Plate Tectonics Part A Theory of Plate Tectonics

Plate Tectonics - Part A - Theory of Plate Tectonics Old 7 th Grade Text Chap 14: 3

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 land mass 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 • Then it 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 Theory of Plate Tectonics

Plate Tectonics • The Earth’s crust and upper mantle are broken into sections. • The sections, called plates, move on a plasticlike 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.

Plate Tectonics • The plates and upper mantle form the lithosphere – The lithosphere is about 100 km 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 into each other, or pull apart, or slide alongside each other • When plates interact, the result of their movement can be seen at the plate boundaries

Plates moving apart. 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.

Plates moving togetherconvergent boundaries • As new crust is added in one place, it disappears below the surface at another.

Plates moving together convergent boundaries • When Ocean and Land Plates meet: – The denser oceanic plates sink under less dense continental plates – The area where the ocean plate moves down is called a subduction zone – This type of convergent boundary forms a deep sea trench

Plates moving togetherconvergent 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

Plates moving togetherconvergent 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.

Plates moving togetherconvergent 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

Plates Sliding Past each other. 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 Earthquakes

Features caused by Plate Tectonics • Mid-Ocean Ridges, Faults and Rift Valleys – are formed at divergent boundaries where plates are pulled apart. • Mountains and Volcanoes – are formed at convergent boundaries where plates collide. • Strike-Slip faults- cause of Earthquakes 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. – Hawaii is moving toward Japan at a rate of about 8. 3 cm per year. – Maryland(? ) is moving away from England at a rate of 1. 7 cm per year • Scientists have observed that the plates move at rates from about 1 cm to 12 cm per year.

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