Geologic and Environmental Hazards

This is the information after Test #1

Alfred Wegener (1880-1930)

• German meteorologist who first proposed the continental drift hypothesis in 1915.

• Published The Origin of Continents and Oceans

Continental Drift Hypothesis

A supercontinent called Pangaea- consisting of all Earth’s landmasses, once existed and it began breaking apart about 180 million years ago.

Pan—> entire gaia—> all land

4 links to evidence of the hypothesis

• Matching profiles of continental margins

• Similar rock types and structures

• Fossil Evidence

• Paleoclimate Belts

Links to evidence of the hypothesis

Matching profiles of continental margins — The “Continental Jigsaw Puzzle”

Links to evidence of the hypothesis

Similar rock types and structures — rock ranges in Canada match Norway and Sweden

Also, Appalachian Mountains match UK mountains

Links to evidence of the hypothesis

Fossil Evidence — identical fossil organisms are found on continents now separated by vast oceans

Links to evidence of the hypothesis

Paleoclimate Belts

Paleomagnetism

Paleo means old or ancient

Paleomagnetism

the study of the Earth’s magnetic field in rocks

Paleomagnetism explanation

• Igneous rocks contain magnetite, an iron-rich mineral affected by Earth’s magnetic field

• The magnetite is then ‘frozen’ in position and indicates the direction of the north pole at the time of rock solidification.

• - Earth’s magnetic field is sufficient to permanently magnetize some surface rocks.

• magnetic field recorded by the rocks changed periodically

  ( Shows in the rocks that the poles (North/South) switched )

Magnetic Reversals

• During a magnetic reversal, Earth’s magnetic field periodically reverses polarity— the north pole becomes the south pole..

• The magnetic time scale shows the sequence of shifts in the polarity of Earth’s magnetic field, determined from lava flows

Magnetic Anomalies

Description of the Crust

Rigid Lithosphere overlies Weak Asthenosphere

• The lithosphere is Earth’s strong, rigid, outermost layer

• The asthenosphere is a hotter, weaker region of the mantle under the lithosphere

• Because of the differences in physical properties, the lithosphere is effectively detached from the asthenosphere

Which one is denser; continental crust or oceanic crust?

Oceanic crust

Plate Boundaries

• Divergent plate boundary — move apart

• Convergent plate boundary — move together; one goes downward

• Transform plate boundary — slide past each other

Divergent Plate Boundaries

• New ocean floor is generated as two plates move apart

• Most located along the crests of oceanic ridges

Divergent Plate Geological Features

• Mid- ocean ridge (90%) with seafloor spreading

• Continental rifting (rift valley)(10%)

Seafloor Spreading

The mechanism that operates along the ridge to create new ocean floor

Mid- ocean ridges form when hot material from the mantle rises up to form a broad ridge, typically with a central rift valley

Mid- ocean ridges location

Mid- ocean ridges along the divergent plate boundaries are found in every ocean basin on Earth.

Form linear submarine mountain chains (Mid-Altantic Ridge)

Continetal Rifting

Occurs when a divergent plate boundary occurs within a continent

Convergent Plate Boundaries

Two plates move toward each other at these destructive plate margins, where the high density plates are returned to mantle

• The leading edge of one plate is bent downward, as it slides beneath the other at subduction zones

• Because of high friction between the plates, they get stuck together and the leading edge of the overlying plate gets dragged backwards

Deep- ocean trenches

The topographic depressions of the seafloor produced at subduction zones

EX: Peru- Chile Trench and Mariana Trench

Types of Convergent Boundaries

• Oceanic continental convergence

• Oceanic- oceanic convergence

• Continental- continental

Oceanic- continental convergence

The high density oceanic crust sinks beneath the continental block into the athenosphere. At 100 kilometers, partial melting occurs when water from the subduction plate mixes with the hot rocks of the asthenosphere, generating magma.

• Creates continental volcanic arc

Oceanic- oceanic convergence

When two oceanic crusts collide, the older one (which is cooler and high density) descends beneath the other. Partial melting initiates volcanic activity

• Creates volcanic island arc

Continental- continental- convergence

Continued collision can bring two continents together. NO SUBDUCTION. Produces mountains

• Himalayas

Transform Plate Boundary

Plates slide past one another along a vertical fault.

EX: San Andreas Fault

Fracture zones

Most joint two segments of an oceanic ridge system along breaks in the oceanic crust

Features related to Plate Boundaries

• Earthquakes: IN ALL THREE TYPES

• Volcanism: Convergent boundaries with subduction and Divergent boundaries ( EX: Alaska and Japan)

Hot spots

The surface expression of a mantle plume ( which is an area of volcanism)

Mantle Plumes

Cylindrically shaped upwelling of hot rock

What drives Plate Motion?

Convective flow in the mantle is the basic driving force of plate tectonics

Other Forces that drive Plate Motion

• Slab-pull force

• Ridge-push force

Slab- pull force

The subduction of cold oceanic lithosphere (density driven)

Ridge-push force

An elevated lithosphere on an oceanic ridge will slide down due to gravity (gravity driven)

The Himalayas are associated with which of the following tectonic plate boundaries?

Continent-continent convergence (collision)

Reconstructing all of the continents into a single supercontinent called Pangaea helps to explain better the occurrence of the same fossil plants and animals on different continents.

True or False?

True

Why aren't volcanoes associated with continent-continent convergence?

Both plates are too buoyant to sink into the asthenosphere.

Which of the following is TRUE about plate tectonics?

Which of the following is TRUE about plate tectonics?

Explains that the earth's lithosphere is split into large pieces that move on top of a weak and hot layer called the asthenosphere.

The slow movement of the lithosphere is

a key feature in the theory of plate tectonics.

What does paleomagnetism study?

magnetism of rocks when their magnetic properties formed

What would you NOT expect to see at locations where tectonic plates are sinking?

Undersea mountain range that wraps around the Earth like seams of a baseball

Tectonic plates are actively separating at convergent plate boundaries and subduction zones.

False

The characteristic type of tectonic plate motion associated with a transform plate boundary is

plates move laterally past each other with horizontal motion.

Which tectonic plate setting is associated with the following features: light to moderate earthquakes; nonexplosive volcanic eruptions; new oceanic lithosphere produced? Mid-Atlantic Ridge is a geographic example.

divergent plate boundary

All of the following are forces that may contribute to plate movements EXCEPT

• the gravitational pull of the Earth on the subducting slabs of oceanic lithosphere.

• the gravitational pull of the Earth on plates away from the mid-ocean ridges.

• convection currents within the asthenosphere

• the forces from the rotation of the Earth.

the forces from the rotation of the Earth.

Where would you find ridge-push, a possible mechanism for driving the motion of tectonic plates?

Divergent boundary

Which of the following hazards would you expect to see at a transform plate boundary?

large earthquakes; no volcanoes; flooding if the plate boundary is hilly

The Hawaiian Islands are an example of

Hotspots

Scientific investigations of the 1906 San Francisco earthquake led to the identification of which fault?

San Andreas Fault

Earthquake

The vibration of Earth produced by the rapid release of energy in the form of mechanical waves

Fault

Rocks don’t always bend, sometimes they break. When the rock moves and breaks it is called:

( any surface or zone in the brittle lithosphere)

Two sides of Fault

• Hanging wall

• Foot wall

Hanging wall

the rock above the fault zone

Foot wall

The rock below the fault zone

Fault zone

Brittle structure that slip occurs on several faults within a band of definable width

Stress

A force that results from plate tectonics movements

Rocks undergo strain or deformation because of stresses

• The three basic types of stress are compressive (push together), tensional (pull apart), and shear (tearing).

Strain

The change in shape or location of the rocks due to the applied stress

• Elastic materials: deformations is temporary

• Plastic material: deformation is permanent

Strike

The direction of the line formed by the intersection of a bed or fault and a horizontal plane.

Dip

The angle between the horizontal plane and titled surface (bed or fault), a measure of the steepness of slope of the place

Dip- slip fault

One in which the displacement is vertical, up or down in the direction of dip.

• Normal fault

• Thrust/ reverse fault

Strike-slip fault

One along which the displacement is parallel to the strike (horizontal) (straight line)

• Transform fault

Normal Fault

Downward movements of the hanging wall relative to the footwall

• Follows the gravitational pull

• Divergent Plate Boundaries

  What stress causes it? Tensional Force

Reverse Fault

An upward relative movement of the hanging wall

• Against the gravitational pull

• Creates some of the world’s highest mountains

• Convergent plate boundary

  What stress causes it? Compressional Force

Strike-slip faults

The movement is parallel to the fault strike (horizontal direction)

• Transform Plate Boundary

• Vertical fault plane

• San Andreas Fault

  Which stress causes it? Shearing Force

Focus (hypocenter)

The point of initial breaking or rupturing within the Earth

Epicenter

An earthquake is the surface location directly above the focus

• Most earthquakes, the epicenter is the point where the greatest damage takes place

Elastic band theory

Rocks behaving elastically like a stretched rubber band

• All objects, including rocks, can be deformed elastically up to a certain point before the stress on them surpasses the material’s internal strength

Foreshocks and Aftershocks

Smaller earthquakes that precede and postdate the main shock

• Only know AFTER the big earthquake occur

• DOES NOT ALWAYS OCCUR

Wavelength

the distance from crest to crest (or trough to trough)

Amplitude

the distance of crest ( or trough) from the midpoint

Period

the time it takes a wave to travel a distance equal to a wavelength

Frequency

the number of waves that passes a fixed point per sec

Velocity

distance traveled by a wave crest in one period

Seismic Waves

When a fault ruptures, rocks break apart suddenly and violently , releasing stored elastic strain energy in the form of seismic waves.

Seismic waves: Body waves

Travel within the body of the earth

• P- waves

• S-waves

Seismic Waves: Surface waves

travel along the surface

• R- waves

• L- waves

P- waves

Primary waves; fastest waves

• Pass through solids, liquids, and gases

S- waves

Shear waves or secondary waves ; produce an up-and-down motion (sideways shear)

• Can only pass through solids

• S-waves have a higher amplitude than P-waves

Which seismic wave is slower?

Surface waves

R (Rayleigh) waves

cause the ground to move up and down as the wave front passes

L (Love ) waves

cause the ground to move side to side as the wave front passes

Seismometers

Earthquake detection and reading instruments. A modern one records the movement of Earth in relation to a stationary mass on a magnetic tape

Finding an earthquake Epicenter

Locate the earthquake epicenter with three different station recording

• each station: time interval of P-waves and S-wave at that location '

• A travel time graph determines each station’s distance to the epicenter

• A circle with a radius equal to he distance to the epicenter is drawn around each station

• The point where all three circle intersect is the epicenter

Richter magnitude scale

Immediately following an earthquake, the first magnitude estimates were made by the Richter Scale

• Measures max ground shaking due to S-waves at the location of seismometer.

• Richter magnitudes are not an absolute measurement of the size of earthquake

Moment Magnitude Scale

Developed in 1979 to determined the absolute size of earthquake

• Takes days to months to calculate precisely

Modifies Mercalli Intensity Scale

Developed by Mercalli using California buildings as its standard (1931)

Local Geological Conditions

Different materials respond differently to an earthquake

• Seismic waves move faster through consolidated bedrock

• Move slower through unconsolidated sediment

• Move slowest through unconsolidated materials with high water content