Transcript Birth of a Theory - Catawba County Schools

Birth of a Theory
Continental Drift and Plate Tectonics
Wegener’s Hypothesis
Alfred Wegener proposed that all the continents
were once joined together in a single super
continent called Pangaea.
He proposed that it began to break apart
around 200 million years ago.
This is the continental drift hypothesis
Evidence for his theory
 Continental Puzzle
 Fossil evidence of several organisms of the
same species on different landmasses.(
Glossopteris Fern, Mesosaurus Reptile)
 Rock types and structures
 And climate evidence.
Problems with his theory
 Couldn’t provide a mechanism for how the
continents moved.
A new Theory
 Plate Tectonic theory- says the earth is
made up of rigid plates that contain the crust
and upper mantle also known as the
lithosphere moving long the
asthenosphere.
 Lithosphere- the crust and upper mantle
 Asthenosphere- Plastic like layer of mantle
beneath the crust
Plate Boundaries
Divergent boundaries (also called spreading
centers) are the place where two plates move
apart.
Convergent boundaries form where two plates
move together.
Transform fault boundaries are margins
where two plates grind past each other
without the production or destruction of the
lithosphere.
Evidence for Plate Tectonics
 Mid-Ocean ridges and rift valleys that cause
sea floor spreading provided a mechanism for
the plate tectonic theory.
Seafloor spreading process that produces new
oceanic lithosphere.
subduction zone occurs when one oceanic
plate is forced down into the mantle beneath
a second plate.
Convergent Plate Boundaries
Oceanic-ContinentalContinental volcanic arcs form in part by
volcanic activity caused by the subduction of
oceanic lithosphere beneath a continent.
 Examples include the Andes, Cascades, and
the Sierra Nevadas
Types of Stress
 The three types of stresses that rocks
commonly undergo are tensional stress,
compressional stress, and shear stress
Faults
 Normal faults occur when the hanging wall block
moves down relative to the footwall block.
 Reverse faults are faults in which the hanging
wall block moves up relative to the footwall block.
 Reverse Thrust faults are reverse faults in which
hanging wall moves up and over the footwall.
 Strike-slip faults are faults in which the
movement is horizontal and parallel to the trend,
or strike, of the fault surface.
Earthquakes
 earthquake is the vibration of Earth produced
by the rapid release of energy
 Focus is the point within Earth where the
earthquake starts.
 Epicenter is the location on the surface
directly above the focus
 Faults are fractures in Earth where
movement has occurred
Cause of Earthquakes
 Most earthquakes are produced by the rapid
release of elastic energy stored in rock that
has been subjected to great forces.
 When the strength of the rock is exceeded, it
suddenly breaks, causing the vibrations of an
earthquake.
Earthquakes cont.
 An aftershock is a small earthquake that
follows the main earthquake.
 A foreshock is a small earthquake that often
precedes a major earthquake
Earthquake Waves
 Seismographs are instruments that record
earthquake waves.
 Seismograms are traces of amplified,
electronically recorded ground motion made
by seismographs
 Surface waves are seismic waves that travel
along Earth’s outer layer.
Earthquake Waves cont.
 P waves- push-pull waves that push
(compress) and pull (expand) in the direction
that the waves travel.
 Travel through solids, liquids, and gases
 Travel the fastest
Earthquake Waves Cont.
 S waves-Seismic waves that travel along
Earth’s outer layer.
 Shake particles at right angles to the direction
that they travel.
 Travel only through solids
 Slower velocity than P waves
Richter Scale
 Based on the amplitude of the largest seismic
wave
 Each unit of Richter magnitude equates to
roughly a 32-fold energy increase
 Does not estimate adequately the size of very
large earthquakes
Momentum Magnitude
 Derived from the amount of displacement that
occurs along the fault zone
 Moment magnitude is the most widely used
measurement for earthquakes because it is
the only magnitude scale that estimates the
energy released by earthquakes
 Measures very large earthquakes
Volcanoes
 Factors that determine the violence of an
eruption
 Composition of the magma
 Temperature of the magma
 Dissolved gases in the magma
 Viscosity is the measure of a material's
resistance to flow.
Factors affecting Eruptions
 Temperature (hotter magmas are less
viscous)
 Composition (silica content
 Gases
 A vent is an opening in the surface of Earth
through which molten rock and gases are
released.
Volcanic Materials
 Pyroclastic materials is the name given to
particles produced in volcanic eruptions.
 Types of pyroclastic material
 Ash and Dust- very fine in sized
 Cinders- Pea Sized
 Bombs- large sized
Types of Volcanoes
 Shield volcanoes are broad, gently sloping
volcanoes built from fluid basaltic lavas
 Cinder cones are small volcanoes built
primarily of pyroclastic material ejected from
a single vent.
 Composite cones are volcanoes composed
of both lava flows and pyroclastic material
 Most violent type of activity