Plate Tectonics The Unifying Theory of Geology Lecture notes start on page 151 – Plate Tectonics and Geocycles are one lecture in two parts A more or less photo-free version of this PowerPoint will be linked from my webpage Warning: The next 3 slides are not in your lecture notes The 4 “Big Ideas” in Geology The Rock Cycle (Friday’s lecture) – By the mid-1700s Antiquityof Earth - “Deep Time” (Fr. Stang’s W 11/10 Lecture) – By the early 1800s, millions of years Faunal Succession (Fossil Record – Core 6) – By mid-1800s Plate Tectonics – By late 1960s The Big Ideas Earth is Old and Dynamic Rather than Young and Static •Recall: The Scientific Method Observations – discovery, look for patterns Hypothesis (a testable explanation) – Includes testing by prediction More observations (testing) -> support Eventually multiple hypotheses -> Theory – Not a “hunch” nor a “fact”, but the best available explanation using natural causes – A Theory is an explanatory framework of the highest level and includes many supported hypotheses Plate Tectonics is a great example of discovery-driven science that was eventually supported by many hypothesis-driven predictions A sequence of observations about Earth throughout human history… The Ancient Greeks Knew that Earth was round (not discovered by Columbus!) by 300 B.C. In fact Eratosthenes (ca. 240 B.C.) measured its circumference Photo Not Available St. Augustine, ca. 400 A.D. Not only knew that Earth was a sphere… But worried about the theological issue of the Antipodes (the other side of the globe) Romans 1:8; 10:18; 16:25-26; Col. 1:6, 23; I Tim. 3:16 – So the physical issue of a spherical Earth was well established in Western thought by this time. “Early” replaced with “Renaissance” Renaissance Observations FirstEuropean explorers started to gather knowledge on world geography Early 1500s: E.g., Magellan & da Gama Renaissance Observations Francis Bacon (1561-1626) first noted how coasts of Africa and South America fit. Yikes!!!… bacon! Today it’s known: Fit is best along their continental shelves The 1800s Bettermaps available Some people proposed that all continents could fit together In 1872, British Challenger began mapping ocean floor by soundings Similarities in rocks of NW Europe and NE America were discovered About 1910, the Mid-Atlantic Ridge was discovered The Three “Paleos” Paleontology The study of fossils Evidence from Paleontology Similar fossils in South America and Africa Glossopteris, a seed fern whose seeds are too large to be carried far by wind Mesosaurus, a fresh water reptile that couldn’t swim across the open sea Paleoclimatology The study of ancient climates Evidence of glaciers - Striations Evidence of glaciers in South America, Africa, India, and Australia... At same time the Northern Hemisphere had lush swamps... and the pieces fit together like a puzzle... “Gondwana” Coal Deposits found in Antarctica brrrrr…. Coal requires a warm, lush climate ===> What’s Antarctica like today? ===> Paleomagnetism Magnetic minerals in molten rock align with Earth’s magnetic field Paleomagnetism When igneous rock cools, magnetism is “frozen in”, like little compasses... “Polar Wandering” = apparent change in position of poles over time Polar Wandering Different continents indicate different position of North Pole - very strange... Only logical explanation is that the CONTINENTS have moved since the rocks were formed WEGENER and CONTINENTAL DRIFT In the 1910s Alfred Wegener put all these pieces of evidence together and made the hypothesis of CONTINENTAL DRIFT... The Origin of Continents and Oceans (1915) Hard to give up old ways of thought – Earth should be reliable, always there, and unmoving, “terra firma” – E pur si muove : Core 2 CONTINENTAL DRIFT When Wegener proposed to fit the continental shelves together, rather than the coastlines, we got a better fit for all modern continents... Idea: All continents have moved to their present positions from one “supercontinent” he called PANGAEA - 200 Ma Scientific Consensus is that Earth is about (this and next two slides not in notes) 4.5 billion years old (4.5 Ga), so 200 million years ago isn’t so long ago – about 5 % of Earth’s history Perspective, context Breakup of Pangaea Pangaea at 200 Ma Is still just 200/4500 = 4.5% of Earth’s history! There is evidence of plate movements well before this time 1912 - 1945 A few geologists looked for more evidence during this period... Seismologists began studying Earth’s deeper layers and discovered a dense mantle and liquid outer core WWII and SONAR Sonar, developed to find enemy subs, was used in the decade after WWII to map the deep sea floor... SONAR Previously, most geologists thought the sea floor was rather flat and featureless They were wrong... A diverse topography was discovered Topography of the Sea Floor Topography of the Sea Floor Researchers found an undersea mountain range 40,000 miles long And a trenches seven times deeper than the Grand Canyon Even more striking were the geophysical findings... Some Geophysics Thousands of drilling samples were taken... Geophysics Paleomagnetism in the deep sea floor rocks indicated that many episodes of magnetic reversals had taken place... These reversals occur in parallel paired bands on opposite sides of a mid-ocean ridge. Radiometric dating showed the rocks get older the farther you get from the ridge. Sea-Floor Spreading In 1962, these data were collected into a theory called SEA-FLOOR SPREADING – New crust forms at mid-ocean ridges... – Oceanic crust pushes outwards from the ridge and (perhaps) takes the continents along Earthquakes and Volcanoes Seismology is the study of earthquakes Remember the 2004 Indian Ocean Tsunami? Earthquakes and Volcanoes Data indicate that earthquakes and volcanoes do not occur at random locations, rather... Earthquakes and volcanoes occur mostly along or near deep ocean trenches and mid-ocean ridges... E.g., Pacific “Ring of Fire” Seismology studies located a zone of weakness where seismic waves travel more slowly It was called the asthenosphere. The rigid lithosphere sits on top of this Finally, a unifying theory... In 1968, seismologists at Columbia put all the evidence together and came up with the theory of PLATE TECTONICS... This combined the sub-theories of CONTINENTAL DRIFT and SEA- FLOOR SPREADING PLATE TECTONICS The rigid upper 40 miles or so of Earth (lithosphere) is broken up into a dozen or so plates, which can slide around on the zone of weakness (asthenosphere). MECHANISM Current idea: plates move as a result of mantle convection, driven by Earth’s internal heat Why does Earth have internal heat? And our Moon doesn’t? Why doesn’t the Moon have Plate Tectonics? – or volcanoes, earthquakes, or a magnetic field? Answer: Earth still has a lot of internal heat And our Moon doesn’t Why? Moon is much smaller than Earth It’s cooled off – Deep Time Consequences of Plate Tectonics Earthquakes Volcanoes Mountain Ranges Why it’s a “Unifying Theory” First a review of definitions... Types of Plate Boundaries DIVERGENT - pulling apart – Mid-Atlantic Ridge – Great Rift Valley of Africa Great Rift Valley of Africa CONVERGENT - coming together – Japan – Himalaya mountains Himalaya mountains formed when India collided with Asia TRANSFORM - slide past each other –San Andreas Fault San Andreas Fault Volcanoes Are associated with divergent and convergent plate boundaries ... Why ? Pacific rim “Ring of Fire” Convergent and Divergent boundaries Earthquakes Are associated with ALL boundary types ... Why ? Movement! Mountain Building “Tectonics” “Tectonics” Comes from the Greek word for “building” - For example, architect “master builder” or technology “the study of built things” Some ages And other facts… Mountain Ranges Occur in linear patterns that parallel current or former plate boundaries Appalachians Started forming 400 million years ago (400 Ma) Folded sediments and hard rock, but evidently worn down by erosion Highest elevation = Mt. Mitchell - 6,684 ft Cumberland Gap Rockies Started forming 60 Ma Highest elevation = 14,440 ft (Mt. Elbert) Maroon bells, CO Himalayas Started forming 30 Ma Highest elevation = 29,035 ft (Mt. Everest) Laurentians (in Quebec) Canadian “Shield” > 500 Ma Hard rock “roots” of an ancient mountain range Highest elevation = 3,825 feet Folded Strata Rockies Appalachians MOUNTAIN BUILDING and PLATE TECTONICS CONVERGENT BOUNDARY - the Real Deal for mountain building Complex mountain system develops ---> Continent Grows Example: The Andes The Andes Geocycles Rocks, Water, Air GEOCYCLES Recycling of matter at or near Earth’s surface Rock cycle –Lithosphere Water cycle –Hydrosphere Weather and Climate –Atmosphere First, the ROCK CYCLE Nice Factoids to know – How big is our planet? Earth is about 8,000 miles in diameter and 25,000 miles in circumference So, it will take you about 20 hours in a jetliner at 600 mph to get from Spain to New Zealand 4 concentric layers Inner core (solid) Outer core (liquid) Mantle (gooey in places) Crust (solid) 3-40 miles thick What is the Lithosphere and what are the “Plates”? The crust, plus the upper 40 miles of the mantle, form the lithosphere, broken into plates which slowly slide around on the asthenosphere (zone of weakness)… via energy from Earth’s internal heat IGNEOUS RXS “Fire-formed” Liquid rock is called magma or lava, depending on amount of dissolved gas INTRUSIVE - cooled under surface – Examples: Granite, Gabbro VOLCANIC - cooled at surface – Examples: Basalt, Obsidian, Scoria SEDIMENTARY RX “Settling” Clastic - formed from pieces “clasts”, which come from the weathering and erosion of other rock. – Examples: Shale, Sandstone, Conglomerate Bio-chemical and Organic – microskeletons, e.g. in coral reefs: Limestone – Salts in stagnant pools, e.g., Halite – Organic - decayed remains of plants: coal METAMORPHIC RX “Changed form” Depends on original rock type and amount of heat and pressure Shale ----> Slate or Schist (more heat) Granite or “dirty” sandstone ---> Gneiss Sandstone ----> Quartzite Limestone ----> Marble IGNEOUS RXS and ABSOLUTE DATING Some igneous rocks can be radiometrically “dated” – i.e., have an ABSOLUTE AGE assigned (+/-) On the other hand… SEDIMENTARY ROCKS Are most likely to contain fossils But, SEDIMENTARY ROCKS Can only be relatively dated… RELATIVE DATING RELATIVE DATING DEEP TIME Oceans vs. Puddles AIR and WATER Water Facts... 70 % of Earth’s surface is covered by H2O 70 % of human body’s weight is H2O Water can exist in 3 states on Earth’s surface – exists only as solid and vapor on Mars – only vapor on Venus Mars is too cold... Venus is too hot... Earth is just right... More Water Facts... Water freezes at 0 degrees Celsius and boils at 100 degrees Celsius –This is higher than similar substances because of hydrogen bonding –Water is most dense at 4 oC. This explains why ice floats, which prevents lakes from freezing solid in Winter Hydrogen Bonding ........O H ........ / \ / H H ---- O | | \ O O H ......... / \ / \ H H H H ......... | | | | Water... Itsheat capacity is higher than similar substances – “A watched pot never boils.” Moderates Earth’s temperatures – Compare San Francisco and St. Louis AIR Main gases in present atmosphere: Nitrogen (78%), Oxygen (21%), and Argon (0.9%), variable amounts of water vapor Average sea level atmospheric pressure is 14.7 pounds per square inch and decreases with altitude DISTRIBUTION of EARTH’S WATER and AIR Origin Origin How: Impact of Icy Comets and Degassing of volatiles ... bottle Earth’s atmosphere was forming 4.4 to 4.0 Ga It then (probably) consisted mainly of water vapor, carbon dioxide, ammonia, methane (Some controversy) Mt. Erebus, Antarctica Brief History Icy Comet impacts ---> add H2O to atmosphere and down to about 70km Earth cools ---> Water condenses Lots of rain; Volcanoes add CO2 Most of CO2 dissolved in oceans Photosynthesizing organisms evolve – CO2 + H2O ------> C6H12O6 + O2 The Water Cycle How Earth’s water and air interact ... Natural “recycling” process The Water Cycle ZONES OF CONCENTRATION Of natural waters ZONES OF CONCENTRATION Marine (saltwater)- The oceans contain over 97 % of earth’s water -- salt water Brackish (mixture of salt and fresh): Found in estuaries, where fresh and salt water meet – Important to aquatic life – 3rd most productive ecosystem – Example: Chesapeake Bay Fresh Water Less than 3 % of Earth’s water is fresh Glaciers contain over 75 % of Earth’s fresh water, but this source is not usable by life Fresh Water Lakes and rivers contain less than 1 % Groundwater (including soil moisture) accounts for the remaining 24 % If all Earth’s water fit in a gallon jug, there would be one tablespoon of available fresh water Structure of the Atmosphere Layers classified by temperature changes ... The Atmosphere All weather occurs in the troposphere = “churning sphere” Convection causes the “churning” Temperature decreases with altitude Convection in Humid Air Thunderhead Stratosphere The ozone layer is contained in the stratosphere ... No “churning” because of a temperature inversion, i.e., Temperature increases with altitude... A l t i t Stratosphere ---> u d e Troposphere ---> Temperature Good Ozone and Bad Ozone The same molecule, O3 The Ozone Layer Ozone, O3 , is an air pollutant in the troposphere, but in the stratosphere protects us from UV radiation : O3 + UV --------> O2 + O Chlorine radicals (like from CFCs) interfere with this process: Cl + O --------> ClO WEATHER and CLIMATE - Atmosphere Geocycles Coriolis Effect Caused by an unattached atmosphere over a rotating Earth Which way does Earth rotate? East to West Or West to East? Coriolis Effect Coriolis Effect on Winds Coriolis Effect on Winds NON-ROTATING ACTUAL PREVAILING WINDS The direction the winds blow most of the time ... Wind direction changes temporarily due to weather systems ... o o In the middle latitudes (30 - 60 ), the prevailing winds are from the west. These affect climate as we shall see in a moment ... PREVAILING WINDS OCEAN CURRENTS (Surface) currents caused by prevailing winds and Coriolis Effect ... They play a major role in redistributing Earth’s heat ... They are called warm or cold, compared with other water at that latitude ... –Example: The Gulf Stream is a warm current. OCEAN CURRENTS Effects on Climate Many factors affect climate Prevailing winds and ocean currents are only two of these Example: Why is the weather in Paris more pleasant than in Quebec City ? Gulf Stream + Prevailing westerlies Which is farther North? Minneapolis or Venice Minneapolis: 44.58o N Venice: 45.27o N Who’d ‘a thunk it?
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