Presentation on theme: "Lecture 3 Outline: Plate Tectonics – from Hypothesis to Theory Learning Objectives: What is the evidence for continental drift? What is the evidence for."— Presentation transcript:
Lecture 3 Outline: Plate Tectonics – from Hypothesis to Theory Learning Objectives: What is the evidence for continental drift? What is the evidence for seafloor spreading? What is the evidence for subduction? How does plate tectonics explain this evidence?
How was Plate Tectonics Discovered? Using scientific method how researchers work collectively over time to develop accurate and reliable explanations By observing, hypothesizing, and testing
What is the Scientific Method? 1.Observe: gather information 2.Hypothesize: attempt to explain observations 3.Predict: use hypothesis to make testable predictions 4.Test: gather additional evidence from observations or experiments to support or refute hypothesis 5.Modify: modify hypothesis in light of new evidence Repeat steps 3-5 until hypothesis consistent with all available evidence and conceivable tests
How does a Hypothesis become a Theory? Hypothesis gradually gains widespread acceptance by repeated testing and modification Theory = hypothesis that withstands scrutiny over time predictions tested and shown to be accurate
What is a Theory? Everyday “theory”: possible explanation or educated guess Scientific “theory”: explanation consistent with available evidence Example: Theory of Evolution supported by large body of scientific evidence succession of organisms in fossil record genetic relationships among modern organisms defined and observable mechanisms of variation and heredity
How was Plate Tectonics Discovered? Plate tectonics too slow to directly observe So how do we know plates move? Three independent lines of evidence 1.Continental Drift 2.Seafloor Spreading 3.Subduction
What is Continental Drift? Early geologists knew continents moved vertically Fossil sea shells high above sea level Uplift and subsidence following earthquakes Can continents also move laterally? Hypothesis proposed by Wegener (1920s) Continents once joined as one land mass Pangaea Broke apart and fragments - modern continents - “drifted” to current locations
What was Wegener’s Evidence for Continental Drift? Fit of coastlines Distribution of fossils Similar types/ages of rocks on widely separated coastlines
Wegener could not explain how continents move through solid rock of ocean floor Why was Wegener’s Hypothesis of Continental Drift Rejected? Discoveries about Earth’s magnetic field and mapping of ocean floor in 1950s provide mechanism for continental drift Seafloor spreading
What was Discovered when the Seafloor was Mapped? Major topographic features Ridges huge underwater mountain ranges through all ocean basins Trenches narrow but very deep, mostly encircle Pacific Ocean Mapping sea floor from ships
trenches ridge trench What was Discovered when the Seafloor was Mapped?
Age of seafloor shows orderly distribution Rocks youngest along ridges Progressively older away from ridges No old oceanic crust Oldest rocks on continents (~4 billion yrs) nearly as old as Earth Nowhere on seafloor are rocks older than ~200 million years Observations suggest ocean crust made at ridge Proven by pattern of magnetism recorded in ocean crust What was Discovered when the Seafloor was Mapped?
Youngest rock in red - oldest in blue Symmetry in ages on either side of ridges What was Discovered when the Seafloor was Mapped?
What is Earth’s Magnetic Field? Generated by Earth’s rotation and molten iron in outer core Two poles of opposite polarity Magnetic poles near rotational (geographic) poles because field generated by rotation
What are Magnetic Reversals? Magnetic field flips polarity (direction) Normal: magnetic north near geographic North Pole Reverse: magnetic north near geographic South Pole Importance of this discovery not realized until seafloor mapped
How do Rocks Record Changes in Earth’s Magnetic Field? Iron minerals align parallel to ambient field as they crystallize Indicates polarity - direction to magnetic north Polarity locked in as rock cools Preserves record of magnetic field polarity at time rock formed High temperature: magnetic minerals randomly oriented During cooling: magnetic minerals align with field After cooling: magnetic orientation recorded in rock will not change as long as rock remains cool – even if magnetic field changes
How are Magnetic Polarity Reversals Recorded on the Seafloor? Polarity along ridge south of Iceland Red: rocks formed during current period of normal polarity Other colors: rocks formed during earlier periods of normal polarity White: rocks with reversed polarity Bands of alternating normal and reversed polarity Symmetric on either side of ridge Ridge axis
How Does Seafloor Spreading Explain these Observations? Youngest rock at ridge formed during current period of normal polarity (N) Slightly older rock further from ridge formed during last time field had reverse polarity (R) Even older rock further from ridge formed during previous period of normal polarity (N) Produces magnetization pattern symmetric with respect to ridge
How Does Seafloor Spreading Explain these Observations? Seafloor spreading results in Crust older with distance from ridge Stripes of normal and reverse magnetization symmetric to ridge
Provides mechanism for continental drift Seafloor acts as conveyor belt to move continents Amount of new oceanic crust produced at ridges must be balanced by equal amount destroyed elsewhere Where is oceanic crust destroyed? How Does Seafloor Spreading Explain these Observations?
What is the Global Distribution of Earthquakes? Not randomly distributed Concentrated near trenches
Trench Subduction = process that removes old oceanic crust Discovered by relationship of deep earthquakes to trenches Earthquakes deeper with distance from trench How was Subduction Discovered? HotCold
How was Subduction Discovered? Earthquakes along and “inland” of trench - deeper with distance Mark descent of old oceanic crust into mantle Process = subduction Places where it occurs = subduction zones Top left: Map view shows earthquakes become progressively deeper to west of Tonga trench Remaining panels: Vertical cross sections showing descending earthquake belts trench T T T
What is the Global Distribution of Volcanoes? Volcanoes not randomly distributed either Concentrated in chains (arcs) inland of trenches Occur above depth where subducting crust causes melting in mantle Pacific “Ring of Fire” Earthquakes and volcanoes associated with subduction
How Does Plate Tectonics Explain Earthquakes and Volcanoes? Compare location of plate boundaries to global distribution of earthquakes and volcanoes Earthquakes and volcanoes mostly at plate boundaries convergence and divergence Earthquakes Plate Boundaries Volcanoes
Ridge new oceanic crust produced by upwelling of asthenosphere seafloor spreading What Happens Where Plates Diverge?
Trench denser oceanic crust sinks beneath lighter continental crust subduction What Happens Where Plates Converge?
Average about 5 cm per year Roughly rate fingernails grow Not very fast, but over long periods of time plates move great distances 50 km per 1,000,000 yr Portland - Salem 5000 km per 100,000,000 yr Portland - Boston How Fast are Plate Motions?
We can now directly measure plate motions using GPS
1.What is the scientific method? What is the difference between a hypothesis and a scientific theory? 2.Explain the geologic evidence that led to the discovery of a) continental drift, b) sea floor spreading, and c) subduction. How does plate tectonics explain this evidence? 3.Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that shows seafloor spreading? 4.What causes plates to move? Questions for Review