Continental Drift

Basic Premise

At one point in history all continents were combined in one big “supercontinent”

For some reason the supercontinent split apart and the smaller land masses slowly drifted to there current positions

Early Idea

Continental Drift had been suggested by numerous scientists throughout history

Edward Seuss (1800)

Frank Taylor (1910)

Alfred Wegner (1912)

Alexander du Toit (1937)

What would make people think this?

Alfred Wegener 1912

Proposed that all landmasses were originally united into a supercontinent he named Pangaea from the Greek meaning “all land”

Evidence:GeologicPaleontologicalClimatologic evidence

Geologic EvidenceShorelines of continents fit together

matching marine, nonmarine and glacial rock sequences of for all five Gondwana continents including Antarctica

Mountain rangesmatch up when continents are united into a single landmass

Paleontological Evidence

Eventually Wegner would include even more paleontologic evidence that was harder to dispute.

Climatic Evidence

Glacial evidenceAs glaciers expand they move outward in all directions from a central point.

Glacial Striations, produced by this movement can be used to locate spreading centers

Additional Support for Continental Drift

Alexander du Toit (South African geologist, 1937)

Proposed that a northern landmass he called Laurasia consisted of present-day

North America

Greenland

Europe

and Asia (except India).

Provided additional fossil evidence for Continental drift

Then WWII

With the start of WWII Continental Drift became sidelined

Interest renewed in the 1950s when researchers use surplus war technologies to study ocean basins

MappingPaleomagnetic studies

Mapping the OceansUsing war time sonar Marie Tharp develops first map of ocean floor.

Discovers largest Mountain Range on Earth

65,000 km long Ridge SystemThe Mid-Atlantic Ridge

best known ridge

divides Atlantic Ocean basin in two nearly equal parts

The Mid Atlantic Ridge

Slight Tangent:Earth’s Magnetic Field

Similar to a giant dipole magnet being located in the core

magnetic poles essentially coincide with the geographic polesResult from rotation of metals in outer core

Strength and orientation of the magnetic field varies

inclination and strength increase from the equator to the poles

weak and horizontal at the equatorstrong and vertical at the poles

Paleomagnetism

Paleomagnetism is a remnant magnetism locked in ancient rocks

When magma cools below the Curie Point, magnetic, iron-bearing minerals align with Earth’s magnetic field.

Back to Continental Drift

Researches discover that Earth’s Magnetic poles apparently moved over time.

The trails produced by this movement were called polar wandering paths.

One problem, different continents had different paths.

Sea Floor Spreading1962, Harry Hess proposed the hypothesis of seafloor spreading

– Continents and oceanic crust move together

– Seafloor separates at oceanic ridges

– Thermal convection cells in the mantle provide the mechanism• hot magma rises from mantle to form new

crust at ridges• cold crust subducts into the mantle at

oceanic trenches, where it is heated and recycled

Conformation for Hess…and the drifters

• In addition to mapping mid-ocean ridges, ocean research also revealed magnetic anomalies on the sea floor– A magnetic anomaly is a deviation from

the average strength of Earth’s Magnetic field

Magnetic Reversals

• Earlier finding, by Bernard Brunnes, showed that Earths magnetic field had reversed– Because this work involved lava flows

the reversals could be dated

Conformation for Hess

Age of Oceanic Crust• Seafloor spreading theory indicates that

oceanic crust is geologically young because it forms during spreading and is destroyed during subduction

• Radiometric dating confirms the youth of the oceanic crust– reveals that the youngest oceanic crust occurs

at mid-ocean ridges – oldest oceanic crust is found near continents

and is less than 180 million years old• whereas oldest continental crust is 3.96 billion yeas

old

Plate Tectonics (the Unifying Theory)

• A unifying theory is one that helps explain a broad range of diverse observations and interpret many aspects of a science on a grand scale.

• Plate tectonics helps explain – earthquakes– volcanic eruptions– formation of

mountains– location of

continents – location of ocean

basins

The Theory of Plate Tectonics

• Plate tectonic theory is based on a simple model – the lithosphere is a rigid structure that consists of

variable-sized pieces called plates which move as a unit

• Plates can be either Continental or Oceanic– Oceanic Plates consist of oceanic crust and upper

mantle• Regions containing oceanic crust are up to 100 km thick

– Continental Plates consist of continental crust and upper mantle• Regions containing continental crust are up to 250 km

thick

Numbers represent average rates of relative movement, cm/yr

How it all works

• The lithospheric plates overlie hotter and weaker semi plastic asthenosphere– Movement of the asthenosphere results

from some type of heat-transfer system

• As plates move over the asthenosphere they: – Separate, mostly at oceanic ridges– Collide, in areas such as oceanic trenches

where they may be subducted back into the mantle

– Slide past each other along transform faults

Divergent Plate Boundaries

• Divergent plate boundaries – occur where plates are separating and new

oceanic lithosphere is forming.

• Crust bulges due to upwelling magma, is extended thinned and fractured

– The magma originates from partial melting of the mantle and is basaltic in composition

Divergent Boundaries• Successive injections of magma:

– cool and solidify to form new oceanic crust– As it cools it records the intensity and orientation of

Earth’s magnetic field

• Divergent boundaries most commonly occur along the crests of oceanic ridges such as the Mid-Atlantic Ridge

– Ridges have• rugged topography resulting from nonuniform displacement of

rocks along large fractures• shallow earthquakes

Features of Ridges (divergent boundaries)

• Ridges also have high heat flow and basaltic flows or pillow lavas

Divergent Boundaries

• Divergent boundaries are also present under continents during the early stages of continental breakup causing the crust to be initially elevated, stretched and thinned– The stretching produces fractures and rift valleys.– Examples:

• Africa

Evidence• What features in the

rock record can geologists use to recognize ancient rifting?

• faults• dikes• sills• lava flows• thick sedimentary

sequences within rift valleys

– Example:• Triassic age fault basins in

eastern US• Palisade Sill

Convergent Plate Boundaries

• Older oceanic crust must be destroyed at convergent boundaries so that Earth’s surface area remains the same

• Where two plates collide, if at least one is oceanic, subduction occurs– During subduction, oceanic plate descends

beneath the margin of another plate• the subducting plate moves into the

asthenosphere where it is heated and is incorporated back into the mantle

Convergent Boundaries• Convergent boundaries are characterized by:

– deformation - folding and faulting of strata– andesitic volcanism (except at continental collisions)– mountain building– metamorphism– earthquake activity– important mineral deposits

• Three types of Convergent boundaries – oceanic-oceanic– oceanic-continental– continental-continental (continental collisions)

Oceanic-Oceanic

• When two oceanic plates converge, one is subducted beneath the other along an oceanic-oceanic plate boundary

– an oceanic trench forms – a subduction complex forms

• composed of slices of folded and faulted sediments and oceanic lithosphere scraped off the subducting plate

Volcanic Arcs

• As the plate subducts into the mantle, it is heated and partially melted generating magma of an andesitic composition – the magma rises to the surface because it is less

dense than the surrounding mantle rocks– At the surface of the non-subducting plate, the

magma forms an andesitic volcanic island arc

Back-arc basin

• A back-arc basin forms when the lithosphere on the landward side of the island arc is stretched and thinned

Oceanic-Continental

• An oceanic-continental plate boundary occurs when a denser oceanic plate subducts under less dense continental lithosphere

• Magma generated by subduction rises into the continental crust to form large igneous bodies called plutons or erupts to form a volcanic arc of andesitic volcanoes

• Example: Pacific coast of South America (Andes Mountains, Peru)

Continental-Continental• Two approaching continents are initially separated by ocean floor

that is being subducted under one of them, which, thus, has a volcanic arc

• When the continents collide – Density of the plates are equal so no subduction occurs,

though one may wedge beneath the other

• The plates are welded together at a continent-continent plate boundary, – along the site of former subduction an interior mountain belt

forms consisting of• deformed sedimentary rocks• igneous intrusions• metamorphic rocks• fragments of oceanic crust

Continental-Continental

Identifying Convergent Boundaries

– Andesitic magma erupted, • forming island arc volcanoes and continental volcanoes

– The subduction complex results in • a zone of intensely deformed rocks • between the trench and the area of igneous activity

– Sediments and submarine rocks are folded, faulted and metamorphosed• making a chaotic mixture of rocks termed a mélange

– Slices of oceanic lithosphere may be accreted • to the continent edge and are called ophiolites

Ohiolites

• Ophiolites consist of layers – representing parts

of the oceanic crust and upper mantle.

• The sediments include– graywacke– black shale– chert

• Ophiolites are key to detecting old subduction zones

Transform Boundaries• Occur where plates slide

laterally past each other– roughly parallel to the

direction of plate movement

• Movement results in– zone of intensely shattered

rock– numerous shallow

earthquakes

• The majority of transform faults – connect two oceanic ridge

segments– and are at fracture zones

Hot Spots

• Hot spots are locations where stationary columns of magma, originating deep within the mantle, called mantle plumes, slowly rise to the surface.

• Mantle plumes remain stationary– although some evidence suggests they may move

somewhat

• When plates move over them, hot spots leave trails of extinct progressively older volcanoes called aseismic ridges which record the movement of the plates

The Mechanism…

• Most geologists accept some type of convective heat system as the basic cause of plate motion

• In one possible model, thermal convection cells are restricted to the asthenosphere

The Mechanism• In a second model, the

entire mantle is involved in thermal convection.

• In both models, – spreading ridges mark the

rising limbs of neighboring convection cells

– trenches occur where the convection cells descend back into Earth’s interior

The Mechanism

• In addition to thermal convection cells, some geologists think that movement may be aided by– slab-pull

• the slab is cold and dense and pulls the plate

– ridge-push• rising magma pushes the

ridges up

• and gravity pushes the ocean floor toward the trench

Plate Tectonics and Life• Present distribution of plants and animals is

largely controlled by climate and geographic barriers

• Barriers create biotic provinces – each province is a region characterized by a

distinctive assemblage of plants and animals

• Plate movements largely control barriers– When continents break up, new provinces form– When continents come together, fewer provinces

result– As continents move north or south they move

across temperature barriers