2.0 Plate Tectonics: The Unifying Theory of the S olid Earth

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Pre-requisites Learning Objectives Table of Contents Why Should I Care? Does the Earth’s surface look like this?

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Pre-requisites None! Most of the other topics you will investigate in this course are best examined in the context of Plate Tectonics. It all starts here! Does the Earth’s surface look like this?

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Learning Objectives: After you complete this chapter you will be able to: 1. View the Earth as Constantly Changing and Evolving Understand Why the Surface of the Earth Consists of Slowly Moving Continents and Oceans 2.1 Understand How Mountains and Valleys are Created, and Why they are Located in their Current Positions Describe Relative Movement at Divergent, Transform, and Convergent Margins, and Why All Three Types of Tectonic Boundaries are Necessary on a Sphere 4. Describe How Processes in the Interior of the Earth Produce Plate Tectonics at the Surface

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Table of Contents: 2.0 The Unifying Theory of the Solid Earth 2.1 The History Behind the Science 2.11 Early Theories for the Formation of Mountain Belts 2.12 Initial Evidence for Continental Drift and Complications 2.13 WWII and Evidence from the Oceans 2.2 Key Elements of Plate Tectonic Theory 2.21 Divergent Plate Boundaries (Red Sea) 2.22 Convergent Plate Boundaries (Cascade Mountains) 2.23 Transform Plate Boundaries (San Andreas Fault) 2.24 Modern Evidence for Plate Movement 2.3 Complications to Plate Tectonics Theory 2.31 Mantle Plumes (Hawaii, Yellowstone) 2.4 Driving Mechanisms for Plate Tectonics 2.41 The Earth as a Convective Heat Engine 2.42 Ridge Push and Slab Pull 2.43 Models of the Earth’s Interior Note: all items in the TOC will be hot links to each particular section.

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Why Should I Care? Have you ever wondered: Why some countries have lots of natural resources, but others have few? Why California has many earthquakes, but Ohio has very few? Why the only active volcanoes in the continental US are in the Northwest? Why most beaches are white sand, but Hawaii has black sand beaches? Why some Himalayan mountains reach 28,000 ft. in elevation, but the Appalachian mountains are all less than 7000 ft.? All of these questions (and more!) can be answered with reference to the theory of Plate Tectonics. After completing this chapter, you will be able to answer them yourself! Return to Table of Contents

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Plate Tectonics is the first theory to provide a comprehensive view of the processes that produced Earth’s major surface features, including the continents and ocean basins. Within the framework of this theory, geologists have found explanations for the basic causes and distribution of earthquakes, volcanoes, and mountain belts. Further, we are now better able to explain the distribution of plants and animals in the geologic past, as well as the distribution of economically significant mineral deposits. Return to Table of Contents USGS

2.0 Plate Tectonics: The Unifying Theory of the Solid Earth Plate Tectonics is the first theory to provide a comprehensive view of the processes that produced Earth’s major surface features, including the continents and ocean basins. Within the framework of this theory, geologists have found explanations for the basic causes and distribution of earthquakes, volcanoes, and mountain belts. Further, we are now better able to explain the distribution of plants and animals in the geologic past, as well as the distribution of economically significant mineral deposits. Tectonics Study of large-scale movement & deformation of the earth’s outer layers (mountain building) Plate Tectonics Relates deformation to existence & movement of rigid “plates” over a weak or partly flowing “plastic” layer in the earth’s upper mantle Return to Table of Contents

2.1 Plate Tectonics: The Unifying Theory of the Solid Earth To start our investigation of the theory of Plate Tectonics let’s start with two fundamental questions: How are mountains and mountain ranges formed? Where are mountain ranges located? Text box for students to enter their thoughts. Text box for students to enter their thoughts. Early geologists had these same questions, and the next few Pages will examine some of their hypotheses and theories. Return to Table of Contents

(1831-1914) 2.11 Plate Tectonics: The History Behind the Science Eduard Suess Thermal Contraction Wrote Das Antlitz der Erde (1885-1909), translated into English as The Face of the Earth (1904-25) Proposed thermal contractionas a mechanism for vertical height differences of mountains vs. valleys The “dessicatedorange”theory… Note: insert picture of dessicated orange with caption: “When an orange dries out it shrinks, leaving ridges. The thermal contraction theory contended that as the Earth cooled it shrunk, leaving mountain ridges. Return to Table of Contents

(1813-1895) 2.11 Plate Tectonics: The History Behind the Science James D. Dana Permanence of oceans Wrote “Geological results of the Earth’s contraction in consequence of cooling” - 1847 Supported the thermal cooling theory, and therefore concluded that Oceans are permanent, continents are stable Return to Table of Contents

(1831-1914) 2.11 Plate Tectonics: The History Behind the Science Eduard Suess Gondwanaland Documented the Glossopteris fern on present-day continents: Glossopteris Proposed that Gondwanaland was a former continent where Some of today’s continents were connected by ancient land bridges Return to Table of Contents

(1849-1937) Mountainsidefolds in Norway 2.11 Plate Tectonics: The History Behind the Science Albert Heim Alpine terrain, horizontal folds Working in the Alps, Heim documented mountain-scale folds and large horizontaldisplacements. This did not support the vertical folding of thermal contraction. Return to Table of Contents

2.12 Plate Tectonics: The History Behind the Science Antonio Snider-Pellegrini Continental fit and “drift” 1858 - “Creation and its Mysteries Resolved” Noah’s flood caused the continents to break up and drift apart Return to Table of Contents S. America and Africa break up and drift apart

2.12 Plate Tectonics: The History Behind the Science Frank BursleyTaylor Drift of continents toward equator 1910 - “Bearing of the Tertiary mountain belt on the origin of the Earth’s plan” Tidal forces induced movement of continents toward equator Return to Table of Contents

(1880-1930) 2.12 Plate Tectonics: The History Behind the Science Alfred Lothar Wegener Paleoclimatic evidence Evidence for inlandglaciation on several present-day continents. The distribution of glaciation and warm, swampy regions make much more sense when the continents are positioned next to each other. Return to Table of Contents

(1880-1930) 2.12 Plate Tectonics: The History Behind the Science Alfred Lothar Wegener Theory of Continental Drift 1915 - “Die Entstehungder Kontinenteund Ozeane” 1924 - “The Origin of Continents and Oceans” Wegener’s continental reconstruction (1924) Return to Table of Contents

(1879-1940) 2.12 Plate Tectonics: The History Behind the Science Emile Argand Gondwanaland reconstruction 1916 - “Sur l'arcdes Alpesoccidentales” 1922 - “La Tectonique de l'Asia” Wegenerwasn’t the only scientist suggesting a continental reconstruction Return to Table of Contents

2.12 Plate Tectonics: The History Behind the Science Alexander Du Toit South America – Africa rock connections (1878-1948) 1927 “A Geological Comparison of South America with South Africa” 1937 “Our Wandering Continents” The same fossils andthe same rock units are found inboth Africa and South America Return to Table of Contents

2.12 Plate Tectonics: The History Behind the Science Summary of key data supporting Continental Drift But what was the driving mechanism for Continental Drift? Jigsaw puzzle fit of the Atlantic coast continents Fossil evidence linking continents Return to Table of Contents Paleoclimatic evidence Rock evidence that links continents

2.12 Plate Tectonics: The History Behind the Science Arthur Holmes Convection in a molten substrate (1890-1965) 1945 - “Principles of Physical Geology” The best model for Continental Drift from the first half of the 20th century Return to Table of Contents

2.12 Plate Tectonics: The History Behind the Science Most American geologists did not support the theory of Continental Drift, because they thought the theory lacked a convincing mechanism or model for the movement of continents. World War II initiated the theory of Plate Tectonics, so let’s again start with some conceptual questions: How can continents plow through oceans? Was the evidence for Continental Drift found on continents or in the oceans? Text box for students to enter their thoughts. Text box for students to enter their thoughts. Return to Table of Contents

2.12 Plate Tectonics: The History Behind the Science 2.1 Concept Check: 1. How did the theory of thermal contraction explain the formation of mountain ranges? 2. What was the evidence for the theory of Continental Drift? 3. What poorly understood part of the theory of Continental Drift led to the lack of support by American geologists? Text box for students to enter their answers. Text box for students to enter their answers. Text box for students to enter their answers. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: The Submarine Telegraph Cable Samuel Morse invented the telegraph in the 1830’s, but it would only work on land. Messages to/from Europe still required ships, but in 1858 a transoceanic cable was laid across the Atlantic ocean floor.However, the ship’s crew was surprised to discover that the ocean floor was not as deep as expected in the middle. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: The Challenger Expedition of 1872 HMS Challenger was the first dedicated oceanographic expedition vessel. The expedition of 1872 took soundings of the ocean depth and collected samples from the ocean floor. Shipboard scientists discovered that ocean depths, sediments, and fossils varied widely, contrary to current thought. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: SONAR – Seafloor Topography Harry Hess, naval reserve officer called up during World War II, used the SONAR on his ship to collect data on shape of the sea floor. Not only did the depth vary, but so did the thickness of the sediment layers on top of the sea floor. Return to Table of Contents

Local magnetic field Disrupted field lines 2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor During WWII, the renewed U-boat threat lead to improvements in SONAR, but other detection methods were developed:RADAR could detect surfaced subs, but deep-diving subs could escape even SONAR. So, the disruption of the local magnetic field was sought as a way of detecting submerged U-boats. Local magnetic field Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor The first magnetic detectors were towed behind a ship, but later were made more compact to fit in aircraft. More sensitive magnetometers began to detect reversals in the local magnetic field preserved in the sea floor rocks. Magnetometer boom Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor Vine and Matthews found linear patterns of these variations, with mirror images of the patterns on opposite sides of ocean ridges. These same rocks were of similar age on opposite sides of the ridge. Patterns were also offset by apparent faults. Interpreted data from several traverses Raw data from a single ship traverse Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor Describe any patterns that you notice Text box for students to enter their thoughts. Interpreted data from several traverses Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor Now the patterns are color coded by age. What do you notice? Text box for students to enter their thoughts. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor From this image and data from other oceans, oceanographers determined that magnetic stripes of normal (colored in the image) and reverse (white stripes in the image) polarity were bilaterally symmetrical around a central (red) normal polarity stripe. In addition, after determining the age of the basalt stripes, they realized the the youngest rocks were along the center stripe, and the rocks got older as one moved perpendicularly away from the center stripe. Coupled with data that showed elevated heat along the center stripe, researchers realized that they had discovered Wegener’s driving mechanism for Continental Drift! Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor New ocean floor (crust) was being created at these mid-ocean ridges (central stripes), which then moved away from the ridge though time. These “seafloor spreading”zones are the mechanism by which oceans are created/enlarged and by which continents can “drift” apart from each other! This needs to be an animation (Declan has one) In addition, scientists realized that the reversing magnetic stripes meant that the Earth’s magnetic field periodically “flipped”, such that sometimes magnetic north was located around the geographic south pole! Links to section 2.21 More on Seafloor Spreading These were unanticipated, but fundamental discoveries! Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Magnetic Variations in the Seafloor Concept question: If new crust is being created at mid-ocean ridges, what might you deduce about the surface area of the Earth? This needs to be an animation (Declan has one) Text box for students to enter their thoughts. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Deep Sea Trenches Scientists soon discovered ocean “trenches” –deep regions where ocean crust was plunging underneath other crust. In effect this “subducting” ocean crust was descending back into the interior of the Earth and being consumed. Problem solved! Oceanic crust is created at mid-ocean ridges and consumed at subduction zones (trenches.) Therefore, the surface of the Earth isn’t getting any larger; it has essentially remained the same size. Bathymetric image of the region around the world’s deepest trench – Challenger Deep, over 11,000 meters below sea level. This trench is deeper than Mt. Everest is high! (image from http://earthobservatory.nasa.gov/IOTD/view.php?id=77640) Links to section 2.22 More on Subduction Zones Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Seafloor Spreading and Subduction Zones Concept question: Given that the Earth is a sphere and not a cylinder, can the surface area of the Earth stay a constant size with only seafloor spreading zones and subduction zones? Text box for students to enter their thoughts. Return to Table of Contents

(Morgan, 1968) 2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Transform Boundaries In 1965 J. Tuzo Wilson published “A new Class of Faults and their Bearing on Continental Drift” in which he proposed that a 3rd type of boundary was necessary to accommodate movement of continents on a spherical Earth. He called these boundaries “transform faults” - locations where regions of crust slid past each other, without being created or consumed. He also showed how transform faults often offset segments of mid-ocean ridges. Links to section 2.23 More on Transform Faults Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Plate Boundaries Now we have discussed all three tectonic boundaries: Seafloor Spreading Zones - oceanic crust is created - occur along mid-ocean ridges - example: Mid-Atlantic Ridge Subduction Zones - oceanic crust is consumed - occur at deep sea trenches - example: Challenger Deep Transform Faults - crustal blocks slide past each other - offset mid-ocean ridges - example: San Andreas Fault Where are these tectonic boundaries currently located on the Earth? These all link to Google Earth tour locations Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Plate Boundaries Now we have discussed all three tectonic boundaries: Seafloor Spreading Zones - oceanic crust is created - occur along mid-ocean ridges - example: Mid-Atlantic Ridge Subduction Zones - oceanic crust is consumed - occur at deep sea trenches - example: Challenger Deep Transform Faults - crustal blocks slide past each other - offset mid-ocean ridges - example: San Andreas Fault This will be an interactive globe where students can explore plate boundaries, seafloor ages, areas of plates, volcanoes, earthquakes, etc. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Plate Boundaries Concept question: Based on the tectonic boundaries around North America, why do you think scientists discarded the term “Continental Drift”? Text box for students to enter their thoughts. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Plate Boundaries The tectonic boundaries around North America extend from the Pacific margin of California to the middle of the Atlantic ocean. This area defines a crustal “plate”, which moves as a unit. Since this unit encompasses both continental crust and oceanic crust, we now call the theory “Plate Tectonics” instead of Continental Drift. Return to Table of Contents

2.13 Plate Tectonics: The History Behind the Science Evidence from the oceans: Plate Boundaries Obviously, there is lots more to go with this chapter, but I think this is enough to discuss tomorrow. Return to Table of Contents

2.0 Plate Tectonics: The Unifying Theory of the S olid Earth