mirror images across the ridges

v Dating determines that the rocks age away from ridge

Tying the Ends Together

v Wegener’s Continental Drift may be combined with Sea-Floor Spreading

v Combination provides a mechanism for movement of continents suggested by Wegener

v Combination has been developed into the Theory of Plate Tectonics

Plate tectonics: A modern version of an old idea

v Earth’s major plates

v Associated with Earth's strong, rigid outer layer

v Known as the lithosphere

v Consists of uppermost mantle and overlying crust

v Overlies a weaker region in the mantle called the asthenosphere

v Earth’s major plates

v Seven major lithospheric plates

v Plates are in motion and continually changing in shape and size

v Largest plate is the Pacific plate

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

v Earth’s major plates

v Plates move relative to each other at a very slow but continuous rate

v Average about 5 centimeters (2 inches) per year

v Cooler, denser slabs of oceanic lithosphere descend into the mantle

v Plate boundaries

v All major interactions among individual plates occur along their boundaries

v Types of plate boundaries

v Divergent plate boundaries (constructive margins)

v Convergent plate boundaries (destructive margins)

v Transform fault boundaries (conservative margins)

v Each plate is bounded by a combination of these three types of boundaries

v New plate boundaries can be created in response to changes in the forces acting on these rigid slabs

Divergent plate boundaries

v Most are located along the crests of oceanic ridges and can be thought of as constructive plate margins

v Oceanic ridges and seafloor spreading

v Along well-developed divergent plate boundaries, the seafloor is elevated forming oceanic ridges

v Seafloor spreading occurs along the oceanic ridge system

Divergent boundaries are located mainly along oceanic ridges

v Spreading rates and ridge topography

v Topographic differences are controlled by spreading rates

v 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)

v At intermediate spreading rates (5-9 centimeters per year), rift valleys that develop are shallow with subdued topography

v At spreading rates greater than 9 centimeters per year no median rift valley develops and these areas are usually narrow and extensively faulted

v Continental rifts

v Splits landmasses into two or more smaller segments

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

v Produced by extensional forces acting on the lithospheric plates

v Not all rift valleys develop into full-fledged spreading centers

Convergent plate boundaries

v Older portions of oceanic plates are returned to the mantle in these destructive plate margins

v Surface expression of the descending plate is an ocean trench

v Called subduction zones

v Average angle at which oceanic lithosphere descends into the mantle is about 45°

v Types of convergent boundaries

v Oceanic-continental convergence

v Denser oceanic slab sinks into the asthenosphere

v As the plate descends, partial melting of mantle rock generates magmas having a basaltic or, occasionally andesitic composition

v Mountains produced in part by volcanic activity associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)

v Types of convergent boundaries

v Oceanic-oceanic convergence

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

v Often forms volcanoes on the ocean floor

v If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

v Types of convergent boundaries

v Continental-continental convergence

v Continued subduction can bring two continents together

v Less dense, buoyant continental lithosphere does not subduct

v Result is a collision between two continental blocks

v Process produces mountains (Himalayas, Alps, Appalachians)

The collision of India and Asia produced the Himalayas

Transform fault boundaries

v The third type of plate boundary

v Plates slide past one another and no new lithosphere is created or destroyed

v Transform faults

v Most join two segments of a mid-ocean ridge as parts of prominent linear breaks in the oceanic crust known as fracture zones

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

Testing the plate tectonics model

v Paleomagnetism

v Polar wandering

v The apparent movement of the magnetic poles illustrated in magnetized rocks indicates that the continents have moved

v Polar wandering curves for North America and Europe have similar paths but are separated by about 24° of longitude

v Different paths can be reconciled if the continents are placed next to one another

Apparent polar-wandering paths

v Plate tectonics and earthquakes

v Plate tectonics model accounts for the global distribution of earthquakes

v Absence of deep-focus earthquakes along the oceanic ridge is consistent with plate tectonics theory

v Deep-focus earthquakes are closely associated with subduction zones

v 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 at the the Japan trench

v Evidence from ocean drilling

v Some of the most convincing evidence confirming seafloor spreading has come from drilling directly into ocean-floor sediment

v Age of deepest sediments

v Thickness of ocean-floor sediments verifies seafloor spreading

v Hot spots

v Caused by rising plumes of heat from the at least the mantle (some say the core)

v Volcanoes can form over them (Hawaiian Island chain)

v Most mantle plumes are long-lived structures and at least some originate at great depth, perhaps at the mantle-core boundary

The Hawaiian Islands: a stationary hot spot

Measuring plate motions

v A number of methods have been employed to establish the direction and rate of plate motion

v Volcanic chains

v Paleomagnetism

v Very Long Baseline Interferometry (VLBI)

v Global Positioning System (GPS)

v Calculations show that

v Hawaii is moving in a northwesterly direction and approaching Japan at 8.3 centimeters per year

v A site located in Maryland is retreating from one in England at a rate of about 1.7 centimeters per year

The driving mechanism

v No one driving mechanism accounts for all major facets of plate tectonics

v Researchers agree that convective flow in the rocky 2,900 km-thick mantle is the basic driving force of plate tectonics

v Several mechanisms generate forces that contribute to plate motion

v Slab-pull

v Ridge-push

v Models of plate-mantle convection

v Any model describing mantle convection must explain why basalts that erupt along the oceanic ridge

v Models

v Layering at 660 kilometers

v Whole-mantle convection

v Deep-layer model

Mountain belts

v Orogenesis – the processes that col-lectively produce a mountain belt

v Includes folding, thrust faulting, metamorphism, and igneous activity

v Mountain building has occurred during the recent geologic past

v Alpine-Himalayan chain

v American Cordillera

v Mountainous terrains of the western Pacific

Earth’s major mountain belts

v Older Paleozoic- and Precambrian-age mountains

v Appalachians

v Urals in Russia

v Several hypotheses have been proposed for the formations of Earth’s mountain belts

Mountain building at convergent boundaries

v Plate tectonics provides a model for orogenesis

v Mountain building occurs at convergent plate boundaries

v Of particular interest are active subduction zones

v Volcanic arcs are typified by the Aleutian Islands and the Andean arc of western South America

v Aleutian-type mountain building

v Where two ocean plates converge and one is subducted beneath the other

v Volcanic island arcs result from the steady subduction of oceanic lithosphere

v Most are found in the Pacific

v Active island arcs include the Mariana, New Hebrides, Tonga, and Aleutian arcs

v Continued development can result in the formation of mountainous topography consisting of igneous and metamorphic rocks

Formation of a volcanic island arc

v Andean-type mountain building

v Mountain building along continental margins

v Involves the convergence of an oceanic plate and a plate whose leading edge contains continental crust

v Exemplified by the Andes Mountains

v Andean-type mountain building

v Stages of development - passive margin

v First stage

v Continental margin is part of the same plate as the adjoining oceanic crust

v Deposition of sediment on the continental shelf is producing a thick wedge of shallow-water sediments

v Turbidity currents are depositing sediment on the continental rise and slope

v Andean-type mountain building

v Stages of development – active continental margins

v Subduction zone forms

v Deformation process begins

v Convergence of the continental block and the subducting oceanic plate leads to deformation and metamorphism of the continental margin

v Continental volcanic arc develops

v Andean-type mountain building

v Stages of development – active continental margins

v Accretionary wedge may form

v Chaotic accumulation of sedimentary rocks and metamorphic rocks with occasional scraps of ocean crust

v Can become large enough to stand above sea level

v Andean-type mountain building

v Composed of roughly two parallel zones

v Volcanic arc

v Develops on the continental block

v Consists of large intrusive bodies intermixed with high-temperature metamorphic rocks

v Accretionary wedge

v Seaward segment

v Consists of folded, faulted, and metamorphosed sediments and volcanic debris

Orogenesis along an Andean-type subduction zone

v Andean-type mountain building

v Sierra Nevada and Coast Ranges

v One of the best examples of an active Andean-type orogenic belt

v Subduction of the Pacific Basin under the western edge of the North American plate

v Sierra Nevada batholith is a remnant of a portion of the continental volcanic arc

Basin and Range Created by Tension

Possible Tensional Explanation

v Continental collisions

v Two lithospheric plates, both carrying continental crust

v The Himalayan Mountains are a youthful mountain range formed from the collision of India with the Eurasian plate about 45 MYA

v Spreading center that propelled India northward is still active

v Similar but older collision occurred when the European continent collided with the Asian continent to produce the Ural mountains

Plate relationships prior to the collision of India with Eurasia

Position of India and Eurasia over time

Formation of the Himalayas

v Continental accretion and mountain building

v A third mechanism of orogenesis

v Small crustal fragments collide and merge with continental margins

v Responsible for many of the mountainous regions rimming the Pacific

v Accreted crustal blocks are called terranes

v Terranes consist of any crustal fragments whose geologic history is distinct from that of the adjoining terranes

v As oceanic plates move, they carry embedded oceanic plateaus, volcanic island arcs and microcontinents to an Andean-type subduction zone

Exotic Terranes

How Orogenesis Ends

Vertical movements of the crust

v In addition to the horizontal movements of lithospheric plates, vertical movement also occurs along plate margins as well as the interiors of continents far from plate boundaries

v Isostatic adjustment

v Less dense crust floats on top of the denser and deformable rocks of the mantle

v Concept of floating crust in gravitational balance is called isostasy

v If weight is added or removed from the crust, isostatic adjustment will take place as the crust subsides or rebounds (Hudson Bay rebound, continental shelf subsidence)

The principle of isostasy

Key Terms Chapter 4

Continental drift

Sea-floor spreading

Plate tectonics

Divergent margin

Convergent margin

Conservative (transform) margin

Subduction zone

Convection

Tectonic cycle

Volcanic arc

Island arc

Isostasy