Continental Drift German scientist Alfred Wegener proposed the
idea of continental drift which stated that Earths continents had
once been joined as a single landmass Called this landmass Pangea,
greek for all the Earth Wegener proposed Pangea broke apart 200
million years ago
Slide 4
Evidence from Rock Formations As Pangea broke apart, mountain
ranges would have fractured So similar rock types should be found
on opposite sides of the Atlantic Ocean Appalachian Mountains (US)
share similar rocks in Greenland and Europe All older than 200
million years
Slide 5
Evidence from Fossils Similar fossils of different plants and
animals have been found all over the world on widely separated
continents Fossils are older than 200 million years
Slide 6
Ancient Climatic Evidence Rocks provide clues to past
environments and climates Evidence of coal was found on Antarctica
Coal forms from dead swamp plants Glacial deposits found on Africa,
India, Antarctica
Slide 7
A Rejected Hypothesis Wegeners continental drift hypothesis was
not widely accepted until the 1960s Wegener could not prove how or
why the continents moved before his death in 1930
Slide 8
Seafloor Spreading Section 17.2
Slide 9
Help from Technology Most people thought oceanic crust was
unchanging and older than continental crust Echo-sounding
technology (SONAR) sent sound waves to the ocean floor to measure
depth A magnetometer is a device that detects small changes in
magnetic fields
Slide 10
Ocean Floor Topography Maps made from data showed vast,
underwater mountain chains and ocean ridges Also discovered
earthquakes and volcanoes along the ridges
Slide 11
Ocean Floor Topography Cont There are also deep- sea trenches,
which are long, narrow depressions The deepest trench, the Mariana
Trench, is in the Pacific ocean and is 11 km deep
Slide 12
Magnetism Rocks (like basalt) with iron provide record of
Earths magnetic field The study of the magnetic record is
paleomagnetism Magnetic Reversals or changes in Earths magnetic
field were found in basalt flows Normal polarity is when the
magnetic field is aligned with earths present field Opposite is
called reversed
Slide 13
Magnetic Symmetry As data was collected, scientists noticed a
series of positive and negative stripes parallel to ocean ridges
The patter was identical on each side of the ridge
Slide 14
Seafloor Spreading Seafloor spreading states that the new ocean
crust is formed at ocean ridges and destroyed at deep sea trenches
Magma is less dense than oceanic crust so its forced up, fills the
gap and hardens As spreading continues, magma forces up and hardens
as well http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p
_seafloorspreading.html
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p
_seafloorspreading.html
Slide 15
Theory of Plate Tectonics Section 17.3
Slide 16
Theory of Plate Tectonics The theory states that Earths crust
and rigid upper mantel are broken into enormous slabs called plates
Plates move in different directions and at different rates
Slide 17
Plate Boundaries Tectonic plates interact at places called
plate boundaries Plates either come together (converge), move away
from each other (diverge) or slide past each other
Slide 18
Divergent Boundaries Places where two tectonic plates move
apart from one another are called divergent boundaries Most are
found on the seafloor where they form ocean ridges and where
seafloor spreading occurs Over millions of years, seafloor
spreading at divergent boundaries causes the ocean basin to grow
wider
Slide 19
Convergent Boundaries Places where two tectonic plates are
moving toward each other are convergent boundaries There are three
types of convergent boundaries At oceanic-oceanic boundaries, one
plate descends beneath the other in a process called subduction
Arcs of volcanic islands often form around this boundary
Slide 20
Convergent Boundaries Cont At oceanic-continental convergence
boundaries, subduction Creates a volcanic mountain range rather
than a volcano island arc
Slide 21
Convergent Boundaries Cont The third convergent boundary is
continental-continental Continental plates are pulled into
subduction zones but are too buoyant to be forced under The
colliding edges crumple and uplift the form a mountain range
Slide 22
Transform Boundaries Transform boundaries occur where two
plates slide horizontally past each other, usually seen as long
faults At divergent boundaries new crust is formed At convergent
boundaries old crust is destroyed At transform boundaries crust is
deformed or fractured *San Andreas Fault- Most studied fault in the
world; produces CA earthquakes