CHAPTER 2 Plate Tectonics and the Ocean Floor Fig. 2-32

Thin, rigid blocks move horizontally Plate tectonics Thin, rigid blocks move horizontally Interactions of plates build major features of Earth’s crust Mountain ranges, trenches, volcanoes, etc.

Theory of Plate Tectonics explains: Global distribution of Volcanoes Earthquakes Faults Mountain belts Features of seafloor Evolution of continents and oceans http://www.geo.lsa.umich.edu/~crlb/COURSES/270/plate_tectonics.jpeg

Continental drift Alfred Wegener proposed continental drift hypothesis in 1912 stating: One large continent – Pangaea Surrounded by single large ocean Panthalassa About 200 million years ago As you can imagine, was NOT widely accepted at first http://pubs.usgs.gov/publications/graphics/wegener.gif

Evidence for continental drift Puzzle-like fit of continents Sir Edward Bullard fit continents at 2000m water depth (1965) Used edge of continental shelves Better fit than Wegener’s using coastlines Fig. 2.3

Evidence for continental drift Wegener matched sequences of rocks and mountain chains Similar age, rock types, structures http://www.lee.edu/~cguldenzopf/Images/Historical/Historicalmw32.jpg http://geology.csupomona.edu/drjessey/class/Gsc101

Evidence for continental drift Wegener noted glacial ages and other climate evidence Ancient glaciation in modern tropical regions If there is evidence of ancient glacial activity, that area had to be closer to poles at some point Direction of glacial flow

Similar fossils on separate southern continents Evidence for continental drift Distribution of organisms Same land plants and animals distributed in different continents (e.g., South America and Africa) Similar fossils on separate southern continents

Similar fossils on separate southern continents http://pubs.usgs.gov/publications/graphics

Objections to Wegener’s continental drift hypothesis: Continents cannot “plow” through ocean crust Celestial mechanism –gravitational forces associated with tides too small Proposed a celestial mechanism for moving plates – incorrrect , lost favor in 1930’s More data was needed, collected using new technologies sonar for mapping ocean bottom measurements of Earth’s magnetism

Evidence for plate tectonics Echo-sounding Showed mountain ranges in middle of oceans, deep trenches near continents Radiometric dating of bottom cores Ocean is 4 billion years old but ocean rocks are less than 180 million years old – why? http://oceanexplorer.noaa.gov/history/quotes/tech/media

http://www.fau.edu/divdept/physics/jordanrg/LLS

Evidence for plate tectonics Earth’s magnetic field Paleomagnetism – study of earth’s ancient magnetism N or S magnetic alignment of magnetite particles when rock hardens Magnetic inclination (magnetic dip) Latitude If rocks move on plates, the latitude of origin is reflected in the rocks magnetic dip

Apparent polar wandering When rocks from different continents were aligned as Pangea, polar wandering curves matched, showing a single stationary pole, but moving plates Fig. 2.8

Magnetic polarity reversals Reversals aged by radiometric dating About 1000 reversals over last 76 my (once per 200K years) Fig. 2.9

Sea floor spreading Harry Hess (1962)

Fig. 2.11 More recent

Seafloor Spreading and Magnetization To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Magnetic anomalies Fig. 2.12

Evidence to support sea floor spreading Heat flow is highest at crest of mid-ocean ridge Most large earthquakes occur along plate margins

Global distribution of earthquakes Fig. 2.13

Plate Boundary Features To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Plate tectonics theory Lithospheric plates “float” on the asthenosphere Large scale geologic features occur at plate boundaries Two major tectonic forces Slab pull Generated by subducting plate that pulls rest behind it Slab suction Subducting plate drags across mantle and sucks it down toward subduction zone

Types of plate boundaries Divergent Convergent Transform Fig. 2.14

Divergent boundary features Fig. 2.15 Plates move apart Mid-ocean ridge Rift valley New ocean floor created Shallow earthquakes As opposed to large, deep earthquakes found at convergent boundaries

Divergent boundary features Fig. 2.17 Divergent boundary features

Divergent Boundaries Types of spreading centers Oceanic rise Fast-spreading Gentle slopes Oceanic ridge Slow-spreading Steep slopes Ultra-slow Deep rift valley Widely scattered volcanoes For example, not all of the mid-Atlantic ridge moves at same rate, some areas are “rises” same are “ridges”

Convergent boundary features Plates move toward each other Oceanic crust destroyed Ocean trench Volcanic arc Deep earthquakes Types: Oceanic-continental Continental-continental

Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Fig. 2.20

Types of convergent boundaries Oceanic-continental convergence Ocean plate subducted under continental Continental arc Find volcanoes on continent Oceanic trench Deep earthquakes

Fig. 2.21a,b

Tectonic Settings and Volcanic Activity To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Types of convergent boundaries Continental-continental convergence Uplifted mountain ranges Deep earthquakes

Transform boundary features Offsets oriented perpendicular to mid-ocean ridge Segments of plates slide past each other Offsets permit mid-ocean ridge to move apart at different rates Shallow but strong earthquakes

Types of transform faults Oceanic—wholly in ocean floor Continental—extends from mid-ocean ridge across continent San Andreas Fault Fig. 2-23

Motion at Plate Boundaries To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Applications of plate tectonics model to intraplate features Mantle plumes and hotspots Volcanic islands within a plate Island chains Systematic variation of age Record ancient plate motions As plate moves over hot spot, new islands are formed, notice how the island of Hawaii is newer than Kauai

Fig. 2.24

Tectonic Settings and Volcanic Activity To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Applications of plate tectonics model to intraplate features Seamounts and tablemounts Seamounts – conical on top Tablemounts – flat on top Wave erosion may flatten seamount

Applications of plate tectonics model to intraplate features Coral reefs associated with subsiding seafloor Fringing Barrier Atoll

Movement from mid-ocean ridge, mountain submersed Fig. 2.27

Seamounts/Tablemounts and Coral Reef Stages To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Measuring plate motion by satellites Fig. 2.30

Paleoceanography Reconstructing paleogeography Continental accretion Continental material added to edges of continents through plate motion Bits and pieces of volcanoes, islands, etc added to landmasses Continental separation or rifting Continents move apart

Paleo- reconstructions Fig. 2.31

Global Geography Through Geologic Time To view this animation, click “View” and then “Slide Show” on the top navigation bar.

Future predictions Future positions of continents and oceans Assume same direction and rate of plate motions as now

World map 50 million years from now Fig. 2.32

Misconceptions All rocks and planets were formed at the same time. Rocks became polarized by the North Pole. The magnetism of the Earth has always been the same.

Ocean Literacy Standards 1a - The ocean is the dominant physical feature on our planet Earth—covering approximately 70% of the planet’s surface. There is one ocean with many ocean basins, such as the North Pacific, South Pacific, North Atlantic, South Atlantic, Indian and Arctic. 1b - An ocean basin’s size, shape and features (islands, trenches, mid-ocean ridges, rift valleys) vary due to the movement of Earth’s lithospheric plates. Earth’s highest peaks, deepest valleys and flattest vast plains are all in the ocean.

Sunshine State Standards SC.7.E.6.1 Describe the layers of the solid Earth, including the lithosphere, the hot convecting mantle, and the dense metallic liquid and solid cores. SC.7.E.6.3 Identify current methods for measuring the age of Earth and its parts, including the law of superposition and radioactive dating. SC.7.E.6.4 Explain and give examples of how physical evidence supports scientific theories that Earth has evolved over geologic time due to natural processes. SC.7.E.6.5 Explore the scientific theory of plate tectonics by describing how the movement of Earth's crustal plates causes both slow and rapid changes in Earth's surface, including volcanic eruptions, earthquakes, and mountain building. SC.7.E.6.7 Recognize that heat flow and movement of material within Earth causes earthquakes and volcanic eruptions, and creates mountains and ocean basins. SC.912.E.6.1 Describe and differentiate the layers of Earth and the interactions among them. SC.912.E.6.3 Analyze the scientific theory of plate tectonics and identify related major processes and features as a result of moving plates. SC.912.E.6.5 Describe the geologic development of the present day oceans and identify commonly found features.