Plate Plate TectonicsTectonics

Section 1: Continental DriftSection 1: Continental Drift

Alfred Wegener suggested that all continents were Alfred Wegener suggested that all continents were joined together at some time in the past. This is called joined together at some time in the past. This is called the hypothesis of “the hypothesis of “continental drift”continental drift”..

According to continental drift, continents have moved According to continental drift, continents have moved slowly to their current locations.slowly to their current locations.

He called this large landmass as He called this large landmass as PangeaPangea (means all (means all land)land)

Besides the puzzlelike fit of continents, fossils Besides the puzzlelike fit of continents, fossils provided support for continental drift.provided support for continental drift.

For example; fossils of reptile For example; fossils of reptile MesosaurusMesosaurus have have been found in south America and Africa. been found in south America and Africa.

Another fossil that supports the continental Another fossil that supports the continental drift is drift is GlossopterisGlossopteris which has been found in which has been found in Africa, Australia, India, South America, and Africa, Australia, India, South America, and Antarctica.Antarctica.

Climate Clues Climate Clues

Fossils of warm weather plants were found in arctic Fossils of warm weather plants were found in arctic ocean.ocean.

Glacial deposits and rock surfaces scoured and Glacial deposits and rock surfaces scoured and polished by glaciers are found in South America, polished by glaciers are found in South America, Africa, India and Australia. Africa, India and Australia.

Similar rock structures are found on different Similar rock structures are found on different continents. For example, parts of Appalachian continents. For example, parts of Appalachian mountains are similar to those found in Greenland mountains are similar to those found in Greenland and western Europe. and western Europe.

Although Wegener provided evidence to Although Wegener provided evidence to support his hypothesis, he couldn’t answer support his hypothesis, he couldn’t answer two questionstwo questions

1.1. What was causing the continents to move?What was causing the continents to move? Wegener said that this force might be the rotation of Wegener said that this force might be the rotation of

Earth, however, physicists were able to show that this Earth, however, physicists were able to show that this force was not great enough to move continents.force was not great enough to move continents.

2.2. How were the continents moving?How were the continents moving? Wegener proposed that the continents were Wegener proposed that the continents were

plowing through a stationary ocean floor. But, plowing through a stationary ocean floor. But, his peers argued that continents could not push his peers argued that continents could not push through the ocean floor without fracturing, through the ocean floor without fracturing, because crustal rock is too brittle. And no because crustal rock is too brittle. And no evidence of fracturing had been found.evidence of fracturing had been found.

Plate TectonicsPlate Tectonics

Section 2: SSection 2: Seaflooreafloor S Spreadingpreading

Harry Hess

Mapping The Ocean FloorMapping The Ocean Floor

Scientists began using sound waves on moving Scientists began using sound waves on moving ships to map large areas of ocean floor in ships to map large areas of ocean floor in detail.detail.

Sound waves echo off the ocean bottom – the Sound waves echo off the ocean bottom – the longer the sound waves take to return to the longer the sound waves take to return to the ship, the deeper the water is.ship, the deeper the water is.

In the early 1960s, Harry Hess proposed that hot, In the early 1960s, Harry Hess proposed that hot, less dense material below Earth’s crust rises toward less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows the surface at the mid-ocean ridges. Then, it flows sideways, carrying seafloor away from the ridge in sideways, carrying seafloor away from the ridge in both directions. This theory is known “both directions. This theory is known “seafloor seafloor spreadingspreading”.”.

In 1968, a research ship, In 1968, a research ship, Glomar Challenger, Glomar Challenger, began began gathering rock samples from mid-ocean ridges. gathering rock samples from mid-ocean ridges.

They made a remarkable discovery as they studied the ages They made a remarkable discovery as they studied the ages of rock samples.of rock samples.

The younger rocks are closer to the mid-ocean ridges and The younger rocks are closer to the mid-ocean ridges and older rocks are farther from the ridges.older rocks are farther from the ridges.

Magnetic CluesMagnetic Clues

Iron-bearing minerals, Iron-bearing minerals, such as magnetite, that such as magnetite, that are found in the rocks of are found in the rocks of the seafloor can record the seafloor can record Earth’s magnetic field Earth’s magnetic field direction when they direction when they form. form.

The magnetic alignment The magnetic alignment in the rocks reverses in the rocks reverses back and forth over time back and forth over time in strips parallel to the in strips parallel to the mid-ocean ridges.mid-ocean ridges.

Seafloor Seafloor SpreadingSpreading

Section 3: Section 3: Theory of Plate TectonicsTheory of Plate Tectonics

In the 1960s, scientists developed a In the 1960s, scientists developed a new theory that combined continental new theory that combined continental drift and seafloor spreading .drift and seafloor spreading .

According to the theory of According to the theory of plate plate tectonicstectonics, Earth’s crust and part of , Earth’s crust and part of upper mantle are broken into sections. upper mantle are broken into sections. These sections are called plates, move These sections are called plates, move on a plasticlike layer of the mantle.on a plasticlike layer of the mantle.

Earth’s crust and a part of upper mantle combined Earth’s crust and a part of upper mantle combined are the are the lithospherelithosphere..(100km=62 mile thick)(100km=62 mile thick)

The plasticlike layer below the lithosphere is The plasticlike layer below the lithosphere is called called asthenosphere.asthenosphere. The rigid plates of the The rigid plates of the lithosphere float and move around on the lithosphere float and move around on the asthenosphere.asthenosphere.

Causes of Plate TectonicsCauses of Plate Tectonics Convection currents cause the movements of platesConvection currents cause the movements of plates Hot, less dense liquid or gas is forced upward, as it Hot, less dense liquid or gas is forced upward, as it

reaches the surface, it cools down and sinks back reaches the surface, it cools down and sinks back down. This entire cycle of heating, rising, cooling down. This entire cycle of heating, rising, cooling and sinking is called a and sinking is called a convection currentconvection current..

Plate BoundariesPlate Boundaries When plates move, they can interact in several When plates move, they can interact in several

ways. Three different moving types of plates are:ways. Three different moving types of plates are: Plates moving apartPlates moving apart Plates moving togetherPlates moving together Plates slide past each otherPlates slide past each other