GEOL 102 Historical Geology
Spring Semester 2012
Another Geography: Plate Tectonics
These facts, unknown to the vulgar, but well known to all who observe nature, force the physical scientist to recognize that all the
surface of our globe has changed; that it has had other seas, other continents, another geography -- Nicholas Boulanger (1722-1759)
Some of the original evidence of mobile continents was based on geography (
matching coastlines between continents, for instance), but much was based on historical geology:
- Similarity of Coal Age (Pennsylvanian Period) coal flora in Europe and North America
- The Glossopteris flora (a particular suite of plants, characterized by
the primitive seed plant shrub
Glossopteris) of the late Paleozoic of India,
Australia, South Africa, and South America (and later Antarctica) and distribution of other
late Paleozoic/early Mesozoic fossils
- Glacial striations of India, Australia, Madagascar, South Africa, and South America,
which only line up if these continents were joined
- Deposits of the Karoo Supergroup (late Paleozoic and early Mesozoic Eras) of
southern Africa, which thicken towards (and have coarser, less rounded sediments towards)
The first of these points suggested some kind of linkage between Europe and North America,
the latter points suggesting that the southern continents (South America, Antarctica,
Africa, Madagascar, India, and Australia) were once linked together (named Gondwana
by Suess in 1885).
Alternative hypothesis: land bridges, intercontinental connections between landmasses
which have now subsided (still would not explain the glacial striation patterns).
Alfred Wegener proposed model of continental drift (in 1915):
- All continents unified into a single landmass (Pangaea), and all oceans a
single ocean (Panthalassa) during the Triassic
- Some force broke up the continents, which drifted away from each other
- Wegener's model suggested that the continents somehow skidded over the oceanic rocks
Wegener was NOT laughed at by all geologists (as some think): his ideas caught on in the
southern continents and had some followers in Europe. An early follower was Alexander du Toit (of South Africa)
- Shared presence of the freshwater reptile Mesosaurus in South Africa and
South America; of the protomammals Lystrosaurus, Cynognathus and
Thrinaxodon and related forms in South Africa, South America (and later discoveries
in Madagascar, Australia, India, and Antarctica)
- Also, some mountain ranges which are now separated but which have similar geology
become parts of the SAME mountain range if the contients are reassembled
- Proposed the name Laurasia for the northern equivalent of Gondwana: North
America, Greenland, Europe & Asia
However, big flaws in continental drift:
- What mechanism drives the drift?
- How can the continents glide over the ocean basins? (Physically impossible)
Idea was considered unlikely by northern (and especially U.S.) geologists.
Predictions about the oceanic basins of the two models:
|Record ||Land Bridges ||Continental Drift
|Seafloor Bedrock ||At least some is granitic ||Basaltic
|Seafloor Age ||At least as old as continents ||At least as old as continents
|Seafloor Fossils ||At least some terrestrial/continental ||All marine
New data from the sea floor, using World War II (and later) technology:
- "Polar Wander":
- In 1950s geologists discovered that rocks which had been heated to the Curie point
would retain the orientation of the magnetic field they cooled in. Found that:
- Few rocks pointed at the modern magnetic poles
- Rocks of different continents of some time periods (Cenozoic, later Mesozoic, early
Paleozoic) pointed at more than one apparent pole, but those of the late Paleozoic and
early Mesozoic pointed at the same pole (and most Paleozoic Gondwanan rocks pointed at
the same pole as each other)
- Was the magnetic pole of the Earth wandering?
- If the continents were reassembled in various configurations, found that the
"polar wander tracks" would converge: the poles weren't moving, but the continents were!
- Sea-floor Spreading:
- American geologist Harry Hess re-examined sonar soundings of the Pacific
(taken when he was an officer on a U.S. Navy vessel in WWII). Found:
- Many guyots (flat-topped seamounts) in deep sea: shape suggested that they had
once been eroded at sea level
- Later cores drilled from these guyots showed shallow-water Cretaceous fossils
- Depths of guyots (and sea floor in general) got deeper away from mid-ocean ridges
- Amount of oozes on bottom of sea floor only 1/16th what it should be if ocean basins
were 4 billion years old
- Number of volcanic islands in oceans only about 1/16th what it should be if ocean
basins were 4 billion years old
- Suggested a new model of continent motion: sea-floor spreading, with predictions:
- Ocean basins are much younger than continents
- New ocean basin rock is generated at mid-ocean ridges, spreading out from these
by convection cells in the mantle
- As the rock moves away from ridges it cools and sinks: islands become guyots
- Old ocean crust sinks back into mantle at deep-sea trenches
- Continents do not plow through oceans or over oceans: they move with oceans!
- Radiometric and biostratigraphic data confirmed: no ocean rocks older than Jurassic
Period, and most are MUCH younger!
In 1960s, the models of continental drift and sea-floor spreading were combined by
John Tuzo Wilson and colleagues to form
- Earth's surface is comprised of numerous rigid lithospheric plates
- Plates themselves carry thick continental and/or thin oceanic crustal rock
- New material generated at divergent boundaries (mid-ocean ridges in sea,
rift valleys on land)
- Plates slide past each other at transform boundaries
- Oceanic crust is lost under other ocean crust or continents at subduction zones
(site of deep-sea trenches, earthquakes, volcanoes, etc.)
- Two continental masses meet at collisional boundaries
Plate boundaries, from online USGS pamplet "This Dynamic Earth".
Each has particular type of geological record.
Differences plate tectonics has from Wegener's "continental drift":
|Record ||Plate Tectonics
|Seafloor Bedrock ||Basaltic
|Seafloor Age ||Younger than vast majority of continental material
|Seafloor Fossils ||All marine
Big change from the 1960s-1970s model: now recognize there are LOTS of little plates
(terranes) rather than just a few big plates.
To Next Lecture.
To Previous Lecture.
Last modified: 19 January 2012