plate tectonics chapter - 8 the lithosphere is broken into rigid plates that move in relationship to...

Plate TectonicsChapter - 8

The lithosphere is broken into rigid plates that move in relationship to one another on the asthenosphere

(see page 712-713 in textbook)

Objectives:◦ Identify the evidence that A. Wegener used to

support Continental Drift

◦Explain how the theory of Plate Tectonics helps predict the locations of earthquakes and volcanoes

◦ ◦AZ Science Standard: Demonstrate the

relationships among earthquakes, volcanoes, mountain ranges, mid-ocean ridges, deep sea trenches, and tectonic plates

8.1 What is Plate Tectonics?

The theory that describes the formation, movements, and interactions of these plates.

The lithosphere is broken into rigid plates that move in relationship to one another on the asthenosphere.


The lithosphere is our crust that sits on top of the asthenosphere.


Plate Tectonics Theory◦Early evidence included shapes of the

continents (South America and Africa)◦Similar fossil record◦Similar locations of rivers flowing into the

ocean◦Similar plants growing◦Similar coastal rock formation.

Initially, there was little acceptance of Wegner’s theory because he couldn’t explain why they moved.

The theory of plate tectonics helps explain the locations of earthquakes and volcanoes.

The theory of plate tectonics helps explain the locations of earthquakes and volcanoes.

Therefore, characteristic features associated with plate boundaries:◦Earthquake activity◦Volcanic activity◦High heat flow


DEFINE:Plate Tectonics - a theory that describes

the formation, movement, and interactions of the plates on the lithosphere.

Continental Drift – movement of continents.

Mid-Ocean Ridge - a long chain of volcanic mountains on the ocean floor.


What observations support the continental drift hypothesis?◦Earthquakes and volcanoes occur in

concentrated belts that correspond to present tectonic plate boundaries.


What evidence in support of plate tectonics is provided by studies of the ocean floor?◦Magnetic reversals in rocks on either side of the

Mid-Atlantic Ridge; High heat flow along ridges; Youngest crust is close to ridge axis.


Magnetic Reversals

http://www.iki.rssi.ru/mirrors/stern/earthmag/reversal.htm

Objectives:Discuss verbally the differences among

the three types of plate boundariesContrast the three different types of

convergent boundaries

8.2 Types of Plate Boundaries

Minerals in igneous rocks can show the direction of Earth’s polarity field at the time the rocks were formed.


Transform Boundary


Convergent Boundary


Divergent Boundary


Mid Atlantic Ridge◦ Hydrothermal vents◦ New rocks are forming◦ Earthquakes occur here


Types of Plate Boundaries summary

Boundary Process involved

Features Current Example

Divergent Sea-floor spreading •Mid-ocean ridges•Rift valleys•Earthquake activity•Volcanic activity

•Mid-Atlantic Ridge•East pacific Rise

Convergent Ocean-ocean •Deep sea trenches•Volcanic island arcs•Earthquake activity

•Island of Indonesia•Mariana Islands

Ocean-Continent subduction

•Deep sea trench bordering continent•Volcanoes along coast-line•Earthquake activity

•Western coast of South America

Transform Plates sliding past each other

•Earthquake activity •San Andreas Fault•North Anatolian Fault (Turkey)•Fracture zones along mid-ocean ridges


New oceanic crust is formed at a divergent boundary when:◦Step 1 - Molten rock forces its way upward into the

rifts that form when two lithospheric plates separate.

◦Step 2 - As the molten rock cools, new oceanic crust forms.

◦Step 3 - The older oceanic crust moves away from the mid-ocean ridge.


Two different types of subduction boundaries are:◦Oceanic-oceanic plate convergence – two

features are a deep-sea trench and a volcanic island arc on the overriding plate. (Marianna trench)

◦Oceanic-continental plate convergence – a deep-sea trench and an inland mountain chain and volcanoes characterize this type of subduction boundary. (the Andes)


Describe what happens at a collision boundary, identify a collision boundary:◦Two Continental plates collide, forming a

single, larger continent and pushing up the crust to form a mountain range.

◦Ex: Himalayan and Appalachian Mountains


When two oceanic plates converge, the denser plate is subducted.

A chain of volcanoes commonly forms when an oceanic plate subducts under a continental plate.

Fracture zones that offset portions of mid-ocean ridges are examples of divergent boundaries.


Describe the movement of plates at a transform boundary, give example.◦Plates slide past each other at a transform

boundary.◦Ex: San Andreas fault; and the fracture zones

along mid-ocean ridges.

Objectives:Discuss mantle convection as a possible

cause of plate movements Compare and contrast ridge push and slab

pull

8.3 Causes of Plate Movements

Three hypotheses for plate movement:

◦Mantle convection - convection currents within the mantle are pushing the plates.

◦Ridge push - hot mantle at ridge is lighter and lifts up at the ridge, as it spreads, it cools, and denser material slides off ridge.

◦Slab pull – sub-ducted material “pulls” the plate with it as it slides downward under other plate.

Explain how Earth’s landmasses have changed positions over the past 200 million years

Discuss the roles of plate tectonics, igneous activity, and deposition in the formation of continental landmasses

8.4 Plate Movements and Continetal Growth

The breakup of Pangaea is a process that is continuing today.


North American Craton◦An expanse of

ancient rock at the core of the North American Continent formed 2.5 billion yrs ago.


Sources of Growth Material

◦ Deep sea sediments most likely contribute to continent growth by being scraped off in a subduction zone and left at the surface.

◦ Igneous rock from plutons and volcano ash, lava, and rock material.

◦ Terranes are large blocks of lithospheric plates that have been moved over thousands of kilometers and attached to the edge of a continent – three characteristics:1. Each terrane block is bound by major faults2. Rocks and fossils found in the terrane do not match those of

neighboring terranes3. Magnetic record found in the terrane does not match that of

neighboring terranes.

8.4 Plate Movements and Continental Growth

Density plays a big part in plate tectonicsBasic density info:

◦Density is mass per unit volume◦Usually as grams per cubic centimeter◦g/cc◦An object that weighs 12 grams and takes up 6

cubic centimeters has a density of (12grams/6cc)=2g/cc

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