Plate Tectonics as the
Unifying Concept of Earth Science
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

• 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.
http://pubs.usgs.gov/gip/dynamic/historical.html
• 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.”
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
• 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)

• Current: Contracting Earth
• 1912: Continental Drift
Observations
• Fit of Continents
• Geology
• Paleontology
• Climate belts
• Pangaea (“all lands”) 300 to 200 Ma
• Breakup 180 Ma
Alfred Wegener • Rigid bodies moving through yielding seafloor

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 (where?)

• Mapping the seafloor 1947-
1959
• Lockney Texas
• Rice University Trained
• UTMB Division of Earth and Planetary Sciences of the Marine
Biomedical Institute

• 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 midocean 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….”

• 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
 You are Here!
• 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 … take 5,000 to
10,000 per reversal; last 10’s of thousands to millions of years … review your storage media …

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

Seafloor as a magnetic tape recorder

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
A Final Blow …
• Transform faults: opposite sense of movement than expected.
• Proven correct (Sykes)
• Sealed theory of sea-floor spreading and plate tectonics for most scientists

• 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

• 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

• Stationary magma chambers under mobile plates …

Plate Movement Rates using Hot Spots http://www.classzone.com/books/earth_science/terc
/content/investigations/es0810/es0810page01.cfm?c
hapter_no=investigation

Do you recognize either of these locations?