EARTHQUAKES AND VOLCANOESMs. Pollock

Earth and Space Science

Spring 2008

Earthquakes

Shaking and trembling that results from sudden movement of part of Earth’s crust

Most common cause faulting Break in Earth’s crust along which rocks

move On sea floor can cause tsunamis Ground rises and falls with seismic waves Energy released during rock movements

Seismic Waves

Faults possible at Earth’s surface or deep within – most less than 74 km deep

Point where rocks break and move focus (underground point of focus)

Surface point directly above focus epicenter; source of most violent shaking

Earthquakes known as seismic waves Tell about interior of Earth Three main types

Seismic Waves

Primary (P) travel fastest; travel through solid, liquid, and gas; push-pull waves; cause rock particles to move back and forth

Secondary (S) travel through solids, but not liquids or gases; not recorded at all locations, since cannot pass through molten interior; cause rock particles to move side to side

Surface (L) waves slowest; travel from focus directly up to epicenter; most damaging, as they bend and twist Earth’s surface

Seismographs

Instrument that detects and measures seismic waves; weight attached to spring or wire

Not attached directly to Earth, so only moves when Earth quakes

Creates waves on paper Seismologists determine strength of

earthquake based on heights of wavy lines Measured according to Richter scale –

measure of energy released by earthquake Any number above 6 very destructive What determines the amount of damage

done?

Predicting Earthquakes

Earthquakes studied in hope of accurate prediction in future

Useful prediction must be reliable and complete.

Prediction far enough in advance could make city planning easier. San Francisco’s earthquake-proof buildings

Warning signals discovered by scientists Change in speed of S and P waves Small rise or fall of land near faults Changes in water levels in wells

Formation of a Volcano

Magma deep within Earth under intense heat and pressure

Located in pockets called magma chambers Magma constantly in motion through cracks

or by melting rocks Called lava at Earth’s surface Can build up to form cone-shaped mountain Location where lava reaches Earth’s surface

called a volcano

Volcanic Eruptions

Different types of eruptions for different types of volcanoes

Some quiet, some very violent Location of lava eruption called vent Often multiple vents Chemical composition of magma

determined by analyzing mineral makeup of lava

Types of Lava

Dark with lot of water Forms basalt Rich in iron and magnesium

Light with little water Rich in iron and magnesium with silica Forms rhyolite

Intermediate composition Composition between dark and light Forms andesite

Large amounts of gases Forms rock with many holes, such as pumice and

scoria

Types of Lava and Eruptions

Dark lava – quiet flows Runny and smooth Islands of Hawaii and Iceland

Light lava – violent eruptions Hardens in vents Steam and new rocks build up beneath

vents Great pressure to cause explosions

Volcanic Eruptions

Rock fragments blown into air Smallest particles volcanic dust; very fine Particles size of rice grains volcanic ash;

falls to Earth and forms small rocks Large rock particles volcanic bombs; can

be size of large boulders Smaller bombs called cinders Molten when leaving volcano; harden in air

Types of Volcanoes

Type of eruption affected by type of volcano

Some quiet and over large area Some violent Some combinations of quiet and violent

Cinder Cones

Made mostly of cinders and other rock particles

Formed from explosive eruption Not high; narrow base with steep sides Paricutin in Mexico

Shield Volcanoes

Quiet lava flows Large area Gently sloped, dome shaped mountain Mauna Loa in Hawaii

Composite Volcanoes

Alternating layers of rock particles and lava

Beginning violent eruption with bombs, cinders, and ash from vent

Followed by quiet eruption with lava flow that covers rock particles

Large cone-shaped mountain result of many alternating eruptions

Mount Vesuvius in Italy and Mount Etna in Sicily

Volcanic Structure

Often crater at top – funnel-shaped pit or depression

If the crater becomes larger due to the collapse of walls, it is called a caldera.

Volcano and Earthquake Zones Often volcano and earthquake zones in

same areas of world Sometimes one is the result of the other. Most major occurrences in three zones of

world.

Volcano and Earthquake Zones Ring of Fire

Around edge of Pacific New Zealand, the Philippines, Japan, Alaska,

western coast of America affected Near Mediterranean

Asia, India, Italy, Greece Turkey Site of many volcanic eruptions

Iceland to middle Atlantic Long ridge of under-ocean volcanic

mountains Formation of new parts of Earth’s crust

Continental Drift

Early 1900s, Alfred Wegener Earth’s continents once joined in giant

landmass, called Pangaea Parts of supercontinent “drifted” to

current positions Theory called continental drift 50 years of study and gathering

evidence before acceptance

Evidence From Fossils

Glossopteris – seed fern that grew in South Africa, Australia, and India 250 million years ago Seeds to large to be carried over ocean to

different continents Mesosaurus – freshwater reptile found in

South America and Africa Only able to survive in shallow fresh water;

could not survive to swim across Atlantic When the landmass separated, some of the

animals were left on each part.

Evidence From Rocks

Rock structure Cape Mountains of South Africa folded

mountains – formed by crumpling of Earth’s crust End abruptly at Atlantic Ocean, matching rocks

in Buenos Aires, Argentina Glacial deposits

Rocks left behind by rock sheet; America, Africa, India, Australia, Antarctica similar

Mineral deposits Salt, coal, limestone derived from coral reefs in

unusual locations

Ocean Floor Spreading

Some scientists not accepting of continental drift until 1950s and 1960s

Better observations of ocean floor and new mapping techniques

Discovery of midocean ridges and rift valleys Much volcanic and earthquake activity at

midocean ridges Lava erupting from rift valleys pushes ocean

floor away as new seafloor forms Deep-sea drilling

Rocks near midocean ridges found to be younger than rocks farther away from the ridge

Magnetic Stripes

Magnetic stripes in ocean-floor rocks Magnetic minerals lined up in molten

rock, becoming permanent magnets as the rocks harden

Discovered shifts in polarity of Earth Pattern identical on both sides of

midocean ridge – half moving in each direction

Plate Tectonics

Combination of continental drift and seafloor spreading

Lithosphere composed of seven major plates, named after surface features

Edges of continents not always boundaries; most boundaries on ocean floor

Three types of plate boundaries

Plate Boundaries

Divergent – plates moving apart; also called constructive; new material being made; midocean ridges

Convergent – plates coming together; also called destructive; material being subducted; trenches; violent reactions; Ring of Fire

Transform fault – usually found at right angles to midocean ridges; plates grinding and sliding; site of many earthquakes

Plate Motion

Believed that convection currents within Earth cause plate movements

Density of material altered due to extreme temperatures within Earth; hotter, less dense matter rises and pushes down cooler matter

Plates move on top of molten material Oceanic crust more dense than continental crust;

oceanic plates subducted beneath continental plates

Plate collisions cause folding with volcanoes, mountains, and trenches

Undersea volcanoes rising above surface of ocean create island arcs, like the Aleutians of Alaska

Past and Future Drifting

Plates moving different speeds and directions

Plates believed separate 510 million years ago, then formed Pangaea

Pangaea surrounded by single large ocean, Panthalassa

200 million years ago breaking of Pangaea

Current drift rate 1 to 5 cm per year