Birth of a Theory Continental Drift and Plate Tectonics.

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-Continental-

Continental 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