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., and dredging of the ocean floor found that rocks are younger than 180 Million Years. They also observed the tensional faults at the ocean ridges and high heat flow.
Sea Floor Spreading Hypothesis: by Hess & Dietz ‐
Harry Hess & Dietz (1960s) proposes sea floor spreading hypothesis: new “slivers” of ocean crust created at ridges and crust is consumed/renewed at trenches (like a conveyor belt) due to convective flow within the Earth. Continents are carried passively by the horizontal part of convective flow. This wasn’t the only idea of the time. There was another Idea that had been proposed: Expanding Earth Hypothesis: by Carey – The continents moved as the Earth expanded. However, there wasn’t enough evidence to support this and it didn’t fit with the available evidence. Until paleomagnetism became known, the expanding Earth hypothesis was not fully discounted.
The Basic Premise of Plate Tectonics
• Lithosphere is broken up into plates "floating" on asthenosphere in relative motion with respect to one another.
• Plates are basically rigid.
• Most action (earthquakes, volcanoes and mountain building) occurs at or near the boundaries of plates.
Tuzo Wilson put missing puzzle pieces together in 1965. Tuzo Wilson saw that the plates were separated by large faults and that there are three (3) types of plate margins (boundaries). This missing piece of the puzzle helped put it all together into one theory – plate tectonics. Wegener’s original ideas finally became accepted.
Plate Boundary Types
• Divergent – Plates pull apart / diverge.
• Convergent – Plates collide / converge
• Transform – Plates slide past each other
Divergent Boundaries ‐ Plates pull apart at continental rifts (East African Rift System – Example) and mid‐ocean ridges; new lithosphere created; volcanism and earthquakes. Oceans are created as plates diverge and sea water fills in the “gap”.
Convergent Boundaries‐ 3 types
• Oceanic‐Oceanic ‐ At Subduction zones (oceanic trenches) “old”, denser lithosphere is consumed back into the asthenosphere. These are characterized by volcanism (volcanic island arcs) and earthquakes. (Example: Marianas Trench‐ Pacific and Philippine plate converge)
• Oceanic‐Continental ‐ At Subduction zones (oceanic trenches) old, denser lithosphere consumed back into the asthenosphere. Characterized by volcanism (volcanic arcs) and earthquakes. (Example 1: (Nazca‐South America plate converge creating the Peru‐Chile Trench and the Andes Mountains. Example 2: The Juan de Fuca and the North American plates converge in the Pacific Northwest of USA forming the Cascade Mountains and Volcanoes) • Continental‐Continental – There is no subduction and there are no volcanoes. These are characterized by great earthquakes and mountain belts. (Example: India and Asia converge creating the Himalayan Mountains)
Transform Boundaries ‐ Plates slide past one another. Form fracture zones in ocean floor (and where they cross continents – e.g. San Andreas Fault). The lithosphere is neither created nor destroyed along transform boundaries. There are earthquakes but no volcanism. (Example: San Andreas Fault of California. Everything west of it is on the Pacific Plate sliding northwest relative to the rest of North America)
Plate Tectonic Theory
The Critical Tests‐ 1960’s
• Earth’s Magnetic Field Polarity
• Magnetization of Successive lava flows
• Geomagnetic Polarity Time Scale Developed using magnetic polarity of lava flows of known age
• Sea Floor Magnetic Anomalies Vine‐Mathews hypothesis ‐magnetic stripes support “sea floor spreading”‐ Sea Floor Magnetic Anomalies Explained
KNOW THIS ‐ How to calculate plate velocities from magnetic anomalies. This will be reviewed in class.
In class we will review how to determine how quickly the plates are moving (velocity) and how long ago plate spreading began. You can also see a diagram, for practice in the “Things to Know to Pass this Class section” under “Exam 1 Essay Questions”.
Plate Dynamics
Internal Heat Engine ‐ The Driving Mechanism of Plate Motion
•Heat within the Earth‐ temperature increases with depth (=Geothermal gradient). Heat is related to:
• Primordial heat ‐ leftover from early formation of Earth. Rocks lose heat very slowly.
• Also..Heat from decay of radioactive materials
When internal heat is gone the tectonic plates will stop moving. Mountains will erode and the Earth becomes flat….like Kansas or Nebraska.
Thermal convection drives plate motion and: • Causes Earthquakes, volcanoes, and mountain building
• Produces igneous and metamorphic rocks
Convective flow causes warm rock to rises under ridges and cooler rock to sink into subduction zones. The plates are driven by unequal distribution of heat within the Earth.
In addition to the heat generated at ocean ridges and subduction zones, there are Hot Spots (e.g. the Hawaiian Islands, Yellowstone National Park and San Francisco Peaks in Flagstaff) where weaknesses in the crust allow heat to reach the surface. Hot spots often occur on the plates, as opposed to at plate boundaries. They generally:
• Create chain of inactive volcanoes with the oldest in direction of plate motion.
• The active volcano indicates the position of the hot spot.
• Can also calculate plate velocities using hot spot tracks as well.
Calculating velocities and the direction of plate movement using hot spots ‐‐ KNOW THIS ‐ This will be reviewed in class.
In class we will review how to determine how quickly the plates are moving (velocity) and the direction of plate movement using hot spots. You can also see a diagram, for practice in the “Things to Know to Pass this Class section” under “Exam 1 Essay Questions”.
Earth's External Solar Powered Heat Engine and the Hydrologic Cycle
External Thermal Convection‐ The Hydrologic Cycle KNOW THIS
The cycle: Evaporation ‐> Condensation ‐> Precipitation ‐> Runoff
Why is it important?
•Role of Hydrologic Cycle in geology ‐ weathering and erosion act to level topography; produces sedimentary rocks
The interaction of the Earth’s Engines = The Rock Cycle
In brief……This is how the Earth Works……….
1. Earth’s internal heat engine causes plate motions, mountain building and creation of new continental crust. 2. The external solar engine drives the hydrologic cycle that erodes mountains and changes landscapes.
• The landscapes and features that we see on Earth today result from the interaction between these two engines.
• This is sometimes referred to as a Dynamic Equilibrium system because both engines are in constant motion.
• The Earth’s internal heat is slowly being lost, however, so eventually new mountains will cease to form and erosion will level the landscape.
So……What are Rocks?
• Rocks ‐ solid aggregates of crystals (xls) with a couple of exceptions (glass).
• Crystals are minerals, the natural chemicals of the Earth
How are rocks classified?
• Rocks are classified primarily on how they formed based on the Principle of Uniformitarianism (present is the key to the past/ past is key to future) – James Hutton from Scotland – late 1700s
• There are three (3) major categories of rocks:
igneous, sedimentary and metamorphic.
Igneous Rocks: formed from cooling of molten material (magma/lava) Texture is interlocking xls or glass. Associated with convergent and divergent plate boundaries
• Two Types: Volcanic & Plutonic
Igneous Rocks: Volcanic = Extrusive formed by rapid cooling at or near the surface
Igneous Rocks: Plutonic = Intrusive formed by slow cooling beneath the Earth’s surface
Sedimentary Rocks: These are formed at Earth's surface and are typically layered rocks deposited horizontally with the oldest layer on the bottom. They can form anywhere but are typically preserved when deposited in large basins such as near or in oceans. They can also “tell us” about ancient geography.
• Three General Types:
Detrital, Chemical & Biochemical/Organic Detrital (e.g. gravel and sandstone)
• Made up of detritus (bits or clasts of other rocks, or crystals weathered from other rocks.
• Clasts are cemented together by crystals grown from chemical precipitation in empty space between clasts.
Chemical (e.g. rock salt and limestone) = crystals formed by chemical precipitation due to chemical environment
Biochemical/Organic (e.g. limestone‐biochemical(coquina or chalk) and organic‐coal) = crystals formed by biological activity or compacted and altered organic material (i.e. coal)
Metamorphic: These are rocks that have been changed into a new rock type. They are the changed form of any other rock type (igneous, sedimentary and other metamorphic rocks). The change primarily occurs in a solid state (non‐molten). Crystal growth and changes are induced by changes in the pressure, temperature or both.
• Although metamorphic rocks occur in a variety of places, large continuous regional belts are typically associated with convergent plate boundaries
Geologic Time
• Most geologic processes occur very very very slowly.
• Rocks can be dated relatively using principles of superposition and faunal succession
Principle of Superposition – The youngest layer is on top and the oldest layer is on the bottom. This applies primarily to materials deposited at Earth’s surface (e.g. sedimentary rocks and volcanic lava flows).
Principle of Faunal Succession – fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.
How Long is Geologic Time? We can use “relative dating” to help us find out.
In addition to superposition and faunal succession, we also use the principles of
Original Horizontality (layers are originally laid down horizontally), and
Lateral Continuity (layers tend to be laterally continuous), and
Unconformities = time gaps in the rock record
To help us determine how old rocks are. Later in the year we will look at methods that are used to determine more precise dates when rocks were formed. What we now know
Earth is very old ‐ about 4.6 Ga. ("Ga." stands for Giga annum or billion years)
General Geologic Time Scale
Precambrian (oldest) ‐‐‐‐ Paleozoic ‐‐‐‐‐Mesozoic & Cenozoic (youngest)
8/2011