earthquakes and volcanoes ms. pollock earth and space science spring 2008
TRANSCRIPT
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