Plate Tectonics

Chapter 10-1 Continental Drift

Objectives:Describe the hypothesis of continental drift.Identify evidence supporting continental drift.

The Theory of Continental Drift

The theory of continental drift, stated by Wegner, a German meteorologist, says that Earth once had a single landmass that broke up into pieces, which have since drifted apart.Scientists call this giant landmass, Pangaea, which means all Earth.

Wegener’s Evidence

Wegener’s evidence for continental drift came form four areas:Fit of the coastlinesFossilRocksClimate

Fit of the Coastlines

Many of the landmasses, such as South America and Africa, appear to fit together as if they are pieces of a puzzle.

Evidence from Fossils

Evidence from identical fossils found in both South America and Africa supports Wegner’s theory.Fossils of Glossopteris, an extinct plant, found in rocks from South Africa, Australia, India and Antarctica provide support. The seeds of this plant were too large to have been carried by the wind and too fragile to have survived the ocean waves.

Antarctica

The presence of Glossopteris fossils in the frozen wasteland of Antarctica also indicate that the climate must have changed greatly. Antarctica must have changed position.

Evidence from Rock

An ancient folded mountain chain formed in Africa lines up with matching folded mountains in South America.Coal field layers found in Brazil line up with coal field layers in Africa.

Evidence from Climate

Glacial deposits found in areas of warm climate also provide evidence.Salt deposits normally formed between 35 and 10 degrees North and South of the equator are found as far north as Michigan.

Break Up of Panagaea

How did the continents move?

Wegener could not explain how the continents moved. He though they plowed through the ocean floor. It did not seem possible that the continents could plow through the harder basaltic rock of the ocean floor. Most scientists did not believe his theory until later it was explained how the continents moved.

Chapter 10-2Sea floor Spreading

Objectives:Explain seafloor spreading.Recognize how age and magnetic clues support seafloor spreading.

Mapping the Ocean Floor

During World War I German scientists introduced the idea of using sound waves to detect submarines. In the 1940s scientists began to use sound waves to map the ocean floor.This is sometimes called echo sounding.

Earth’s Spreading Ocean Floor

Scientists discovered a large system of underwater mountains that have a deep crack, called a rift valley, running through their center, known as the mid-ocean ridges. A great deal of volcanic activity occurs at the mid-ocean ridges. Lava erupts from the rift valley. The hardened lava forms new ocean floor. This process is known as sea-floor spreading.

Faults along the Mid-Ocean Ridge

Ocean-floor spreading helps to explain how continents drift. As a piece of the ocean floor moves, it takes its continent (if it has one) with it.Individual sections of mid-ocean ridges are straight but the ridge curves. It curves because the straight sections are offset by thin cracks known as faults.The youngest rock is near the mid-ocean ridge. As the floor spreads, older rocks move farther away from the ridge.

Magnetic Stripes

A permanent record of the Earth’s magnetism remains in the rocks.Scientist discovered that the Earth’s magnetic poles reverse themselves from time to time.Studies show that during the past 3.5 million years, the magnetic poles have reversed themselves nine times. The pattern is identical on both sides of the mid-ocean ridge.

Ocean Floor

The ocean floor is divided into three big areas: margin, basin and ridge.

The ridge is an underwater mountain chain formed by cooling magma that comes out the division between plates. As they cool, they smoke and are called black smokers.

Mid Ocean Ridge

The mid ocean ridge is the longest topographic feature on Earth.There is always a rift valley on the top of an ocean ridge system.

The Continental Margin

The margin is made up of the shelf, the slope and the rise.The shelf is the submerged part of the continent.The slope is the area that drops off into the ocean floor.The rise is made up of the sand that is dropped at the bottom of the slope.

Subduction

As the older ocean floor moves away from the mid-ocean ridges, it will eventually move down deep into the Earth along the trenches, (a long narrow valley on the ocean floor). When the rocks are pushed deep enough, they are melted by the heat of the Earth. Some of it will rise up through the crust and produce volcanoes along the trench but most of the molten rock will become part of the mantle.

Chapter 10-3The Theory of Plate Tectonics

Objectives:Compare and contrast different types of plate boundaries.Explain how the heat inside the Earth causes plate tectonics.Recognize features caused by plate tectonics.

Plate Tectonics

In the 1960s scientists developed a new theory that combined continental drift and seafloor spreading. This new theory was called plate tectonics. According to this theory the earth is broken into irregularly shaped sections that move on a plastic like layer called the mantle.

Composition of Lithospheric Plates

Plates are made of crust and a part of the upper mantle. These two parts combined are the lithosphere. It is rigid and about 100 km thick.The plastic like layer below the lithosphere is known as the asthenosphere.

Seven Lithospheric Plates

The Pacific plate covers 1/5 of the Earth’s surface. The other major plates are the North American, South American, Eurasian, African, Indo-Australian and Antarctic plates.There are also smaller plates such as the Caribbean and Arabian plates.

Plate Boundaries

Plates move at different speeds and in different directions. Smaller landmasses move more quickly.In a few cases, the edges of the continents are the boundaries of plates.Most plate boundaries are on the ocean floor.

Types of Plate Boundaries

There are three types of plate boundaries.Divergent BoundariesConvergent BoundariesTransform Boundaries

Divergent Boundaries

The first type occurs at mid-ocean ridges. Because the plates move apart, the ridges are called divergent. These boundaries are also called constructive boundaries. The average rate of seafloor spreading is 5 cm/yr.

Ocean Floor

None of the ocean floor is older than 180 million years.The East Pacific Rise is spreading more rapidly than the Mid Atlantic Ridge.

Continental Rifting

Sometimes there will be a divergent boundary under a land mass. This is happening on the African plate today.This is known as continental rifting.

Red Sea

The Red Sea was a continental rift valley in the past.As Africa and Saudi Arabia moved apart, water moved into the rift valley created and formed a sea.

Convergent Plate Boundaries

There are three types of convergent plate boundaries:Ocean - OceanOcean - ContinentThis type of plate boundary has trenches. Because the plates come together at the trenches, these boundaries are called convergent. They are also called destructive.Continent – ContinentThis does not produce a trench but some land is destroyed.

Convergent BoundariesRing of Fire

The collision of plates at convergent boundaries causes tremendous friction and pressure.Earthquakes and volcanoes often result. This explains why the Ring of Fire follows the major ocean trenches in that area.

Ocean-Continent Plate Boundary

Oceanic plates are more dense than continental plates. When an oceanic plate collides with a continental plate, the continental plate rides over the edge of the oceanic plate and pushes the ocean plate underneath. This is known as subduction.The subduction of the oceanic plate causes the plate to start to melt back into magma. Some of the magma bubbles up through the land and forms continental volcanic mountain ranges (arcs) such as the Andes of South America and the Cascades of North America.

Ocean- Ocean Plate Boundary

When two oceanic plates collide, the older oceanic plate is subducted under the younger. (Plates grow denser as they cool.)The plate being subducted melts. Molten rock then rises up and breaks through the surface. As a result, a string of volcanoes erupts on the ocean floor along the trench. Sometimes they may rise above the ocean’s surface as a string of islands, such as Japan, the Tonga Islands, the Philippines and the Aleutian Islands.

Continent-Continent Plate Boundary

When two continental plates collide, the edges of the continents fold upward to form large mountain ranges, such as the Appalachian Mountains. They were formed when Africa collided with North America during the formation of Pangaea. The Urals formed when Siberian plate collided with the Baltic plate and North America.The Himalayan Mountains formed when India collided with Eurasia after the break up of Pangaea.

Transform Boundaries

The third type of plate boundary is formed when plates move side by side.

No new plate material is destroyed or made.

Earthquakes often occur along strike-slip boundaries, such as the San Andreas Fault.

Pacific-North American Plate Boundary

The Pacific Plate is moving in a northwest direction. The North American plate is moving to the southwest. Eventually the part of California on the Pacific Plate will move up toward Alaska and collide with the Aleutian Islands.

Causes of Plate Tectonics

Today most scientist think plates move as convection currents in the mantle circulate.A convection current is the movement of material caused by differences in temperature. Mantle material close to the core is very hot. Mantle material farther from the core is cooler and more dense. Rising and sinking of magma occurs in a circular motion.

Faults

When rocks break and move along surfaces, a fault occurs.Faults interrupt rock layers by moving them out of place.Entire mountains can form this way and are called fault-block mountains.

Evidence of Plate Movement

Magnetic stripes on ocean floorVolcanoes and earthquakesLasers and satellites

Hawaiian Islands

The Hawaiian Islands show direction of plate movement as well as the rate of movement in the past.The Hawaiian Islands formed over a hot spot in the Pacific Plate. As the plate moves over the hot spot new islands are created with the youngest island being over the hot spot.

Possible Changes in Plates

Any change in one plate or boundary affects all the other plates and boundaries. There are many changes that can occur in plates and their boundaries. Continental plates may fuse together. A trench may switch direction and begin to subduct a formerly overriding plate. New divergent boundaries may form in the center of continents. Plates may also be completely subducted and disappear.

Prediction of our Future World