Download Chapter 9 Plate Tectonics

Chapter 9
Plate Tectonics
Continental Drift
 More than 300 years ago, mapmakers produced world maps that
accurately showed the shapes of the continents.
 Once people had these accurate maps, they began to notice that
some continents fit together like pieces of a puzzle
 In 1915, Alfred Wegener proposed the hypothesis of continental
drift
 Continental Drift  all the continents had at one time been joined
together to form a single super continent
 Pangaea  name given to the super continent; means “all land”
Evidence for Continental Drift
 Evidence for Wegener’s hypothesis includes:
 Similar fossils
 Similar types of rock
 Traces of glaciation
 These were found on coastlines of widely separated
continents like South America and Africa
Matching Fossil
 Fossil Evidence  several similar fossil organisms are found
on different landmasses
 These animals could not have crossed the vast ocean to move
to another continent
 Ex: Mesosaurus
 Found only in southern South America and southern Africa
Rock Types
 Rock Evidence  matching types of rock in several mountain
belts that today are separated by oceans
 Ex:
 The Appalachian mountain belt on the East coast of North America
today ends in Newfoundland (Canada)
 However, there is evidence of the same rock types in a mountain
range in the British Isles and Scandinavia (Sweden/Finland)
Ancient Climates
 Climatological Evidence  various glacial deposits have been
found to show that large ice sheets covered areas of the
Southern Hemisphere that today have tropical climates.
 There is no way these ice sheets could have existed unless the
continents were in different locations in the distant past
Rejection of Wegener’s Hypothesis
 The main objection to Wegener’s hypothesis was that he
could not describe a mechanism capable of moving the
continents
 Luckily, some geologists continued to work on his hypothesis
 As technology got better, out ability to study the Earth also
got better
 This allowed for the theory of Plate Tectonics
Exploring the Ocean Floor
 As scientists began to study the idea of continental drift more
and more, their search led them to the ocean floor
 Where they expected the ocean to be very deep, they found
mountain ranges
 This helped to fuel the need to map the entire ocean floor
 Sonar (SOund NAvigation and Ranging)  a system that uses
sounds waves to calculate the distance to an object
Deep Ocean Trenches
 As scientists mapped the ocean floor, they found long,
curved alleys along the edges of some ocean basins
 Trenches  form the deepest parts of Earth’s oceans
 Ex:
 The Marianas Trench in the Pacific Ocean is over 11 km deep
(about 6.8 miles)
Mid-Ocean Ridges
 By the late 1950’s, scientists had constructed a more
complete map of Earth’s ocean floor
 This map showed that the mountain range found in the
Atlantic Ocean was not an isolated feature
 Mid-Ocean Ridge  a long chain of mountains extending the
length of the ocean
 Rift Valley  a deep, central valley that runs down the center
of a ridge; usually resembles a long canyon
Process of Sea-Floor Spreading
 In the process of sea-floor spreading, new ocean floor forms
along Earth’s mid-ocean ridges, slowly moves outward across
ocean basins, and finally sinks back into the mantle beneath
deep-ocean trenches.
 Sea-floor spreading  new oceanic lithosphere is formed and
the ocean floor gets wider
Subduction at Deep-Ocean Trenches
 Although new ocean floor is constantly being added at the
mid-ocean ridges, our planet is not growing larger
 This means the old ocean floor has to be going somewhere
 Subduction  ocean floor returns to the mantle as it sinks
beneath a deep ocean trench
Evidence for Sea-Floor Spreading
 Evidence for this theory includes:
 Magnetic stripes in ocean-floor rock
 Earthquake patterns
 Measurements of the ages of ocean floor rocks
Magnetic Strips
 Geophysicists learned that Earth’s magnetic field occasionally
reverses polarity
 The north magnetic pole becomes the south magnetic pole
and vice versa
 Scientists graphed these reversals going back millions of years
 Paleomagnetism  as certain rocks form, they acquire the
polarity that Earth’s magnetic field has at the time
Earth’s Moving Plates
 During the 1960’s, scientists realized that sea-floor spreading
explained part of Wegener’s idea of continental drift
 Canadian geologist J. Tuzo Wilson combined this evidence
together to develop a new theory of tectonic plates
 Plates  several huge pieces of Earth’s lithosphere that are
separated by deep faults
 Theory of Plate Tectonics  Earth’s lithospheric plates move
slowly relative to each other, driven by convection currents in
the mantle
Causes and Effects of Plate Motion
 Causes
 Convection currents within Earth drive plate motion
 Hot material deep in the mantle moves upward by convection
 At the same time, cooler, denser slabs of oceanic lithosphere
sink into the mantle
 Effects
 Plate motion averages about 5cm per year
 About as fast as your fingernails grow
 Earthquakes, volcanoes, and mountain building
Types of Plate Boundaries
 Divergent Boundaries  found where two of Earth’s plates
move apart
 Most divergent boundaries are spreading centers located
along the crests of mid-ocean ridges
 Some spreading centers, however, occur on the continents
Types of Plate Boundaries
 Convergent Boundaries  form where two plates move
together
 At convergent boundaries, plates collide and interact,
producing features including trenches, volcanoes, and
mountain ranges
 Continental Volcanic Arc  a range of volcanic mountains
produced in part by the subduction of oceanic lithosphere
 Volcanic Island Arc  newly formed land consisting of an arcshaped chain of small volcanic islands
Types of Plate Boundaries
 Transform Fault Boundaries  occur where two plates grind
past each other
 At a transform fault boundary, plates grind past each other
without destroying the lithosphere
What Causes Plate Motions?
 Convection Current  the continuous flow that occurs in a
fluid because of differences in density
 Warm material is less dense so it rises
 Cooler material is denser so it sinks
 Convection currents in the mantle provide the basic driving
forces for plate motions
Plate Motion Mechanisms
 The sinking of cold ocean lithosphere directly drives the
motions of mantle convection through slab-pull and ridgepush.
 Some scientists think mantle plumes are involved in the
upward flow of rock in the mantle
 Slab-pull  the force of gravity pulls old ocean lithosphere,
which is relatively cold and dense, down into the deep mantle
Plate Motion Mechanisms
 Ridge-push  the stiff ocean lithosphere slides down the
asthenosphere that is elevated near mid-ocean ridges
Plate Motion Mechanisms
 Mantle Plume  a rising column of hot, solid mantle rock