Lecture notes ‐ Bill Engstrom: Instructor Intro & Plate Tectonics GLG 101 – Physical Geology Geology – The scientific study of the Earth. There are many different categories or disciplines that make up the science of Geology. This class will focus on Physical Geology ‐ How the Earth works .... its composition and the processes which operate on and beneath its surface Other categories/disciplines Historical Geology ….how earth has evolved through time Environmental Geology ….how geology relates to our environment, geologic hazards and earth’s resources Why take Science? Companies want people that …. • Can solve problems (Scientists are “detectives”) • Are aware of science and technology – It’s a more complicated technological world Why study geology/science? To learn about things we see and hear about in our lives, including: Geologic hazards e.g. Earthquakes, Volcanoes, Floods, Landslides Earth Resources: Everything we don't grow must be mined And…. use your knowledge of geology as it applies to public Policy, teaching and Business. Geology as a Science Scientific Philosophy – • There are orderly patterns in nature that can be explained naturally (without appeal to the supernatural) with the application of natural physical and chemical principles. Science and Religion: Although religion may be based strictly on a belief in something, scientific principles are based on predictions or ideas that can be tested (see hypothesis, theory and law below). • When an explanation has been discovered it can be used to predict such patterns in the future; or determine their presence in the past. Geology as a Science & the Scientific Method ‐ How do we solve problems as “scientific detectives”? Hypothesis vs. Theory vs. Law • Hypothesis: Proposes a natural explanation for the patterns – prediction must be testable. A hypothesis is an untested model or idea. • Theory: Best explanation of a particular phenomena/pattern accounting for all available observations (data, evidence). Although we can’t be sure theories are absolutely correct, they are the accepted explanation for our observations. When a hypothesis becomes a well tested accepted view it is a theory. • Law: A theory that has stood the "test of time". Not usually known to be violated. Gravity = example We use the scientific method every day to predict what is going to happen next based on our observations (e.g. football, the weather patterns, baseball) whether we realize it or not. The Scientific Method (see diagram below) Observation: notice a pattern Hypothesis: Propose a natural explanation for the pattern. Because of limit to natural processes, hypothesis must make a testable prediction. Prediction: Define a prediction based on hypothesis. If you are right, then… Test or experiment: Test the hypothesis’ prediction. New Observation: results from the experiment. Evaluation of hypothesis: if wrong need new hypothesis; if right test again. Do over until hypothesis explains all available data‐ then you have a theory. How can we apply this to the Earth? When we observe a flat Map of Earth with the Ocean Water Removed we see Patterns that need explanation such as: • Ocean Basins – abyssal plains, ridges, transform faults, trenches (up to 11 Kilometers deep), island arcs, island/seamounts • Ocean Ridges – e.g. Mid‐Atlantic Ridge (Ridges avg. 2 to 3 Kilometers high) • The Continents – Passive margins, shelves, active margins, volcanic arcs, mountain belts • Passive Continental margins ( e.g. East Coast of United States – no trenches or ridges) Continental Shelf = Submerged Continent (e.g. on East Coast of U.S.) We can also see other patterns such as: • Global distribution of earthquakes • Distribution of Volcanoes • Atlantic Continents Seem to Fit Together (fit at seaward edge of the Continental Shelf) • Age of the Sea Floor Alfred Wegener’s Observations & the Original Theory of Continental Drift In 1915 ‐1929 Wegener proposed that…. • The continents were once together in a supercontinent called Pangea. • Pangea broke up and the continents somehow “drifted” to their present positions. Observations: • Fit of the Atlantic‐bordering continents (fit nicely at the seaward edge of continental shelf) ‐ Match of geometry and geologic features • Match of Fossils between Africa & S. America ‐ Fossils of identical creatures found on widely separated continents – Not likely to swim across oceans. • Another observation ‐ Creatures not where they belong. Fossils of creatures whose modern analogs suggest latitudinal dependence are located in areas where they could not have lived. • More fossil evidence: Glossopteris fern – grows in subpolar regions but the fern fossils are found in today’s warmer regions) • Match of Appalachians and Caledonian Mtns – Mountains chains of comparable age and structure which are located on separate continents today were once connected. • Match of 200 Ma Glacial Deposits ‐ Ancient glacial deposits are now located at equitorial lattitudes How do we explain it all? Continental Drift Hypothesis Wegener’s Pangaea ‐ (Pangea = together 200 Million years ago) Wegener’s problem ‐ How do you do it? What caused the continents to move? Although he came up with a hypothesis, he could not find a feasible mechanism. Wegner’s Mistakes Hypothesis: Continents break through ocean crust. Wegner’s Mistake‐ Icebreaker Mechanics Problem: The ocean floor not weak enough to allow for this without deforming continents in the process. Hypothesis: Drift caused by tidal forces of Sun and Moon. Problem – Forces are not strong enough to move continents without stopping Earth’s rotation. However, Wegener did not have the tools available to him to make measurements & test his hypothesis. Rejuvenation of the Continental Drift Hypothesis Post WWII, additional studies were performed using paleomagnetism, geophysics and sea floor exploration The Earth has a geomagnetic field and Earth’s poles approximately define the rotational axis. Paleomagnetism (recording the geomagnetic field). Rocks record the location of geomagnetic field • Magnetisim in the rocks provide a record of the direction to the poles, and the distance to the poles when iron rich rocks (e.g. lava‐basalts) cool and solidify. Earth’s magnetism at that time the rock formed is held in the rocks and can be measure. Apparent Polar Wander – Earth’s magnetic poles appear to have wandered (changed location) over time. Possible Hypotheses….. either (1) the magnetic field has really wandered through time; OR (2) the continents have drifted through time. • Comparison of paleomagnetic poles from different continents strongly favors hypothesis #2 & continental drift Discovery of a Layered Earth ‐ a theory. Observations of earthquake wave behavior, gravity, the magnetic field, in conjunction with astronomical considerations allow us to make the following theory of what's inside the Earth There are two ways to describe Earth’s layers…..by composition and by its mechanical layers Compositional Earth Layers • Core – Probably made of iron and nickel in composition. • Mantle‐ greatest volume of Earth‐ probably made of compounds of silicon, aluminum, iron, magnesium, and oxygen • Crust‐ thin “scum” on top of mantle = The rigid outer shell of the Earth. These are lower density rocks/cooler rigid outer shell Oceanic crust • Average thickness = 7km(5mi); • Crust under deep oceans • Made up of silicon compounds similar to those in mantle; • Denser and thinner than continental crust; • Made up of volcanic rocks ….like those at Sunset Crater by Flagstaff. Continental Crust • Avg thickness = 40km(25mi); • Thicker than oceanic crust • Relatively “light”‐ also made up of silicon compounds but less iron and magnesium • In general….made up of rocks that have compositions similar to the granite used in making landscape rocks. Mechanical Earth Layers and Earth’s Spheres • Inner Core: Solid: Probably iron and nickel in composition. • Outer Core: Liquid: Also most likely iron and nickel in composition. • Mesosphere: Solid: The bulk of the mantle. Sometimes called the lower mantle. • Asthenosphere (weak sphere): Plastic or Gooey upper mantle that is at or very near melting point. May be partially molten to behave like a fluid much like peanut butter in consistency • Lithosphere‐ Solid: Everything above the asthenosphere. Includes uppermost solid mantle AND crusts • Hydrosphere & Atmosphere‐ Liquid and Gas: oxygen, nitrogen, carbon dioxide etc. • Biosphere: Plants/Animals All of Earth’s spheres interact with each other. Conversion: (70 km = ~ 50 mi) Important: Earth radius= ~ 4000 mi or 6400 km Note: Study the diagram of the Earth’s layers in your textbook (Know this….except exact thicknesses. However, you should know the general thicknesses…. E.g. You should know the continental crust is an average of 40 km(25mi) thick and ocean crust is an average of 7Km (5mi) and NOT 100s of Kilometers thick and that the Earth is 4000 mi in radius and NOT 6000 Km.) How did Earth form? • Big Bang ….. 14 Billion years ago – birth of the universe • Formation of Solar System – the Nebular Theory • Gravitational collapse of dust/gas from a Nebula • Formation of rotating disk • Rocky & Metallic material condenses • Coalesce & accrete into asteroids, planets • Earth formed 4.6 Billion years ago. Oldest rocks found are ~ 4+ Bill years Sea Floor Exploration and the Sea Floor Spreading Hypothesis More Post WWII observations. Mapping the Sea Floor – Found features never seen before. The ocean floor was mapped with echo sounders/sonar revealing ocean ridges, trenches, abyssal plains, seamounts, etc.,