Plate Tectonics

A Brief History of a Unifying Theory

Accumulation of Observations -

Evidence· Patterns of continents

· Paleontology· Geology

· Patterns of sea floor ages· Patterns of seafloor depth· Patterns of seafloor sediments · Patterns of magnetism· Patterns of volcanoes· Patterns of earthquakes

Plate Tectonics as the Unifying Concept of Earth Science

http://pubs.usgs.gov/gip/dynamic/historical.html

Earth’s Great Puzzle Pieces

• 1620 – Sir Francis Bacon observed similarities of coasts of Africa and South America … “no mere accidental occurrence.” A few years later it was suggested that they were once one, but had been separated by the Flood.

• 1782 – Benjamin Franklin, based on observed oyster shells on mountain tops “The crust of the Earth must be a shell floating on a fluid interior.... Thus the surface of the globe would be capable of being broken and distorted by the violent movements of the fluids on which it rested.”

• 1799 – Alexander Von Humbolt, German explorer and naturalist, observed the similarities in the geology and features of the west coast of Africa and east coast of South America (separated by a valley filled by the flood)

1858 - Geographer Antonio Snider-Pellegrini made these two maps showing his version of how

the American and African continents may once have fit together, then later separated

A Man and His Model

• Current: Contracting Earth• 1912: Continental Drift• Observations

• Fit of Continents• Geology• Paleontology• Climate belts

• Pangea (“all lands”) 300 -200 Ma• Breakup 180 Ma

• Rigid bodies moving through yielding seafloorAlfred Wegener

Eduard Seuss

The same land plant and animal fossils are found on separate continents!

• 1926• Based on

glossopteris fern

Fossils Match!

Geology Matches, Too!

Scientific Community says:

Scientific Community says:

No mechanism to make continental drift happen

Mechanism for Plate Movement!• Arthur Holmes (Late 1920’s)

• Interior of Earth has sluggish convection (transport of heat from core); hot stuff rises, cool stuff sinks

• New ocean crust injected into ocean floor

But from where?

Maurice Ewing

• Mapping the seafloor 1947-1959

• Lockney Texas

• Rice University Trained

• UTMB - Division of Earth and Planetary Sciences of the Marine Biomedical Institute

Maurice Ewing

• Mapping the seafloor 1947-1959• Surprises:

• Thin sediment• Basalt crust – glasses• Age less than 150 Ma

(hadn’t identified a pattern yet)

• Ridges – later shown to circle globe

• Valley within ridge (Tharp)• Earthquakes along ridges• High heat flow (Bullard)

Harry Hess and Seafloor Spreading

• 1962 – startling new theory “History of the Oceans”

• New ocean crust at mid-ocean ridges

• Ocean crust dragged down at trenches; mountains form here

• Continental crust too light; remains at surface

• Earthquakes occur where crust descends

“It explains everything….”

Rocks and Magnetism - Tools• When magma cools, takes on signature

of Earth’s prevailing magnetic field

• Three magnetic measurements can be taken from rocks

• Inclination - ~ latitude ~distance to the pole

• Declination - ~ direction to the pole

• Positive (normal) or negative (reversed) - depending on what Earth’s field is doing

• Add age = powerful tool

• Earth’s present magnetic field is called normal• magnetic north near the north geographic pole • magnetic south near the south geographic pole

• At various times in the past, Earth’s magnetic field has completely reversed• magnetic south near the north geographic pole • magnetic north near the south geographic pole

• 171 times in last 76 million years … takes 5,000 to 10,000 per reversal. Lasts 10’s of thousands to millions of years

Magnetic Reversals

Symmetric patterns of magnetism on either side of mid-ocean ridge

Vine and Matthews The Final Push – 1962-1963

Seafloor as a magnetic tape recorder

Magnetic Stripes on Seafloor

When magma cools, it takes on signature of Earth’s prevailing magnetic field

magnetic iron-bearing minerals align with Earth’s magnetic field

Oceanic Crust Is Young

Age of Continents

Original copyrighted image removed; there is an image available at http://

www.lithosphere.info/TC1-2006/TC1_Fig2-ages-062006.jpg

that may be copyrighted.

Tuzo Wilson

• Transform faults: opposite sense of movement than expected.

• Proven correct (Sykes)• Sealed theory of sea-floor spreading and plate tectonics

for most scientists • 1960s-1970s

Hydrothermal Vents

• The result of seawater percolating down through fissures in the ocean crust in the vicinity of spreading centers or subduction zones

• The cold seawater is heated by hot magma and reemerges to form the vents. Seawater in hydrothermal vents may reach temperatures of over 340°C (700°F)

• Discovered in 1977 while exploring an oceanic spreading ridge near the Galapagos Islands

Theory of Plate Tectonics• The upper mechanical layer of Earth (lithosphere) is divided into rigid

plates that move away from, toward, and along each other

• Most deformation of Earth’s crust occurs at plate boundaries

How can you calculate the rate of plate movement?

Calculating Plate Movement

• Pick an object and watch it … • Better on glaciers than on slow moving

plates …

• Use magnetic reversals … long time periods

• Date rocks across a mid-ocean ridge really really carefully … tedious

Northern Pacific Basin

Hawaii

HawaiianRidgeMidway

EmperorSeamount

Chain

Hot Spots

• Stationary magma chambers under mobile plates …

Prominent Hot Spots

Plate Movement Rates using Hot Spots

http://www.classzone.com/books/earth_science/terc/content/investigations/es0810/es0810page01.cfm?chapter_no=investigation

Tectonics on Other Planets?

Do you recognize either of these locations?

Mars Topography

Mars Magnetic Field