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Pangaea Theory
21
Transcript
Theories
Evidence Found
There are many theories as to how our Earth formed.
Plate Tectonics
Mesosaurus
Glossopteris
Plate Tectonics
- extinct genus of reptile from the Early Permian of southern Africa and South America
- fossils found in South America, Africa, Antarctica, India and Australia.
Continental Drift
Problems
- leaves were tongue-shaped) is the largest and best-known genus of the extinct order of seed ferns
- found in all of the southern continents provide strong evidence that the continents were once amalgamated into a supercontinent
- 70 different species found
- Abraham Ortelius contemplated idea in 1596
- the concept was independently and more fully developed by Alfred Wegener in 1912
- hypothesis that the continents had once formed a single landmass before breaking apart and drifting to their present locations
- the lithosphere is broken up into tectonic plates
- on Earth, there are seven or eight major plates (depending) and many minor plates → where plates meet, their relative motion determines the type of boundary: convergent, divergent, or transform. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation occur along these plate boundaries
- hypothesis that tectonic plates are composed of oceanic lithosphere and thicker continental lithosphere, each topped by its own kind of crust
Glacial Striations
- these are scratches or gouges cut into bedrock by glacial abrasion
- from the 17th century to the late 19th century, the world experienced a "Little Ice Age," when temperatures were consistently cool enough for significant glacier advances
- are found in Alaska and other various countries
Evidence
Climate Similarities
- continents do not plow through the ocean floor
- both continents and ocean floor form solid plates, which "float" on the asthenosphere
Wegener's hypothesis was specifically unable to provide a verifiable or satisfying mechanism by which continents—with all of their bulk and drag—could move over an underlying mantle that was solid enough in composition to be able to reflect seismic S-waves
- fossil distribution, distinctive patterns in rock strata in now unconnected parts of the world and the distribution of the world's coal
- geologists have found coal under Antarctica's very cold and dry ice caps
- found support and development in the work and writings of South African geologist Alexander Du Toit who discovered a similarity in the fossils found on the coasts of Africa and South America that were seemingly derived from a common source
Apparent polar wander paths provided the first clear geophysical evidence for continental drift, while marine magnetic anomalies did the same for seafloor spreading. Paleomagnetism continues to extend the history of plate tectonics back in time and are applied to the movement of continental fragments, or terranes.
Alfred Wegener
Harry Hess
Because we already know what plate tectonics are, let's discuss other formation theories
Sea-Floor Spreading
- achievements in meteorology and as a pioneer of polar research
- most remembered for advancing the theory of continental drift → hypothesized that the continents were slowly drifting around the Earth
- palaeomagnetism provided strong support for continental drift
- unifying theory of plate tectonics
- best known for his theories on sea floor spreading, specifically work on relationships between island arcs, seafloor gravity anomalies, and serpentinized peridotite
- suggesting that the convection of the Earth's mantle was the driving force behind this process
Arthur Holmes
- when oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere
- process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge
- He pioneered the use of radioactive dating of minerals
- the first earth scientist to grasp the mechanical and thermal implications of mantle convection, which led eventually to the acceptance of plate tectonics.
- earlier theories (by Alfred Wegener) of continental drift were that continents "ploughed" through the sea
- the idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s
- the theory is well-accepted now, and the phenomenon is known to be caused by convection currents in the plastic, very weak upper mantle, or asthenosphere
Mid-Ocean Ridges
Oceanic Trench
What Could Our World Look Like
Geomagnetic Reversals
- underwater mountain system that consists of various mountain ranges, typically having a valley known as a rift running along its spine, formed by plate tectonics
- this type of oceanic ridge is characteristic of what is known as an oceanic spreading center, which is responsible for seafloor spreading
- following the discovery of the world-wide extent of the mid-ocean ridge in the 1950s, geologists discovered and began to propose mechanisms for sea floor spreading
- plate tectonics was a suitable explanation for sea floor spreading, and the acceptance of plate tectonics by the majority of geologists resulted in a major paradigm shift in geological thinking
- hemispheric-scale long but narrow topographic depressions of the sea floor
- articulation of the seafloor spreading hypothesis in the early 1960s and the plate tectonic revolution in the late 1960s the term “trench“ has been redefined with plate tectonic as well as bathymetric connotations
- a change in the Earth's magnetic field such that the positions of magnetic north and magnetic south are interchanged
- it was discovered that some rocks would reverse their magnetic field while cooling, it became apparent that most magnetized volcanic rocks preserved traces of the Earth's magnetic field at the time the rocks had cooled