Plate Tectonics• Alfred Wegener – early 1900’s proposed the concept of

Continental Drift- the continents were once joined a large “super continent” called Pangaea– Evidence

• Continental Jigsaw• Fossils• Ancient Climates- study of Paleozoic Rocks (200-300 million years ago• Mountain Ranges

– Problems• Work was mostly completed in the southern hemisphere- vast majority of

geologists worked in the northern hemisphere• He could not produce a mechanism

Paleoclimatic evidence for Continental Drift

Wegener’s matching of mountain ranges on different continents

Pangaea approximately 200 million years ago

• Breakup of Pangaea figure 19.2

• It was not until 1950’s and 1960’s when Wegener’s theories were revised through the study of the sea floor topographic features, the geophysics of earth interior and paleo-magnetics of the rock on the ocean floor

1950’s and 60’s

• Geophysical data- – seismic data about the earth’s interior– Paleomagnetic data– Topography of the sea floor

• Additional data– Rock samples from the sea floor• Absolute dating of seafloor basalts

– Images of the deep sea floor

Deep Wells• 1970: Kola super deep borehole in Kola Peninsula, Russia 7.6 mile planed

9.3 miles• 1999 Hawaii Scientific Drilling, Project Hilo, Hawaii. 1.8 mile of planned 2.8

miles into flank of Mauna Kea, Hawaii- magma movement and magnetic changes

• 2004: San Andreas Scientific drilling Parksfield California 2.5 miles into San Andreas Fault buried sensors

• 2004: Integrated Ocean Drilling Project: Atlantis Massif Region of the Atlantic Ocean: .9 miles goal to penetrate earth’s mantle but crust proved thicker than anticipated

• 2006:1.5 mile bore hole into an impact crater at the mouth of Chesapeake Bay Cape Charles, Virginia

Data collection

Cameras

The Harry Hess Hypothesis: Sea Floor Spreading ( Mechanism!)

– Hypothesis sea floor moves away from the mid oceanic ridge crests and to the trenches as a result of mantle convection

– Evidence– Geophysical evidence of mantle convection– Ridge crest high heat flow- basalt eruptions– Trenches low heat flow– Ridge Crests (spreading axis) have thin or little palagic

sediment– Age of the sea floor ( 200 million years old)

1960’s The Mechanism- Sea Floor Spreading

• further support of the Plate Tectonic Theory comes from the work of Fred Vine and Drummond Matthews

• Vine and Matthews discovered from gravity and magnetic surveys of the ocean floor that there was a regular pattern of magnetic anomalies that were symmetrical. The pattern of magnetic anomalies of the mid-oceanic ridge was the same on both sides of the ridge.

Vine and Matthews Study of the Magnetism of Seafloor Rocks

– On the ocean floors magnetic reversals occur in a regular pattern that is mirrored on both sides of the mid-oceanic ridges

– – Magnetic reversals have since been dated from

land; the anomalies at the sea floor caused by the reversal is also dated and its distance from the spreading center or ridge is measured and the rate at which it has moved can be calculated

Paleomagnetic reversals recorded by basalt at mid-ocean ridges

• Paleomagnetic Reversals

Predicting the age of the sea floor

• Magnetic reversals are known to have occurred back to Precambrian time.

• The sequence of marine magnetic anomalies has been established for the past 160 million years and forms a distinctive pattern.

• It is documents on the ocean floor as well as on land

Additional Evidence

• Earthquake Patterns- associated with plate boundaries

• Deep sea drilling projects- the first in 1968 –The Glomar Challenger- discovered that the ocean floor was only 180 million years old- mostly made up of basalt- also ocean floor got older as you moved away from the mid-oceanic ridges

• Mantle Hotspots- Hawaii (islands seamounts and guyot)– Emperor seamount chain – 40 million years ago change in direction of Pacific

plate movement

• Re-fit of the continental jigsaw using the continental slope break

Deep-focus earthquakes occur along convergent boundaries

Earthquake foci in the vicinity of the Japan trench

The Hawaiian Islands have formed over a stationary hot spot

• Mantle Hot Spots (figure 19.46)

The Plate Tectonic Model• includes the concepts of:– Continental Drift– Mantle Convection– Sea Floor Spreading– Paleomagnetic Reversals

• supported by data:– age of the ocean floor

– distribution for earth quakes and volcanoes– relative thickness and types of sediment

• Plate Tectonic Theory!

The PT Model

• The upper part of the upper mantle and the crust make up the lithosphere which is rigid or solid

• The lower part of the upper mantle is the asthenosphere which is weaker (plastic)

• The lithosphere is broken into a series of rigid plates which moving and changing size and shape. The lithosphere is balanced on the asthenosphere

Plates

• Plates move at a rate of 1-10cm per/yr – The Breakup of Pangaea (figure 19.2)

• Seven big plates and many smaller plates– North American Caribbean– South American Nazca– Pacific-largest Phillippine– African Arabian– Eurasian Cocos– Australian Scotia– Antarctica

Three Types of Plate Boundaries

• Divergent- plates moving apart – mantle is closer to the surface – magma is cooling on the seafloor creating new oceanic crust – Divergent Plate Boundary (figure 19.21)

• Convergent- two plates are coming together- – Oceanic-Continental Convergence to Continental-Continental Convergence

• Transform Fault - Plates moving past each other- strike slip

Divergent Plate Boundaries

• Mid-oceanic ridges- spreading axis• Seafloor spreading-• Rifts and rift valleys- – The Red sea- East African Rift valley

Divergent boundaries are located mainly along oceanic ridges

• Divergent Lithosphere Plate Boundary (figure 19.21)

The East African rift – a divergent boundary on land

The Red Sea -NASA

^N

Convergent Plate Boundaries

• Classified based on the type of plates involved• Oceanic plate are made of basalt more dense

than continental crust. Denser plates moves “subduct” under less dense plate– Continental-Oceanic Convergent Boundary– Oceanic-Oceanic Convergent Boundary– Continental-Continental Convergent Boundary

Oceanic –Continental Convergence• Continental volcanic arcs- – Examples : The Andes Mountains and the Cascade Range, – Oceanic-Continental Convergence (figure 19.32)

Oceanic-Oceanic convergence• Volcanic island arcs- – Atlantic Ocean Lesser Antilles, Sandwich Islands– Pacific Ocean- Aleutian islands, Mariana and Tonga

islands– 120-180 mile arc-trench gap– Oceanic-Oceanic Convergence

Continental-Continental Convergence• Continental crust converge with continental

crust• Mountain building !!!!– Example- Himalayas, Alps. Appalachians, Urals– Continental-Continental Convergence (figure 19.33)

The collision of India and Asia produced the Himalayas ( Breakup of Pangaea)

Transform Fault Lithospheric Plate Boundaries

• Most are fracture zones on mid –oceanic ridges- approximately every 100 km (60 miles)

• San Andres is unusually because it cuts continental crust

Example 4: The San Andreas Fault System

a major transform fault lithospheric plate boundary

UC Santa Barbara ModelsWest Coast North America

• sm01PacN.mov (80 MYA- Present)

• sm02Pac-NoAmflat.mov (40 MYA –Present)

• sm16OldWestNoAm.mov (20 MYA- Present)

• sm03socalcities.mov (20 MYA- Present)

• sm12Plplshortening.mov (model)

UT Models

The Driving Mechanism for Plate Tectonics

• Three concepts:

– Mantle Convection

– Slab-Push and Slab-pull

– Rising Plumb and Descending Slabs