Chapter 3 Plate Chapter 3 Plate Tectonics Tectonics

Chapter 3 Plate Chapter 3 Plate Tectonics Tectonics

3-1 Earth’s Drifting Continents

• Theory-guess• A land bridge once stretched across the

Atlantic Ocean and connected South America and Africa

• Evidence for this includes fossils of plants and animals that could not have possibly crossed the ocean.

• Fossils-Preserved remains of ancient organisms.

• The earth once had a single landmass that broke up into large pieces, which have since drifted apart.

• The name of this giant landmass is Pangaea which means all earth.

• Wegner’s –Theory of continental drift • One supporting piece of evidence is the

fossil Glossopteris( extinct now longer living plant) located in rocks about 250 M years ago have been found in South Africa, Australia, India and Antarctica.

• Presence in Antarctica indicates that at one time the climate for Antarctica was much different than it is today.---end 57

Evidence from rocks• Rocks from Africa and South America piece

together.• An ancient folded mountain chain that

stretches across South Africa links up with an equally ancient folder mountain chain in Argentina.

• Coal fields with distinctive layers in Brazil line up with coal field with identical layers in Africa

• End 58

3-1 Section Review• What is continental drift? • The theory that the Earth had one

giant landmass that split to form today’s continents.

• Who first developed a scientific argument for continental drift?

• Alfred Wegener

• 2. How do scientists explain the existence of fossils of the same plants and animals on continents thousands of kilometers apart?

• Scientists use the theory of continental drift to explain this phenomenon.

3-2 Earth’s Spreading Ocean Floor

• How could the continents plow through hard, solid ocean floor?

• In 1950’s and 60’s they discovered a large system of underwater mountains that have a deep crack, called a rift valley running through their center. These underwater mountains are known as midocean ridges

• End 60

• Midocean ridges form the single largest mountain chain in the world. The chain is approx. 80,000 kilometers long-roughly 20 times the distance from NYC to LAX and 3 KM high.

• A great deal of volcanic activity occurs at the mid-ocean ridges. Lava erupts from the rift valley that runs the length of the ridge.

• As the ocean floor moves away on either side of the ridge lava wells up and hardens

• The hardened lava forms new ocean floor• This process is called ocean-floor

spreading.• Ocean-floor spreading helps to explain

how continents drift.• Although sections of the midocean ridges

are straight, the ridges as a whole curve because straight sections are offset by thin cracks called transform faults

• Rock samples from the ocean floor indicate that rocks next to a midocean ridge are younger than rocks farther away the youngest rocks are in the center of the ridge

• End 61

Section 3-2 Pg 62• Magnetic stripes in ocean-floor rocks

confirmed ocean-floor spreading• The history of the earth’s magnetism is

recorded in rocks, when molten rock hardens a permanent record of the earth’s magnetism remains in the rocks

• As a result, scientist learned that the earth’s magnetic poles reverse themselves from time to time-during the past 3.5 M years they have reversed 9 times

• The pattern of magnetic stripes is identical on both sides of a mid-ocean ridge.

• As magma hardens into rock at a midocean ridge half the rock moves in one direction and the other half moves in the opposite direction.

• The oldest rocks on land are 4 billion years old but the oldest rocks in the ocean are 200 million years old.

• Because the Earth’s surface remains the same size, the ocean floor is being destroyed as fast as it is being formed by ocean-floor spreading.

• End 62

63• Trenches-V shaped valleys that lie

along the bottom of the oceans, they are the deepest parts of the oceans close to continents or near strings of islands such as Alaska’s Aleutian Islands .

• Why is the location of these trenches significant?

• The trenches create the Ring of Fire.

Subduction• Older ocean floor moves away from the

midocean ridges as new floor is formed.• Eventually the older floor moves down deep

into the Earth along trenches.• Subduction-The process in which crust

plunges back into the Earth.• When the rocks are pushed deep enough they

are melted by the heat of the earth.• Some of the molten rock will rise up through

the crust and produce volcanoes,, but most will become part of the mantle.

• As new rocks are formed along the ocean ridges, older rocks are subducted into trenches, the process balances itselt. End 63

Section 3-2 Review• 1. What process helps to explain

how continents drift?• The process of ocean-floor

spreading helps to explain how continents drift

• 2. Where are the youngest rocks on the ocean floor found?

• The youngest rocks on the ocean floor are found at the midocean ridges

Section 3-2 Review• 3. How can the magnetic orientation

of rocks be used to trace the way a continent moved as it drifted?

• The magnetic orientation of rocks of different ages can be compared to get an idea of how the position of a continent has changed over time.

The short, sad life of Kid Crusty

The Life of Kid Crusty

Crusty Celebrates his birthday

• Happy 200 millionth Birthday

Crusty is attacked by subduction

Crusty meets his maker and is born again

3-3 Earth’s Moving Plates

• Plate- irregularly shaped slabs that fit together like paving stones to form the surface layer of the earth.

• Theory of Plate Tectonics-links together ideas of continental drift and ocean-floor spreading, explains how the Earth has evolved over time and helps to explain the formation, movements, collusions, and destruction of the Earth’s crust. 64

Lithospheric Plates• Lithosphere- The topmost solid part of the

earth• There are seven major lithospheric plates and

many smaller plates, some too small to list on a map

• They move at different speeds, and in different directions, small plates that lack landmasses move as much as several centimeters a year-large plates that are weighted down with landmasses may only move a few millimeters a year.

• In a few cases, the edges of the continents are the boundaries of the plates, however most plate boundaries are on the ocean floor. 65

Plate Boundaries• There are 3 types of boundaries• Divergent-Midocean ridges-aka

constructive boundaries-• Convergent-trenches-plates come

together at the trenches-cause pressure and friction-

• Strike-slip boundaries-Two plates grind together and slip past each other horizontally-earthquakes usually occur here

Plate Motion