Introduction To Geology Geology is the study of the Earth Materials Phenomena and Processes Life Lava Lamp Meteorite magnets, glass plate, iron chips 1 Scientific Method Hypothetico-deductive framework Hypotheses have testable consequences We test hypotheses to try to falsify them Karl Popper http://en.wikipedia.org/ wiki/Karl_Popper 1934 Popper Logic of Scientific Discovery Popper: Hypotheses In Science are 2 Falsifiable Paradigm: a supermodel with unexpected predictive power http://en.wikipedia.org/wiki/Thomas_Kuhn http://www.amazon.com/StructureScientific-Revolutions-ThomasKuhn/dp/0226458040/ref=pd_cp_b_1 3 Early Geological Concepts Catastrophism Flood, Earthquake, Tsunamis, Meteors Uniformitarianism Present is key to past, slower processes Erosion, deposition, mountains, glaciers James Hutton (1726-1797) Championed by Charles Lyell Notice these are early Paradigms 4 Modern Synthesis The Plate Tectonics Paradigm Uniformitarian and slow •Mass Extinctions Sudden Catastrophes •Both are correct A 6-7 km meteor hit Yucatan ~65 mya; major extinction event 5 Origin of the Universe The spectral shift of light coming from distant galaxies tells us that the universe is expanding* out of a very small volume began about 13.7 bya An object moving away shifts the absorption line toward the red. An object moving toward you shifts the line toward the violet Yellow-green absorption line of Helium, wavelength 587 nanometers The universe expanded from a state of pure energy, hydrogen atoms evolved through a process called nucleosynthesis * Distant objects are red-shifted http://en.wikipedia.org/wiki/Redshift 6 Element: an atom with a certain number of Protons, e.g. Oxygen has 6 Carbon has 8 Origin of the Elements Hydrogen gas clouds condensed to form main sequence stars. Hydrogen burning forms Helium Main sequence stars form Oxygen and Carbon. 3 Heliums make 1 Carbon Fred Hoyle 7 Water = 2 Hydrogen + 1 Oxygen Origin of Heavy Elements A star more than 9 times the mass of our sun burns faster, then expands into a red super giant star. Betelgeuse, for example. Pressure is high enough to also produce the heavier elements including silicon, magnesium, and iron. Once its fuel is exhausted, a supernova explosion *occurs due to core collapse. 8 *one of many ways Origin of Our Solar System Our solar system with its abundant collection of heavier elements condensed from the gas cloud left after the explosion of a supernova. 9 Supernova ejects matter rich pressure waves into space Local concentrations of dust coalesce Balance between gravity and solar wind For low speeds Force = mass x acceleration F= ma Energy = Force x distance E = mass x accel x distance It takes more energy to move a more massive atom, and/or to move an atom a greater distance Which is why heavy Silicon, Oxygen and Iron planets are near the sun, and lighter Hydrogen Helium planets 10 are further away 1.During coalescence: 1_7 Particles assemble due to gravity – heat up CLAP HANDS 3.DIFFERENTIATION Iron melts and begins to sink 4. 5. Lighter materials concentrate closer to surface 6. Crust and Mantle Crust and Silicate Minerals 7. Core Iron and Nickel; liquid outer core, solid inner core mantle 2.Planetesimals strike growing Earth 2bThe moon formed after a Mars-sized planet hit earth, about 4.6 bya We use that event as Earth’s origin date 8. Interior of Earth is hotter than surface Liquid core Atmosphere Crust Mantle Outer core Inner core 11 Earth’s Internal Structure Earth’s internal layers defined by Chemical composition Physical properties Deduced from Seismographs of Earthquakes Meteorites lend support Layers defined by composition Crust Mantle Core Iron-Nickel Meteorite 12 The density of Earth’s Layers “Three centuries ago, the English scientist Isaac Newton calculated, from his studies of planets and the force of gravity, that the average density of the Earth is twice that of surface rocks and therefore that the Earth's interior must be composed of much denser material. “ http://pubs.usgs.gov/gip/interior/ 13 Earth’s internal structure Main layers of Earth are based on physical properties including mechanical strength Outer layers mostly Silicate Minerals: Crust and Mantle Lithosphere (behaves like a brittle solid) Crust and uppermost mantle Asthenosphere “weak sphere” Rest of Upper Mantle Heat softened, plastic solid Lower Mantle Solid due High Pressures • Inner Layers Core Iron and Nickel, outer above melting point - liquid, inner solid due to high pressures 14 1_8 CRUST (least dense) Upper mantle Continental crust Oceanic crust MANTLE 0 km ~100 km ~350 km Lower mantle Lithosphere Asthenosphere CORE (most dense) Outer core ~5155 km Inner core ~2900 km Earth’s center is 6371 kilometers below the surface, 1 mi = 1.61 km. Equals ~ 3957 miles, or about 4000 miles radius 6371 km x 1 mile/1.61 km = 3957 miles 15 http://pubs.usgs.gov/gip/interior/ “Although the core and mantle are about equal in thickness, the core actually forms only 15 percent of the Earth's volume, whereas the mantle occupies 84 percent. The crust makes up the remaining 1 percent. “ 16 There are three types of Rock IGNEOUS – From molten rock SEDIMENTARY – From rock fragments deposited in water and cemented together. Also from precipitates. METAMORPHIC – Rocks altered by heat, pressure, chemical reactions 17 Sediment Rock Cycle Each type can be formed from any other Weathering, transport, and deposition IGNEOUS ROCK Cementation and compaction Heat and pressure (metamorphism) Weathering, transport, and deposition Cooling and Solidification (crystallization) SEDIMENTARY ROCK Heat and pressure (metamorphism) 1_11 Melting Magma (molten rock) METAMORPHIC 18 ROCK 1_14 Humans a very recent event Note extinctions end of Permian and Cretaceous Modern humans Extinction of dinosaurs, others, 75% Flowering plants and grasses First mammals Earliest dinosaurs Extinction of Trilobites, many others. 83% genera Early reptiles Primitive fish Memorize the Periods and dates when Eras start & end Use Timescale doc, which is simplified 19 The Geology Paradigm The continents fit together WW II sonar reveals mid-ocean ridge 20 Continental drift: An idea supported with tests Why do the continents fit together? Alfred Wegener Summarized evidence in 1915 Published The Origin of Continents and Oceans Wegener’s Continental drift hypothesis Supercontinent Pangaea began breaking apart about 200 million years ago This hypothesis is testable http://en.wikipedia.org/wiki/Alfred_Wegener 21 Wegener’s Continental drift hypothesis Continents "drifted" to present positions Evidence used in support of continental drift hypothesis: Fit of continents Fossil evidence Rock type and mountain belts Paleoclimatic evidence 22 Fit of Continents: Pangaea approximately 200 million years ago Especially good agreement if continental shelf is included. Coastlines Fit Mountain Ranges line up 23 Fossil Evidence Fossil Reptiles similar in South Africa, Antarctica, South America and India Fossil Plants also very similar More about this later 24 Matching of mountain ranges on continents 25 Paleoclimatic evidence for Continental Drift Ancient glaciers line up if continents together 26 Arthur Holmes Wegener had no mechanism. Arthur Holmes proposed that Earth's mantle contained convection cells that dissipated radioactive heat and moved the crust at the surface 27 Some useful background Common lavas contain the mineral Magnetite Magnetite crystals align with earth’s magnetic field Gives us latitude where the lava “froze” ALSO “North” and “south” reverse at irregular intervals Good for aligning events worldwide28 The revolution begins During the 1940s and 1950s technological advances permitted mapping of the ocean floor. There are mountain ranges in the centers of oceans. Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s. http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_hess.html Harry kept his depth sounder on, collected huge amounts of data. Together, the data 29 show mid-ocean ridges. Harry Hess: Mid-ocean ridges are spreading apart due to flow in the mantle. Crust moves apart as if on conveyer belts. Note Mid-ocean Ridges (aka MORs) 30 How can we test the hypothesis? Fred Vine and Drummond Matthews: How about geomagnetic reversals? Earth's magnetic field reverses polarity at irregular intervals– north magnetic pole becomes south magnetic pole, and vice versa Dates when polarity of Earth’s magnetism changed were determined from lava flows 31 Paleomagnetic reversals recorded by new lava rock at mid-ocean ridges This common lava rock is called “Basalt” SHOW SAMPLE The Magnetite crystals align with the magnetic field, forming large bands of north-pointing or southpointing ocean floor. 32 Geomagnetic reversals Geomagnetic reversals are recorded in the ocean crust when hot lava solidifies. Magnetic minerals point to poles In 1963 Fred Vine and Drummond Matthews pointed out that symmetrical magnetic stripes in the ocean crust near ridges were predicted by Hess’s concept of seafloor spreading 33 The Test: Princeton PostDoc Fred Vine So, they checked. NOT FALSE 34 Structure of three Plates 35 Three boundary types, divergent, convergent, and transform Also: Oceanic Crust youngest at ridges Hess model prediction: sea floor youngest at ridges, oldest at edges Also NOT FALSE 36 Oldest ~ 200 my Plate tectonics: The new paradigm Earth’s major plates Associated with Earth's strong, rigid outer layer Known as the lithosphere Consists of uppermost mantle and overlying crust Overlies a weaker region in the mantle called the asthenosphere. The Asthenosphere is hot and plastic, and sheds heat via convective currents. 37 Mid-ocean 1_20 Ridge Origin of new Ocean Floor At the Mid-Ocean Ridge • Mantle material is move to the surface. • Lithosphere (Crust + Upper Mantle) bulges into a midocean ridge. • It cracks, exposing the mantle to low pressures • Some of the Mantle minerals are unstable at atmospheric pressures • The unstable minerals melt forming lavas, and cool into basalt, the main rock of ocean lithosphere. 38 Mantle circulations are an example of convection, heat transfer by moving material This example shows transfer of core heat to the upper mantle and crust 39 180º 90º 0º 90º 180º Mid-Atlantic Ridge 1_15 45º 45º NORTH AMERICAN PLATE JUAN DE FUCA PLATE EURASIAN PLATE ARABIAN PLATE CARIBBEAN PLATE PACIFIC PLATE PHILIPPINE PLATE AFRICAN PLATE COCOS PLATE 0º PACIFIC PLATE FIJI PLATE SOUTH AMERICAN PLATE NAZCA PLATE SCOTIA PLATE 45º 0º Mid-Atlantic Ridge INDIANAUSTRALIAN PLATE 45º ANTARCTIC PLATE 180º Convergent plate boundary Divergent plate boundary Transform plate boundary ANTARCTIC PLATE 90º 180º rate Plates move0ºrelative to each other at90º a very slow but continuous Average about 5 centimeters (2 inches) per year Seven major lithospheric plates Cooler, denser slabs of oceanic lithosphere descend into the mantle Seven or so smaller ones. Plates are in motion and change in shape and size Largest plate is the Pacific plate 40 Several plates include an entire continent plus a large area of seafloor 1_22a Concept caused revelation. Yes, revelation. Earth’s many features were all caused by the same process. Oceanic lithosphere being subducted (a) Water driven out of ocean lithosphere Water hits mantle, which partially melts. Forms a deep basaltic magma 41 Plate boundaries Each plate bounded by combination of all three boundary types: divergent, convergent, transform Edges marked by Earthquakes 42 Three main plate boundaries 43 Okay, lets go back over this in more detail. We saw divergent plate boundaries Most divergent plate boundaries are located along the crests of oceanic ridges Mid ocean ridges are the site of seafloor spreading Seafloor is elevated by asthenosphere upwelling and expansion due to heating, forming oceanic ridges How people learn, teaching method 44 1_20 Mid-ocean ridge The upwelling at the mid-ocean ridges occurs where rising convection currents in the mantle reach the lithosphere and diverge. Sea-level is raised. 45 Divergent boundaries are located mainly along oceanic ridges 46 Divergent boundaries in Continents Continental rifts “RIFT VALLEY” Splits landmasses into two or more smaller segments We are above the rift that opened the Atlantic about 190 mya. Began 200 mya. We sit above the western continental half. Identical rocks (red shales, basalts from lava flows, lake deposits) can be seen in Morocco. Modern examples include East African rifts Produced by extensional forces acting on the lithospheric plates 47 The East African Rift The rift valley collects river and lake sediments. Land animals are preserved as fossils instead of being eroded away 48 Continental Rift into Ocean Basin Rift Valleys and Oceans are the same thing 49 Convergent plate boundaries •On the other side of a plate, opposite the diverging margin, a converging margin is common. •Three different types, formed from pushing together of ocean floors and/or continental plate 50 Types of Convergent Boundaries Ocean-Continent Descending convective cell this side Yields Continental Volcanic Arc Ocean-Ocean Yields Volcanic Island Arc Descending convective cell this side Continent-Continent Yields Collision Mtns. Alps, Himalayas, Appalachians Descending convective51 cell this side 1_22a 1. Ocean - continent convergence A volcano forms as magma reaches the surface Oceanic lithosphere being subducted (a) A Subduction Zone As plate descends into the Subduction Zone, partial melting of mantle rock makes magmas (Molten Rock) These are buoyant, and rise. Volcanic mountains associated with subduction are called volcanic arcs. Andes and Cascades mountains are continental volcanic arcs 52 Types of convergent boundaries: 2. Oceanic-oceanic convergence When two oceanic slabs converge, one descends beneath the other. Often forms volcanoes on the ocean floor above the subduction zone. If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands) 53 3. Continental-continental convergence 1_22b • Continued subduction brings continents together • Less dense, buoyant, thick continental lithosphere does not subduct well •Result is a collision between two continental blocks. Process produces folded mountains (Himalayas, Alps, Appalachians) Collisional mountains Fault and Fold Mountains (b) 54 Rocks deformed in collision The collision of India and Asia produced the Himalayas 55 Transform fault boundaries Third type of plate boundary Plates slide past one another and no new lithosphere is created or destroyed Transform faults Most join two segments of a mid-ocean ridge (MOR) as parts of linear breaks in the oceanic crust known as fracture zones Accommodate simultaneous movement of offset ridges Source of weak (MOR) to fairly strong (San Andreas) earthquakes. 56 Transform faults accommodate movement on offset ridge segments Plates are moving in opposite directions 57 Plate Tectonics Explains It All We now understand mountains, volcanoes, and big earthquakes associated with, for example, the San Andres fault. We understand rift valleys and how oceans form, deep ocean trenches, mid-ocean ridges, why fossils and mountain ranges look alike across vast oceans. 58 Dirk Egbert Vogel