alfred wegner - continental drift hypothesis alfred wegener, a german climatologist, developed the...

Alfred Wegner - Continental Drift Hypothesis

Alfred Wegener, a German climatologist,developed the Continental Drift hypothesis in 1915

Continental Drift

• Continents have drifted to present locations

• Continents once joined as “supercontinent” Pangaea

• Pangaea formed 250 mya

• 200 mya, tectonic forces began pulling Pangaea apart

Some of Wegener’s Evidence at the Time:

Evidence for continentaldrift

Rocks in Newfoudland are same age/type as Sweden, Norway, Scotland.

Problems with Continental Drift Hypothesis

• Continents drift -- but what about the ocean floor?

• What force could move continents?

• Studies of the ocean floor in the 2 decades following WWII led to the development of the plate tectonic theory

Mid-Atlantic Ridge

• Mountain range running N-S on floor of Atlantic Ocean

• Magnetic polarity of ocean floor “striped” with alternating N/S poles– This is called Magnetic striping

• How does this occur?• 1. New ocean forms when basaltic magma from

mantle rises and hardens at the ocean ridge. New magma coming up moves older rock away from ridge like conveyer belt.

• 2. Basalt, rich in iron, becomes magnetic

• 3. Minerals line up with magnetic north of earth

• 4. Earth’s magnetic field flips every 500,000 years

• New portions of the ridge will have reverse polarity

• Result = alternating bands of normal and reverse polarity in rock around ridge

• The Earth is constantly changing

• The Earth’s crust is divided into 8 large plates (and several small plates)

• Almost all major earthquake or volcano activity occurs along the plate boundaries

• Because each plate moves as a unit, the interiors of the plates are generally stable.

• Really not a theory due to overwhelming evidence!!!!

The Theory of Plate Tectonics

Lithosphere (crust + upper mantle) broken into lithospheric plates (tectonic plates)

How plates move - Convection Currents

Mantle convection• Convection in the mantle brings hot material upward in

some places. Elsewhere, cooler rock sinks.• Upwelling hot material can cause lithosphere to rift (split)

and plates drift apart (usually at oceanic ridges)

• The plates pushed apart contact another plate

• This often forms subduction zones– Denser plate (usually oceanic) forced underneath

less dense plate (continental)– A valley, called a trench, is formed

• Subducting plate pulls rest of plate = slab pull

TYPES OF PLATE BOUNDARIES

• Divergent boundaries -- plates move away from each other

• Convergent boundaries -- plates move toward each other

• Transform boundaries -- plates try to slide past each other

A

Divergent

B

Convergent

C

Transform

• plates are moving apart

• new crust is created

• magma is coming to the surface

• plates are coming together

• crust is returning to the mantle

• plates are slipping past each other

• crust is not created or destroyed

Types of plate Boundaries

A

Divergent

B

Convergent

C

Transform

• almost always found under the ocean

•Forms mid-ocean ridges

•Iceland is a rare example of one on land

• usually ocean plate colliding with land plate

• ocean plate goes under land plate

• pushes up mountains and forms deep ocean trenches (subduction zones)

• rare on the planet

• famous one is the San Andreas Fault in California

DIVERGENT BOUNDARIES

• Plates move apart along a system of fractures• As magma rises and cools, it pushes older rock away• Newer rock found closer to spreading oceanic ridge

and older rock farther away• Spreading on land = rift• Blocks of rock are down-dropped along fractures

(faults) - rift valleys• Seafloor spreading : mid-ocean ridges volcanic

activity produces new seafloor as plates drift apart• Examples: E. African Rift, mid-Atlantic ridge

Diverging Plates

Howocean basinsformed

CONVERGENT BOUNDARIES – Oceanic-Continental

• Oceanic plate (dense basalt) subducts under continental plate (less dense granite)

• Subducting plate pushed into mantle and melts…this forms cone volcanos

• Force also causes formation of mountains as continental crust crumples

• Causes smalllarge earthquakes• BC is over a subduction zone: oceanic Juan de

Fuca plate subducting under continental North American plate formed Coast Mountains

Converging Plates - Subduction

CONVERGENT BOUNDARIES – Oceanic-Oceanic

• Rock densities similar, so one plate forced under other

• Volcanoes produced– Form long chain of island = island arc

– Eg: Japan, Aleutian Islands

Earthquake focus increases in depthalong subducting plate

The further from the edge of plate, the deeper the earthquake

CONVERGENT BOUNDARIES – Continental-Continental

• Plates are same density– No subduction!

• Instead, we get mountain building• Plates crumple and fold as they collide• No volcanos formed• Do get earthquakes• Eg: Himilayas (Mt. Everest) formed by India

colliding with Eurasian plate

Converging Continental Plates

TRANSFORM BOUNDARIES• Plates slide past each other in opposite

directions• No volcanoes or mountains• Do get many large, shallow earthquakes• Examples: San Andreas fault zone, southern

CA; – oceanic Pacific Plate sliding past North

American plate• Also found at divergent plate boundaries

Transform Boundaries

Streams offset by San Andreas Fault

GATHERING EVIDENCE

• Field work - geologists sampling rocks, drilling, mapping formations

• Remote Sensing - observing from a distance (satellite photos, sonar mapping of ocean floors)

• Seismology - study of earthquakes and seismic waves

• Volcanology - study of volcanoes