Movement of the Earth

Movement of the Earth Theory of Plate Tectonics(Part 1)

Continental Drift Hypothesis • In 1912 Alfred Wegener suggested that all the continents were once connected as one large landmass now called Pangaea. • Pangaea means “all land”. • The landmass broke apart about 200 million years ago and the continents slowly drifted to their present positions.

Seafloor Spreading-explains the motion of the continents • Hot dense material below the Earth’s crust rises upward to the surface at the mid-ocean ridges. • It then flows sideways, carrying the seafloor away from the ridge. • As the new seafloor spreads apart, magma moves up and flows from the cracks, cools, and forms new seafloor. • Youngest rocks are located at the mid-ocean ridges and become increasingly older the further out you go.

Plate Tectonics • In the 1960’s scientists developed a new theory that combined continental drift and seafloor spreading. • It is called The Theory of Plate Tectonics.

Plate Tectonics • The Earth’s crust and upper mantle are broken into sections. • The sections, called plates, move on a plastic-like layer of the mantle. • Geologists use the term plastic to describe the consistency of rock that flows but is not a liquid like Silly Putty, modeling clay, or toothpaste.

PlateTectonics • The plates and upper mantle form the lithosphere. • The lithosphere is about 100km thick and generally is less dense than the material underneath. • The plastic-like layer is called the asthenosphere. • The rigid plates of the lithosphere float and move around on the asthenosphere.

Plate Boundaries • When plates move, they can interact in several ways. • They can collide, pull apart, or slide alongside each other. • When plates interact, the result of their movement can be seen at the plate boundaries.

Divergent Boundaries • Plates moving apart at the mid-ocean ridges are examples of divergent plate boundaries. • In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates. • The Great Rift Valley in eastern Africa may become a divergent boundary. The continental plate is being pulled apart.

Convergent Boundaries • As new crust is added in one place, it disappears below the surface at another.

Convergent Boundaries • When Ocean and Land Plates Meet: • The denser oceanic plates sink under less dense continental (land) plates. • The area where the ocean plate moves down is called a subduction zone. • This type of convergent boundary forms a deep sea trench.

Convergent Boundaries • When Ocean and Land Plates Meet (continued): • Newly formed hot magma is forced upward along the subduction zones and forms volcanic mountains. • The Andes Mountain Range in South America contains many volcanoes that were formed at the convergent boundary of the Nazca and South American Plates.

Convergent Boundaries • When Two Oceanic Plates Collide: • The older, denser plate will subduct and sink down into the mantle. • The Mariana Islands in the Western Pacific are a chain of volcanic islands formed from two oceanic plates colliding.

Convergent Boundaries • When Two Continental (Land) Plates Collide: • There is usually no subduction because both of these plates are less dense than the material of the asthenosphere. • Plates will crumple up to form mountain ranges. • Earthquakes are common here, but not volcanoes. • The Himalayas in Asia are forming where the Indo-Australian Plate collides with the Eurasian Plate. Eurasian Plate Indo-Australian Plate

Transform Boundary • When one plate slips past another suddenly, earthquakes occur. • The Pacific Plate is sliding past the North American Plate forming the famous San Andreas Fault in California.

Causes of Plate Tectonics • Convection inside the Earth • Cycle of heating, rising, cooling and sinking is called a convection current. • This process occurring inside the mantle of the Earth is the driving force behind plate tectonics.

Convection currents occur inside the mantle. They are the driving force behind plate tectonics.

Features Caused by Plate Tectonics • The interaction of plates produces forces that build mountains, create ocean basins, and cause volcanoes. • When rocks in the Earth’s crust break and move, the energy is released as seismic waves. • We feel this release as an earthquake.

Features Caused by Plate Tectonics • Mid-Ocean Ridges, Faults and Rift Valleys – formed at divergent boundaries where plates are pulled apart • Mountains and Volcanoes – formed at convergent boundaries where plates collide • Strike-Slip Faults- cause of earthquakes and are found at transform boundaries where plates grind past each other

Testing for Plate Tectonics • Using lasers and satellites, scientists can measure movements as little as 1 cm per year. • Scientists have observed that the plates move at rates from about 1cm to 12cm per year. • Hawaii is moving toward Japan at a rate of about 8.3 cm per year. • Europe and North America are moving away from each other at the rate of about 3 cm per year.

Part 1 Assessment Using a separate sheet of paper, answer the following questions. • What happens to plates at a transform boundary? • What occurs at plate boundaries that are associated with seafloor spreading? • Describe the types of plate boundaries where volcanic eruptions can occur. • How are convection currents related to plate tectonics? • Using a flow map, describe the three theories that were discussed at the beginning of the PowerPoint.

Part 1 AssessmentANSWERS One plate slips past another suddenly, resulting in an earthquake. • What happens to plates at a transform boundary? • What occurs at plate boundaries that are associated with seafloor spreading? Oceanic crust is very dense compared to land crust, so it sinks beneath land crust and pulls away from other ocean plates. When this happens, a void is left at the plate boundary. Magma fills the void where it cools to create new oceanic crust. It is a continual process.

Part 1 AssessmentANSWERS(continued) • Describe the types of plate boundaries where volcanic eruptions can occur. • Diverging boundaries (plates pulling apart) either on the ocean floor or on land • Converging boundaries where an ocean plate collides with a land plate or where two ocean plates collide.

Part 1 AssessmentANSWERS(continued) • How are convection currents related to plate tectonics? Magma can heat, rise, cool, and sink just like water can when boiling or air can creating wind. Magma is heated below Earth’s surface. This heated, less dense magma rises towards Earth’s crust where it cools and become more dense. It sinks back down towards Earth’s surface where it’s heated again, starting the cycle over.

Part 1 AssessmentANSWERS(continued) • Using a flow map, describe the three theories that were discussed at the beginning of the PowerPoint. CONTINENTAL DRIFT - In 1912 Alfred Wegener suggested that all the continents were once connected as one large landmass now called Pangaea. The land mass broke apart about 200 million years ago and the continents slowly drifted to their present positions. THE THEORY of PLATE TECTONICS - In the 1960’s scientists developed a new theory that combined continental drift and seafloor spreading. SEAFLOOR SPREADING – Refer to question #2 for description

Movement of the Earth