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 ( SO und NA vigation and R anging)  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  length of the ocean a long chain of mountains extending the 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 slowly relative to each other, driven by convection currents in the mantle  Earth’s lithospheric plates move

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  move apart found where two of Earth’s plates  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  together form where two plates move 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 arc shaped chain of small volcanic islands

Types of Plate Boundaries

 Transform Fault Boundaries  past each other occur where two plates grind  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 ridge push.

 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