Plate TectonicsChapter 10 – Section 1

Ch. 10-1: Continental DriftSummarize Wegener’s hypothesis of

continental drift.Describe the process of sea-floor spreading.Identify how paleomagnetism provides

support for the idea of sea-floor spreading.Explain how sea-floor spreading provides a

mechanism for continental drift.

Wegener’s HypothesisEarly map makers noticed that the edges of the

continents looked like puzzle pieces that may fit together.

Alfred Wegener was a German scientist and in 1912 he proposed the continental drift theory.He hypothesized that all the continents were once

part of a single landmass that he called a supercontinent.

250 million years ago they began to break up and drift to where they are now found today

Speculated that mountain ranges were areas of Earth’s crust that were crumpled up as the land masses moved

Evidence for Wegener’s TheoryFossil Evidence – fossils of certain animals

were found on multiple continents. These animals could not swim and there was no evidence of land bridges.

Evidence from Rock Formations – Mountain ranges of similar age and composition extended across several continents. If lined up, they would have been part of the same chain

Climatic Evidence – Plant and debris showed evidence of glaciers in now tropical areas. Also, subtropical swamps once covered areas that are now much colder

Missing MechanismsWegerner’s theory did not take off during his

lifetime because he was unable to explain how the continents actually drifted. He proposed that continents plowed through

the rock on the ocean floor, but that was easily shown not to be true.

Wegener died in 1930 still trying to find a plausible explanation for continental drift.

Mid-Ocean RidgesIn 1947 the evidence needed to support

Wegener’s theory came to light when a group of scientists began studying the Mid-Atlantic Ridge.Mid-ocean ridges are undersea mountain

ranges w/ a steep, narrow valley running through the center.

They noticed that the sediment covering the ocean floor was thicker the further they travelled from the ridge.

They also found that the closer to the ridge, the younger the sediment and actual rock layers were.

Figure 5, p. 242

Sea-Floor SpreadingFigure 6, p. 243In the late 1950’s, Harry Hess, a geologist,

proposed that at the center of a valley there was a rift, or crack, that would form as the two edges of the ridge pulled apart.Magma would then rise to the surface and fill the

crack, cooling into new rockThe ridge would move apart again and a new crack

would form, magma would rise and cool, newer rock formed again.

Robert Dietz named this process sea-floor spreadingEvidence to support this theory came in the form of

paleomagnetism in the mid 1960s.

PaleomagnetismFigure 7 , p. 244Paleomagnetism is the residual magnetism of

rock as it solidifies from iron-rich minerals in magma.

Magnetic reversals – using patterns of normal polarity and reversed polarity to form the geomagnetic reversal time scale.

Figure 8, p. 245Magnetic symmetry: matching regions of

polarity on either side of the rift valley.

Wegener RedeemedThe reversal patterns of the ocean floor also

matched rocks found on land and supported the continental drift theory.

Now that scientists could see the mechanism behind the theory, they supported Wegener’s original idea.

Plate TectonicsChapter 10 – Section 2

Ch. 10-2 The Theory of Plate TectonicsSummarize the theory of plate tectonics.Identify and describe the three types of plate

boundaries.List and describe three causes of plate

movement.

Plate TectonicsThe theory that explains why and how continents move.

The study of the formation of features in Earth’s crust.Lithosphere is the crust or thin outer shell of Earth.

It is broken into several blocks called tectonic plates, figure 1, p. 247

Aesthenosphere is a layer of “plastic” rock that moves much like silly putty and allows the tectonic plates to move across it.

Oceanic crust is very dense rock that is rich in Mg and Fe

Continental crust is less dense rock and is rich in SiTectonic plates may be one or both types of crust

Tectonic Activity15 major plates have been identified – Figure

2, p. 248Earthquakes – sudden movements or shifts in

plates along the boundaries trigger earthquakes. Plotting locations of quakes can help locate plate boundaries

Volcanoes – locations of these also help locate plate boundaries. Example: Pacific Ring of Fire

Types of Plate BoundariesDivergent – 2 plates are pulling away from each

other. Figure 3, p. 249Forms mid-ocean ridges and rift valleys

Convergent – 2 plates are colliding. Figure 4, p. 250Three types of collisions can occur at convergent

boundariesOceanic & Continental lithospheres collide & oceanic

lithosphere subducts. Deep ocean trenches form at subduction zones.

2 continental plates collide, edges crumple and lift up forming mountain chains

2 oceanic plates collide, one subducts, magma may rise and form an island arc.

Types of Boundaries Cont’dTranform boundaries – 2 plates slide past

each other horizontally. Figure 5, p. 251Transform boundaries do not produce

magma, but they do form fault lines or fracture zones.

Short segments of mid-ocean ridge can be connected by transform boundaries forming a fracture zone

Table 1, p. 251 summarizes all 3 types of plate boundaries and movements that occur at each.

Causes of Plate MotionForce behind plate movement is not fully

understood, but most believe convection to play a part.

The cycle is called a convection cell – Figure 6, p. 252

Mantle convection may occur due to energy generated in Earth’s core and radioactivity w/in the mantle.

Convection currents may drag along the bottom of the plates, pulling/pushing them across the surface of the aesthenosphere.

Causes of Plate MotionRidge Push – Newly formed rock at a mid-

ocean ridge is less dense. As it cools it begins to become more dense, it sinks, and it pushes against the existing plate Figure 7, p. 253

Slab Pull – In an area of a subduction zone, one are of lithosphere is being pulled below another. As the plate begins its downward movement, the rest of the plate is pulled along with it. Figure 8, p. 254.

Plate TectonicsChapter 10 – Section 3

Ch. 10-3: The Changing ContinentsIdentify how movements of tectonic plates

change Earth’s surface.Summarize how movements of tectonic plates

have influenced climates and life on Earth.Describe the supercontinent cycle.

Reshaping Earth’s CrustThe continents are constantly changing in

shape and size. They always have been and always will.Cratons – large areas of stable rocks that are

older than 540 millions years old.Shields – Rocks within the cratons that have

been exposed at Earth’s surface.Rifting and Continental Reduction – Figure 1,

p. 255Rifting is the breaking apart of continentsMay be due to a build-up of heat in Earth’s

interior, as Si is an insulator in continental crust

Terranes & Continental GrowthContinents may also grow by gaining new

material, such as terranesTerranes are pieces of lithosphere that have

unique geologic histories compared to the surrounding lithosphere.

Figure 2, p. 256 show terranes becoming part of a continent at convergent boundaries, a process called accretion.

Types of materials that form terranes may include sea-mounts, atolls, or continental crust (mountain chains)

Effects of Continental ChangeChanges in climate

Location in relation to poles or equatorLocation in relation to oceans, mountains, or

other continentsEvidence shows that even Africa’s Sahara

desert was once covered by glaciers.Changes in life – Figure 3, p. 257

New species can form due to rifting, separating an existing species

Isolation can allow for unique adaptations to occur

The Supercontinent CycleThe process by which supercontinent form

and break apartConvergent plate boundaries collide, heat

builds up underneath continental plates, and they break apart once again. Figure 4, p. 258-259

Pangaea formed 300 million years ago, Panthalassa was the single large ocean that surrounded it.

250 million years ago Pangaea broke apart into Laurasia and Gondwanaland, further rifts and collisions brought continents to current locations.

Geography of the FutureFigure 5, p. 260Continued plate movement at current pace will cause

Africa to collide with Eurasia, closing the Mediterranean Sea

A new ocean will form as east Africa breaks from the rest of Africa. North & South America will move east across the Atlantic Ocean, causing it to close as the continents collide with Africa.

Part of North America will shift to where Alaska sits today, LA will sit north of where San Francisco currently is

In 250 million years the continents will form the next supercontinent

How might climates change??