1) Alfred Wegener first proposed the theory of continental drift, which suggested that the continents were once joined together and have since drifted apart.
2) Wegener provided geological, fossil, and climate evidence to support his theory, but could not explain the force driving continental movement.
3) In the 1960s, the theory of plate tectonics was developed, explaining that convection currents in the mantle cause the movement of tectonic plates. Sea floor spreading provided key evidence that the ocean floor was pulling apart.
introduction of plate tectonics leading to finding the epicenter.
Divergent plate boundary is alsoincluded in the ppt. Search the activity sheet on this topic also uploaded here
introduction of plate tectonics leading to finding the epicenter.
Divergent plate boundary is alsoincluded in the ppt. Search the activity sheet on this topic also uploaded here
Steps to Plate TectonicsStep 1 – Continental Driftwww.mat.docxdessiechisomjj4
Steps to Plate Tectonics:
Step 1 – Continental Drift
www.math.montana.edu / ~nmp / materials / ess / geosphere / inter / activities / plate_calc / pangaea_map.gif
The Continental Drift hypothesis published by Alfred Wegener in his 1915 book “ The Origin of Continents and Oceans”, although this was partially based on the work of earlier investigators.
Continental Drift = the continents were once connected in a single supercontinent called Pangaea. They have since drifted apart and are still moving today.
http://www.kidsgeo.com/geology-for-kids/0042-pangaea.php
Wegener’s hypothesis had several problems:
1) He had no power source – no way to make the continents move.
2) He thought the continents moved through the seafloor just like boats move through the ocean, but there was no evidence of this (no wake)
3) He was a meteorologist so many geologists didn’t take him seriously!
BUT Wegener had lots of evidence to show that the continents were once connected!
1) The jigsaw puzzle-like fit of the continents.
Figure 2.3 in text
Identical fossil assemblages on now widely spaced continents!
best about 250-200 MY ago
become increasingly dissimilar the closer to today we look!
Garrison, 2012, Essentials of Oceanography
3) Sequences of similar rock types on continents which do not now have the same geologic environment!
http://www.geology.ohio-state.edu/~vonfrese/gs100/lect25/index.html
4) Geologic structures (mountain ranges, faults, chains of volcanoes) which match up on either side of oceans but can not be found underwater.
5) Apparent polar wander – paleoclimatic evidence the continents had very different climates 250 MY ago than they do today.
either the continents moved or
the climate bands moved – which means the Earth’s poles of rotation moved.
Earth’s climate zones today are arranged symmetrically around the poles.
http://www.webquest.hawaii.edu/kahihi/sciencedictionary/C/climatezone.php
Paleoclimatic data from ˶300 MY ago, figure 2.5 in text
After Wegener died, his ideas were largely dismissed, until…
Post-Wegener evidence for drifting continents (and plate tectonics)…
6) Apparent Polar Wander – Paleomagnetic evidence.
Figure 2.7 showing that the Earth has a magnetic field very similar to that created by a bar magnet.
Directions of magnets parallel to Earth’s magnetic field lines.
Post-Wegener evidence for drifting continents (and plate tectonics)…
Figure 2.7b showing how magnets align to the Earth’s magnetic field when allowed to move freely.
Rocks containing the mineral magnetite (especially basalt) record the orientation of the Earth’s magnetic field at the time the rocks formed.
Figure 2.8a showing apparent polar wander paths for Europe and North America.
Figure 2.8b showing alignment of polar wander curves if the Atlantic Ocean is “closed”
The polar wander tracks for all the continents show great variation, suggesting it is the continents that moved!
If we put the continents “back to.
Plate Tectonics
Chapter 19
Plate TectonicsPlate tectonics - Earth’s surface composed thick plates that moveIntense geologic activity is concentrated at plate boundariesCombination of continental drift and seafloor spreading hypotheses proposed in late 1960s
Review: Three Types of Plate Boundaries
But how do we
know that plates
move at all ?
Transform Convergent Divergent
(strike-slip) (subduction) (spreading)
Early Case for Continental DriftPuzzle-piece fit of coastlines of Africa and South America has long been known
In early 1900s, Alfred Wegner noted South America, Africa, India, Antarctica, and Australia have almost identical rocks and fossils
Early Case for Continental DriftGlossopteris (plant), Lystrosaurus and Cynognathus (animals) fossils found on all five continents Mesosaurus (reptile) fossils found in Brazil and South Africa only
Glaciers Most of the Earth's ice is found in Antarctic continental glacier. Where are some other continental glaciers ?
FIGURE 10.5 Iceberg calving at Wrangell-St. Elias National Park, Alaska. Calving occurs when huge blocks of ice break off at the edge of a glacier that has moved to a shoreline. [Tom Bean.]
Glacial striations on a rock from stones grinding at the base of a heavy ice sheet leave these shiny linear marks on the bedrock below.
FIGURE 10.18 Glacial striations on bedrock in Glacier Bay National Park, Alaska. Striations are evidence of the direction of ice movement and are especially important clues for reconstructing the movement of continental glaciers. [Carr Clifton.]
Glacial Characteristics Glaciers flow downhill as a solid mass that creates channels, and walls made of ground up rock debris known as a merraine.
Erosional LandscapesErosional landforms produced by valley glaciers include: U-shaped valleys Hanging valleysSmaller tributary glacial valleys left stranded above more quickly eroded central valleys
Early Case for Continental DriftWegner reassembled continents into the supercontinent Pangaea
Late Paleozoic glaciation patterns on southern continents best explained by their reconstruction into (Pangaea) Gondwanaland
Early Case for Continental DriftCoal beds of North America and Europe indicate Laurasia super continent
Continental Drift hypothesis initially rejected Wegener could not come up with viable driving force continents should not be able to “plow through” sea floor rocks
The Earth's Magnetic Field
Can Give Us Clues
Paleomagnetism and Continental Drift RevivedStudies of rock magnetism allowed determination of magnetic pole locations (close to geographic poles) Paleomagnetism uses mineral magnetic alignment and dip angle to determine the distance to the magnetic pole when rocks formedSteeper dip angles indicate rocks formed closer .
Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere.
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4. Wegener searched and found three main pieces of evidence. Geologic - evidence in the layers of rocks across continents Fossil - evidence in the places certain fossils are found Climate - evidence in the changing climates during the past
5. Fit of Continents Across the Atlantic Mountain ranges in South America line up exactly with those in Africa! Geologic evidence
6. Fossil evidence Notice how fossils lined up across continents! http://evolution.berkeley.edu/evolibrary/images/history/fossils3.gif
7. Amazing Facts: Did you know... ...that India was once in the Southern Hemisphere connected to Antarctica? ...that North America was once surrounded by warm, tropical seas? ...that Africa was once covered by glaciers, which were kilometers in thickness? ...that the Sahara desert was once a tropical rain forest? Climate change
8. As North America has drifted to different latitudes, changing climates were experienced. This shows the changes in the state of New Mexico. An example of the proof for changing climate: At one time, the area that is New Mexico (and the rest of the USA was below the equator!
9. Wegener's problem He could not find the force that was causing the continents to drift. Because of this, he could not convince anyone that continents could move. He died in Greenland on an expedition. At the time of his death, no one believed his hypothesis! Technology developed during the 1940’s changed all that!
11. Sea-Floor Spreading m.y. means million years ago Notice this compass . It is important for the next slide
12.
13. As the sea floor spreads, the lava cools according to the magnetic poles at the time. The rocks on the ocean floor have proved that the earth’s magnetic field sometimes reverses. The inner core flips and so the north pole moves to the southern hemisphere! The earth itself does not flip. Amazing proof of that the inner core flips! http://platetectonics.pwnet.org/img/blocks.gif
18. This is a model of sea floor spreading at a divergent boundary called a mid ocean ridge .
19. Did you know that the Earth’s longest mountain range is underwater and is called the mid-ocean ridge? : www.ocean.udel.edu The Mid-Ocean Ridge system, shown above snaking its way between the continents, is more than 56,000 kilometers (35,000 mi) long. It circles the earth like the stitching on a baseball!
24. There are three combinations of how earth's crust can come together. Continental crust to continental Continental crust to oceanic Oceanic crust to oceanic
29. Because one plate gets pushed under another, it is called subduction . This is where volcanoes form! Oceanic crust colliding with oceanic crust Oceanic crust colliding with continental crust all from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270
30. Andes Mts. From space View looking south – ocean to continental
32. The process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle is called subduction
33. Plate tectonics video clip Here is an great link for seeing where each type of boundary is located. Click here for a great hyperlink! Make sure you click on each type of boundary in the key!