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? Observe: gather information Hypothesize: attempt to explain observations Predict: use hypothesis to make testable predictions Test: gather additional evidence from observations or experiments to support or refute hypothesis 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 Continental Drift Seafloor Spreading 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

Why was Wegener’s Hypothesis of Continental Drift Rejected? Wegener could not explain how continents move through solid rock of ocean floor 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

What was Discovered when the Seafloor was Mapped? ridge trenches trench trench ridge ridge 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 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? Bands of alternating normal and reversed polarity Symmetric on either side of ridge Ridge axis 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

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

How Does Seafloor Spreading Explain these Observations? 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?

What is the Global Distribution of Earthquakes? Not randomly distributed Concentrated near trenches

How was Subduction Discovered? Subduction = process that removes old oceanic crust Discovered by relationship of deep earthquakes to trenches Earthquakes deeper with distance from trench Trench Hot Cold

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 trench T Top left: Map view shows earthquakes become progressively deeper to west of Tonga trench Remaining panels: Vertical cross sections showing descending earthquake belts 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? Plate Boundaries Compare location of plate boundaries to global distribution of earthquakes and volcanoes Earthquakes and volcanoes mostly at plate boundaries convergence and divergence Earthquakes Volcanoes

What Happens Where Plates Diverge? Ridge new oceanic crust produced by upwelling of asthenosphere seafloor spreading

What Happens Where Plates Converge? Trench denser oceanic crust sinks beneath lighter continental crust subduction

How Fast are Plate Motions? 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

Questions for Review What is the scientific method? What is the difference between a hypothesis and a scientific theory? 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? 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? What causes plates to move?