Plate Tectonics

lithosphere asthenosphere mesosphere

Lithosphere (hard) Asthenosphere (soft) Mesosphere Lithosphere includes: Crust– oceanic (basalt)- heavier and thinner - continental (granite)- lighter and thicker Lithosphere and asthenosphere differ both physically and chemically Asthenosphere (soft) Mesosphere

Earth formed 4.6 bya Inner Core- 4300oC mostly iron core inner part is so compressed that it is solid Outer Core- 3700oC iron and sulfur liquid Mantle- 1000oC mesosphere Solid ~ 2300 km thick asthenosphere Soft ~3000 km thick lithosphere hard ~100 km thick Crust floats on top of lithosphere continental crust (granite) 20 to 70 km thick oceanic crust (basalt) ~ 8 km thick mantle-crust mass and is probably composed mainly of silicon, magnesium, and oxygen. It probably also contains some iron, calcium, and aluminum. Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites. Transition region: 7.5% of Earth's mass; depth of 400-650 kilometers (250-406 miles) The transition region or mesosphere (for middle mantle), sometimes called the fertile layer, contains 11.1% of the mantle-crust mass and is the source of basaltic magmas. It also contains calcium, aluminum, and garnet, which is a complex aluminum-bearing silicate mineral. This layer is dense when cold because of the garnet. It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma. Upper mantle: 10.3% of Earth's mass; depth of 10-400 kilometers (6 - 250 miles) The upper mantle contains 15.3% of the mantle-crust mass. Fragments have been excavated for our observation by eroded mountain belts and volcanic eruptions. Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been the primary minerals found in this way. These and other minerals are refractory and crystalline at high temperatures; therefore, most settle out of rising magma, either forming new crustal material or never leaving the mantle. Part of the upper mantle called the asthenosphere might be partially molten. Oceanic crust: 0.099% of Earth's mass; depth of 0-10 kilometers (0 - 6 miles) The oceanic crust contains 0.147% of the mantle-crust mass. The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000-kilometer (25,000 mile) network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with basalt. Hawaii and Iceland are two examples of the accumulation of basalt piles. Continental crust: 0.374% of Earth's mass; depth of 0-50 kilometers (0 - 31 miles). The continental crust contains 0.554% of the mantle-crust mass. This is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust (both oceanic and continental) is the surface of the Earth; as such, it is the coldest part of our planet. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).

Principles of plate tectonics The Earth is composed of a mosaic of thin rigid plates (pieces of lithosphere) that move horizontally with respect to one another Plates interact with each other along their plate boundaries Plate boundaries associated with tectonic activity (mountain building, earthquakes, active volcanoes)

Continental Drift Theory German meteorologist, Some truly revolutionary scientific theories may take years or decades to win general acceptance among scientists. This is certainly true of plate tectonics, one of the most important and far-ranging geological theories of all time; when first proposed, it was ridiculed, but steadily accumulating evidence finally prompted its acceptance, with immense consequences for geology, geophysics, oceanography, and paleontology. And the man who first proposed this theory was a brilliant interdisciplinary scientist, Alfred Wegener. Born on November 1, 1880, Alfred Lothar Wegener earned a Ph.D in astronomy from the University of Berlin in 1904. However, he had always been interested in geophysics, and also became fascinated with the developing fields of meteorology and climatology. During his life, Wegener made several key contributions to meteorology: he pioneered the use of balloons to track air circulation, and wrote a textbook that became standard throughout Germany. In 1906 Wegener joined an expedition to Greenland to study polar air circulation. Returning, he accepted a post as tutor at the University of Marburg, taking time to visit Greenland again in 1912-1913. (The above photograph of Wegener was taken during this expedition). In 1914 he was drafted into the German army, but was released from combat duty after being wounded, and served out the war in the Army weather forecasting service. After the war, Wegener returned to Marburg, but became frustrated with the obstacles to advancement placed in his way; in 1924 he accepted a specially created professorship in meteorology and geophysics at the University of Graz, in Austria. Wegener made what was to be his last expedition to Greenland in 1930. While returning from a rescue expedition that brought food to a party of his colleagues camped in the middle of the Greenland icecap, he died, a day or two after his fiftieth birthday. While at Marburg, in the autumn of 1911, Wegener was browsing in the university library when he came across a scientific paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic. Intrigued by this information, Wegener began to look for, and find, more cases of similar organisms separated by great oceans. Orthodox science at the time explained such cases by postulating that land bridges, now sunken, had once connected far-flung continents. But Wegener noticed the close fit between the coastlines of Africa and South America. Might the similarities among organisms be due, not to land bridges, but to the continents having been joined together at one time? As he later wrote: "A conviction of the fundamental soundness of the idea took root in my mind." Such an insight, to be accepted, would require large amounts of supporting evidence. Wegener found that large-scale geological features on separated continents often matched very closely when the continents were brought together. For example, the Appalachian mountains of eastern North America matched with the Scottish Highlands, and the distinctive rock strata of the Karroo system of South Africa were identical to those of the Santa Catarina system in Brazil. Wegener also found that the fossils found in a certain place often indicated a climate utterly different from the climate of today: for example, fossils of tropical plants, such as ferns and cycads, are found today on the Arctic island of Spitsbergen. All of these facts supported Wegener's theory of "continental drift." In 1915 the first edition of The Origin of Continents and Oceans, a book outlining Wegener's theory, was published; expanded editions were published in 1920, 1922, and 1929. About 300 million years ago, claimed Wegener, the continents had formed a single mass, called Pangaea (from the Greek for "all the Earth"). Pangaea had rifted, or split, and its pieces had been moving away from each other ever since. Wegener was not the first to suggest that the continents had once been connected, but he was the first to present extensive evidence from several fields. polar explorer, astronomer, and geologist In 1912 he proposed Continental Drift Theory Wegner suggested that all the earth’s land has once been joined into a supercontinent called Pangae Wegener eventually proposed a mechanism for continental drift that focused on his assertion that the rotation of the earth created a centrifugal force towards the equator.  He believed that Pangaea originated near the south pole and that the centrifugal force of the planet caused the protocontinent to break apart and the resultant continents to drift towards the equator.  He called this the "pole-fleeing force".  This idea was quickly rejected by the scientific community primarily because the actual forces generated by the rotation of the earth were calculated to be insufficient to move continents.  Wegener also tried to explain the westward drift of the Americas by invoking the gravitational forces of the sun and the moon, this idea was also quickly rejected.  Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories Alfred Wegener Proposed Theory of Continental Drift (1915) Failed to provide a mechanism

Evidence for Continental Drift continental shape similar geology fossil evidence (animal and plant) volcano and earthquake zone paleomagnetism Geology: areas of erosion apparently caused by the same glacier in tropical areas now widely separated

His ideas were dismissed as crank Problem– lacked technology that wasn’t available until after WW II Echosounding, radiocarbon dating, submarines

Objections to the continental drift model Wegener envisioned continents plowing through ocean basins Wegener did not provide a plausible mechanism to explain how the continents could have drifted apart Most Earth scientists rejected continental drift because it was Too far-fetched Contrary to the laws of physics Lacked technology

Evidence for continental drift Jigsaw Puzzle Evidence for continental drift Matching coastlines on different continents Puzzle noted by Leonardo DaVinci and others

Evidence for continental drift Similar Geology Evidence for continental drift Matching mountain ranges across oceans Today 300 million years ago

Evidence for continental drift Glacial ages and climate evidence

Fossil Evidence

Fossil Evidence Distribution of fossils such as Mesosaurus Mesosaurus

Permian 225 mya Triassic 200 mya Jurassic 135 mya Cretaceous 65 mya Breakup of Pangae 225 mya Present Day

Marie Tharp “It was very exciting in those days. We were explorers.” Marie Tharp (1920–) Oceanographer Faculty 1948–83 A pioneer of modern oceanography, Tharp was the first to map details of the ocean floor on a global scale.  Her observations became crucial to the eventual acceptance of the theories of plate tectonics and continental drift in the earth sciences. Working with pens, ink and rulers, Tharp drew the underwater cartography, longitude degree by latitude degree, based on data from sonar readings taken by pioneering earth scientist Maurice Ewing and his team.  Piecing maps together in the late 1940s and early 1950s, she and colleague Bruce Heezen discovered a 40,000-mile underwater ridge girdling the globe.  By this finding, they laid the foundation for the conclusion from geophysical data that the sea floor spreads from central ridges and that the continents are in motion with respect to one another—a revolutionary geological theory at the time.  Years later, satellite images proved Tharp’s maps to be accurate. Tharp came to Columbia in 1948 to work as Ewing’s research assistant.  Following him to the new Lamont Geological Observatory, she provided much of the data and analyses for Ewing and Hezeen’s scientific papers. In recent years, she has been honored for her contributions by the Library of Congress, the Women's Committee of the Woods Hole Oceanographic Institution, and the place where it all began, now the Lamont-Doherty Earth Observatory.

Marie Tharp's "World Ocean Floor Map” 1977

Evidence for plate tectonics Pattern of worldwide earthquakes (left) matches plate boundaries (right)

Plate Tectonics & A circle of volcanic geological activity surrounding much of the Pacific Ocean volcanoes and earthquake zones coincide Theory of Plate Tectonics proposed by Tuzo Wilson (1965) Earth’s outer layer consists of dozens of plates Plate movement- ave. 5cm/year

Hess- Convection Cell Theory

Midocean Ridge (Atlantic to Pacific) New seafloor development at mid ocean ridge Spreading center Does this mean that the earth is constantly expanding? No, there are subduction zones 2 football teams: mountain building and trench formation

The 3 types of plate boundaries Divergent Convergent Transform

Divergent plate boundaries The Mid-Atlantic Ridge is a divergent plate boundary where sea floor spreading occurs

Divergent plate boundaries Iceland sits atop a divergent plate boundary where continental rifting occurs

Divergent plate boundaries Formation of an ocean basin by rifting and sea floor spreading

Convergent plate boundaries a. Ocean-continent Convergent plate boundaries vary depending on the type of crust c. Continent-continent b. Ocean-ocean

Convergent plate boundaries An ocean-continent convergent plate boundary produces the Cascadia subduction zone and Cascade Mountains

Convergent plate boundaries A continent-continent convergent plate boundary produces the Himalaya Mountains

Transform plate boundaries Transform plate boundaries occur between segments of the mid-ocean ridge Can also occur on land (ex: San Andreas Fault)

The world as it may look 50 million years in the future

Glomar Challenger (1960’s) Deep sea ocean drilling It was on June 24, 1966, that the Prime Contract between the National Science Foundation and The Regents, University of California was signed. This contract began Phase I of the Deep Sea Drilling Project which was based out of Scripps Institution of Oceanography at the University of California, San Diego. Global Marine, Inc. performed the actual drilling and coring. The Levingston Shipbuilding Company laid the keel of the D/V Glomar Challenger on October 18, 1967, in Orange, Texas. The ship was launched on March 23, 1967, from that city. It sailed down the Sabine River to the Gulf of Mexico, and after a period of testing, the Deep Sea Drilling Project accepted the ship on August 11, 1968. Over the next 30 months, Phase II consisted of drilling and coring in the Atlantic, Pacific, and Indian oceans as well as the Mediterranean and Red Seas. Technical and scientific reports followed during a ten month period. Phase II ended on August 11, 1972, and ship began a successful scientific and engineering career. The success of the Challenger was almost immediate. On Leg 1 Site 2 under a water depth of 1067 m (3500 ft), core samples revealed the existence of salt domes. Oil companies received samples after an agreement to publish their analyses. The potential of oil beneath deep ocean salt domes remains an important avenue for commercial development today. But the purpose of the Glomar Challenger was scientific exploration. One of the most important discoveries was made during Leg 3. The crew drilled 17 holes at 10 different sites along a oceanic ridge between South America and Africa. The core samples retrieved provided definitive proof for continential drift and seafloor renewal at rift zones. This confirmation of Alfred Wegener's theory of continental drift strengthened the proposal of a single, ancient land mass, which is called Pangaea. The samples gave further evidence to support the plate tectonics theory of W. Jason Morgan and Xavier Le Pichon. The theory of these two geologists attempts to explain the formation of mountain ranges, earthquakes, and deep sea trenches. Another discovery was how youthful the ocean floor is in comparison to Earth's geologic history. After analysis of samples, scientists concluded that the ocean floor is probably no older than 200 million years. This is in comparison with the 4.5 billion years of our Earth. As the seafloor spreads from the rifts, it descends again beneath tectonic plates or is pushed upwards to form mountain ranges. The ship retrieved core samples in 30 ft long cores with a diameter of 2.5 in. These cores are stored at the Lamont-Doherty Earth Observatory (LDEO) at Columbia University and at Scripps Institution of Oceanography. After splitting the core in half length-wise, one half was archived and the other is still used as a source to answer specimen requests. Although itself a remarkable engineering feat, the Challenger was the site of many advances in deep ocean drilling. One problem solved was the replacement of worn drill bits. A length of pipe suspended from the ship down to the bottom of the sea might have been as long as 20,483 ft (6243 m)(as was done on Leg 23 Site 222). The maximum depth penetrated through the ocean bottom could have been as great as 4,262 ft (1299 m)(as at Site 222). To replace the bit, the drill string must be raised, a new bit attached, and the string remade down to the bottom. However, the crew must thread this string back into the same drill hole. The technique for this formidable task was accomplished on June 14, 1970, in the Atlantic Ocean in 10,000 ft (3048 m) of water off the coast of New York. This re-entry was accomplished with the use of sonar scanning equipment and a re-entry cone which had a diameter of 16 ft (4.88 m) and height of 14 ft (4.27 m).

Earth's Magnetic Field N S Earth has N and S pole Iron bearing magnetic minerals are found in basaltic magma. As they cool, they align with the earths magnetic field The rotation of the earth, coupled with liquid iron, produces the earths magnetic field As the ocean floo rrises from the asthenosphere, its material is magnetized in the current direction Reverses once every 300,000 to 500,000 years Effect on animals that use magnetic field to navigate unknown

Mid-Ocean Ridge (Atlantic Ocean) Molten rock erupts along a mid-ocean ridge, then cools and freezes to become solid rock. The direction of the magnetic field of the Earth at the time the rock cools is "frozen" in place. This happens because magnetic minerals in the molten rock are free to rotate so that they are aligned with the Earth's magnetic field. After the molten rock cools to a solid, these minerals can no longer rotate freely. At irregular intervals, averaging about 200-thousand years, the Earth's magnetic field reverses. The end of a compass needle that today points to the north will instead point to the south after the next reversal. The oceanic plates act as a giant tape recorder, preserving in their magnetic minerals the orientation of the magnetic field present at the time of their creation. Geologists call the current orientation "normal" and the opposite orientation "reversed." - + - + - + + - + - + - There have been 170 reversal in the last 76 million years. The earth’s present orientation has existed for the past 60,000 years.

Age of the Atlantic