This is Planet Earth / Earth Story. Lecture 5 PLATE TECTONICS Drifting Continents, And Oceans That Open And Close

Transcription

1 This is Planet Earth / Earth Story Lecture 5 PLATE TECTONICS Drifting Continents, And Oceans That Open And Close

2 Earth Structure The Lithosphere: Forms the outermost layer of the Earth. Consists of the crust and top part of the mantle Ranges in thickness from about 7 km (oceanic) to 125 km (continental) Overlies the asthenosphere which is partially melted, whereas the lithosphere is rigid.

3 Plate Tectonics

4 Evidence for moving continents (e.g. Francis Bacon 1620) In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated.

5 Alfred Lothar Wegener ( ) Published The Origin of the Continents and Oceans in 1915 Challenged the Rigid Earth Theory Provided evidence that continents are in relative motion with respect to time (Continental Drift)

6 Wegener s evidence for Continental Drift Continental fit Correlation of rocks and climate Crustal structure Distribution of fossil plants and animals Direct measurements of changing distance between the continents Best to use continental margins rather than coastlines

7 Evidence for Continental Drift Past climates and the distribution of sedimentary rocks According to the continental drift theory, the supercontinent Pangaea began to break up about million years ago, eventually fragmenting into the continents as we know them today.

8 Evidence for Continental Drift Structure of the Crust Isostasy: Areas underlain by continental crust (acidic, low density) stand considerably higher than areas underlain by oceanic crust (basic; high density) Light and thick blocks float high (equivalent to continental crust), dense thin blocks float low (equivalent to oceanic crust),

9 Evidence for Continental Drift Fossil Plants and Animals The locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined.

10 Continental Drift Additional Supporting Evidence Arthur Holmes (1931) suggested that the continents and oceans move together, driven by convection currents in the mantle Technology developed during WW2 was put to use in surveying the ocean floor Mapping of the ocean floor (East Pacific)

11 Exploration of the Ocean Floor Oceanic Ridges Oceanic ridges (shown in red) wind their way between the continents

12 Exploration of the Ocean Floor Ocean Trenches (Subduction Zones) The deepest waters are found in oceanic trenches, which plunge as deep as 11 km below the ocean surface. These trenches are usually long and narrow, and run parallel t and near the oceans margins. They are the surface expression of subduction zones

13 Earthquake Distribution As early as the 1920s, scientists noted that earthquakes are concentrated in very specific narrow zones. In 1954, French seismologist J.P. Rothé published this map showing the concentration of earthquakes along the zones indicated by dots and cross-hatched areas.

14 Earthquake Distribution at Subduction Zones Benioff Zone Image shows earthquakes that occurred beneath the Tonga Trench, Pacific Ocean during 1965 Each red circle represents a single earthquake The earthquake distribution defines a Benioff zone, representing the downward movement of a relatively cool slab of lithosphere.

15 Harry Hess ( ) Naval Officer and Professor of geology at Princeton University, Very influential in setting the stage for the emerging plate tectonics theory in the early 1960s. He believed in many of the observations Wegener used in defending his theory of continental drift, but he had very different views about large-scale movements of the Earth. Credited with the concept of Sea Floor Spreading

16 Plate Movement - Convection Trench Oceanic ridge Hess oceanic rocks are replaced every Ma due to SEA FLOOR SPREADING The oldest oceanic lithosphere is about 180 million years old, which is relatively young

17 Support for Sea Floor Spreading - The Earth s Magnetic Field

18 Magnetic stripes on the sea floor - Iceland

19 Magnetic Reversals (Vine and Matthews Hypothesis)

20 Drilling and Sampling of the Sea Floor The Glomar Challenger was the first research vessel specifically designed in the late 1960s for the purpose of drilling into and taking core samples from the deep ocean floor

21 Recommended Reading Press, F. and Siever, R Understanding Earth (3rd Edition). Freeman (ISBN ). Chapter 20 Skinner, B.J., Porter, S.C. and Park, J Dynamic Earth: An Introduction to Physical Geology (5 th Edition). Wiley (ISBN ). Chapter 2