Chapter 17 Plate Tectonics

Big Idea: Most geologic activity occurs at the boundaries between plates. Essential Questions: 1. Where are the boundaries between tectonic plates? 2. What observations are useful for characterizing plate boundaries

DISCOVERING PLATE BOUNDARIES DAY 1 INTRODUCTION TO CLASSIFICATION In this activity you will “invent” a new classification scheme of your own. There are many “correct” answers to this type of problem! Example: In classifying a large box of donated clothing, you could put shirts in one pile, pants in another, shoes in a third and so on. You could also classify the items by color, you could classify them by size, you could classify by gender (men’s and women’s clothing) etc. In other words, there is no “ONE RIGHT WAY” to classify things!….. Now imagine that you have hundreds of such boxes and helpers to do the sorting for them. In order to make things work, they will have to devise an appropriate classification scheme, and then define it so the others can apply it to their boxes. This requires that they write a sample description of the categories that they created. We also note that they may find items that they do not recognize or cannot put into one of their categories. In that case they may need a new category for items that fit no category!

SCIENTIFIC SPECIALTIES Seismology – seismologists – experts with earthquakes Volcanology – volcanologists – experts with volcanoes Geography – geographers – experts with topographic maps Geochronology – geo-chronologists – experts with seafloor age

DISCOVERING PLATE BOUNDARIES DAY 1 Introduction to classification – continued Now you will work in your scientific “specialty” group and come up with a classification scheme of the plate boundaries of the world based on data provided on one of the four wall maps. Use generic boundary names like “boundary type 1, boundary type 2, etc. Write a description of how you identified your plate boundary types on the back of your plate boundary map. Select a boundary type color and symbol and include it along side the description. Do this for at least 5 different boundary types. Each person should color your maps to locate your group’s boundary types. Now, use your handout and follow Period 1 (Day 1) instructions for Task 1, Task 2, and Task 3.

THINGS TO REMEMBER Each specialty group is to work only with your data map! Your are to OBSERVE rather than INTERPRET For descriptions use words like “wide or narrow”, “straight or curved”, “symmetric or not symmetric”, “deep or shallow”, “ridge or valley”, “active or inactive”, etc. Be as descriptive as possible. Make logical classifications based on your data. Maps will be turned in after you complete Task 3 for Period 1 (day 1).

EXAMPLE SYMBOLS YOU MAY USE TICS SPIKES DOTS ARROWS

DISCOVERING PLATE BOUNDARIES DAY 2

PLATE BOUNDARY PROCESSES

Big Idea: Most geologic activity occurs at the boundaries between plates. Essential Question: What is plate tectonics and how does it affect geologic activity on Earth’s surface?

MAIN IDEA: The shape and geology of the continents suggests that they were once joined together Drifting Continents

The Hypothesis  The Earth’s continents were once joined as a single landmass that broke apart and sent the continents adrift.  This hypothesis is known as Continental Drift.

Who (the Scientist behind the Hypothesis)  Alfred Wegener ( ) – 1912 – Was the first to propose the hypothesis of Continental Drift to the scientific community. On his final expedition to Greenland. He died on this expedition.

Details of the Hypothesis  Wegener called the supercontinent Pangaea (a Greek word meaning All the Earth)  Pangaea began to break apart 200 million years ago (mya)  Continents have been slowly moving since then, to the positions they are in now.

What evidence did Wegener have?  Evidence to support his theory: 1) Shape of the Continents  Wegener noticed that some of the continents seemed to fit together South America and Africa

What evidence did Wegener have?  Rock Formations  Layers of rock in the Appalachian Mountains (U.S.) are identical to layers of rock in similar mountains in Greenland and Europe.  Layers of rock in South America match rock in Africa.

What evidence did Wegener have?  Fossils  Identical land-dwelling plant and animal fossils were found on widely separated continents.  Cynognathus and Lystrasaurus  Mesosaurus (a freshwater reptile)

What evidence did Wegener have?  Climate  Glossopteris (fossil) – seed fern that grows in temperate climates. Fossils were found very wide-spread and in places that are not temperate today.  Coal deposits – found in Antarctica; Wegener said that Antarctica must have been closer to the equator in the geologic past. (form from the compaction and decomposition of accumulations of ancient swamp plants).  Glacial deposits – found in Africa, India, Australia, and South America (all places that today are much too warm for thick ice to accumulate).

Problems with the Theory  There were two problematic, unanswered questions: 1. What forces could cause the movement of the continents? 2. How could continents move through solids?  In the 1960s, new technology revealed new evidence about how continents could move. Wegener’s hypothesis was brought back and reconsidered.

MAIN IDEA: Oceanic crust forms at ocean ridges and becomes part of the seafloor Seafloor Spreading

Seafloor Spreading  Seafloor spreading was discovered in 1960 by Harry Hess, a professor from Princeton University 1. Ocean floor topography was mapped using SONAR 2. Underwater mountain chains called ocean ridges and deep sea trenches run along the Earth like seams on a baseball 3. The thickness of the ocean-floor sediments increases with distance from the ridge

How does Seafloor Spreading Occur? 1. New ocean crust is formed at ocean ridges and destroyed at deep-sea trenches 2. Magma rises and fills in the gaps, then solidifies and new seafloor is created

Evidence to Support Seafloor Spreading  What do magnetic patterns on the seafloor indicate? 1. Earth’s magnetic field reverses every few thousand years. 2. Magnetic reversal – Earth’s magnetic field changes polarity between normal and reversed 3. Iron bearing minerals align with the Earth’s magnetic field 4. Magnetic symmetry is created as the crust moves out from the ridge

Evidence to Support Seafloor Spreading 5. Isochrons (imaginary lines on a map that show points of the same age formed at the same time) could be created to determine the age of the seafloor.

Seafloor Spreading

MAIN IDEA: Volcanoes, mountains, and deep-sea trenches form at the boundaries between the plates Plate Boundaries

Important Information to know before we start The Theory of Plate Tectoncis The crust is made of two different types - oceanic – Basalt (3.0 g/cm 3 ) - thin but dense - continental _ Granite (2.7 g/cm 3 ) - thick with a low density

Plate Boundaries  Tectonic plates – huge pieces of Earth’s crust (the lithosphere) that covers its surface and fit together at their edges. Lithosphere = Crust and the solid upper mantle Asthenosphere = low mantle - plasma

Features found at Plate Boundaries  At the plate boundaries several features can be found: 1. Volcanoes 2. Earthquakes 3. Trenches 4. Mid-Ocean Ridges & Rift Valleys

Types of Plate Boundaries  There are three types of boundaries that are named for how the plates interact with one another.

Types of Plate Boundaries 1. Divergent Boundaries - place where two tectonic plates are moving apart; is associated with volcanism, earthquakes, and high heat flow, and is found primarily on the seafloor a. Results in: a. mid-ocean ridges [long mountain chains formed by divergent volcanism, are very tall] b. rift valleys [long, narrow depressions that form in the center of mid-ocean ridges when the plates separate. b. Diagram:

Types of Plate Boundaries 3. Transform Boundaries - place where two tectonic plates slide past each other; characterized by long faults and shallow earthquakes a. Plates slide against each other b. Results in a fault with many earthquakes c. Diagram:

Types of Plate Boundaries 2. Convergent Boundaries - place where two tectonic plates are moving toward each other; is associated with trenches, island arcs, and folded mountains a. Subduction – when two plates collide, the denser plate eventually descends below the other, less-dense plate b. Diagram:

Theory of Plate Tectonics  Oceanic-Oceanic: where two very dense plates are pushed into each other and one slides underneath  This process is called subduction – the plate that has a higher density will always subduct  Creates trenches and volcanic Islands arcs ( example: Japan)

Theory of Plate Tectonics  three special types of convergent boundaries:  Oceanic – Continental – two plates (one with low and one with high density) collide and the higher density plate slide under  Which plate is more dense? The oceanic (basaltic) or continental (granitic) This process is called suduction  resulting in ocean trenches, and mountain ranges with many volcanoes

Theory of Plate Tectoics

Theory of Plate Tectonics  Continental – Continental:– two plates with low density collide resulting in large mountain ranges

MAIN IDEA: Convection currents in the mantle cause plate motions Causes of Plate Motions

Causes of Plate Motion  Convection is the circulatory motion that occurs in a fluid with a non-uniform temperature. This creates a variation of its density.  The movement of the tectonic plates is driven by the convection currents in the asthenosphere.

Causes of Plate Motion  The transfer of thermal energy, by the movement of heated material from one place to another, moves the tectonic plates. Cooling = increase in density → sinks Heating = decrease in density → floats

Causes of Plate Motion  In the mantle convection currents transfer heat and energy  The Heat source for mantle convection in the core  The mantle material is heated and becomes less dense  It rises and cools becoming more dense  Then sinks and the process begin again

The plates float on top of the mantle and move along with the currents

Causes of Plate Motions