Spring Semester 2017 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
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 Cape
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).
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?
Idea was considered unlikely by northern (and especially U.S.) geologists.
Predictions about the oceanic basins of the two models:
At least some is granitic
At least as old as continents
At least as old as continents
At least some terrestrial/continental
New data from the sea floor, using World War II (and later) technology:
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!
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!
Geologists discovered in continental rocks of the recent geologic past that the magnetic field has changed polarity through time: normal vs. reversed.