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Chapter 21 Section 2Earthquakes & Volcanoes

EARTHQUAKES

• Occur due to plate movement faults

• Introduction to Earthquakes• It’s an Earthquake Song

EARTHQUAKES: FOCUS VS. EPICENTER

• Focus: point UNDER Earth’s surface where waves originate…along the fault line

• Epicenter: point ON Earth’s surface directly above the focus…surface waves begin here and move outward• Focus of an Earthquake

SEISMIC WAVES

•3 types of seismic waves:1.P-waves2.S-waves3.Surface waves

Seismic Waves Song

P-WAVES

• P (primary)-waves: move the fastest• Rocks are squeezed & pushed in direction of

wave (longitudinal)• P-Wave Animation

S-WAVES

• S (secondary/shear)-waves: rocks move at right angles (transverse) to direction of waves

• Slower than P-waves• S-Wave Animation

SURFACE WAVES

• Rocks move sideways and up & down• Slowest of the three types of waves• Most destructive wave

Surface Wave Animation

HOW ARE EARTHQUAKES MEASURED?

• Seismograph: instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings

• Seisomogram: a graph showing the motion of the ground versus time

• Seismograph Animation• Seismograph-ing to the beat• Cali Seismogram Real Time

TRAVEL-TIME CURVES

• Measures the time between P & S-waves (that hit a particular seismic station)• Wave speed differs b/c of

density & rigidity of Earth’s layers

• Tells you the distance of location from epicenter

• As the time between waves increases the distance from epicenter increases• Time & Distance= Direct

relationship!

LOCATING THE EPICENTER

• Use travel-time curve

• Ex. Difference between P & S waves=6.3 min• What’s the distance

from the epicenter?• 4,000 km

BIG DEAL? WHAT DOES THAT TELL US…

• By looking at travel-times of seismic waves to different stations, we can generate a probability of epicenter location (scale distance on map)

• See circles around d1, d2 and d3?

• Where all 3 circles intersect tells us epicenter!

• Quake Movie Trailer

DETERMINING DESTRUCTION• Richter Scale

– measures magnitude (energy of largest seismic wave) on 1-10 scale– looking at the wave’s largest

amplitude

– Each magnitude is 10 times stronger than the previous magnitude

– Ex: magnitude 8 earthquake releases as much energy as detonating 6 million tons of TNT!• Japan March 2011 Tsunami Photos

Nat. Geo.• Tsunami Animation• Largest EQs Since 1900

DETERMINING DESTRUCTION CON’T

• Modified Mercalli Scale• amount of shaking experienced at

different locations (intensitydamage)

• Varies based on:• overall magnitude• how far you are from the fault that

ruptured in the earthquake• rock/ land texture or type (i.e. sand vs.

concrete)

• More meaningful measure of severity to the nonscientist because intensity refers to the effects actually experienced at that place.

VOLCANOES

Mt. St. Helens

ZONES OF VOLCANISM• Volcanism: all the processes associated with the release of

magma, hot fluids, and gas• Fueled by magma

• Rises to surface because less dense • Called lava once at surface

LOCATION OF VOLCANOESNOT RANDOM!

• Mostly determined by plate boundaries• Most at convergent and divergent boundaries• 5% far from boundaries

MAJOR BELT OF CONVERGENT VOLCANOESCircum-Pacific Belt

(Pacific Ring of Fire)– Outlines the

Pacific Plate – Convergent

volcanism:– Plates colliding– Continental

subduction zones

– Characterized by explosive eruptions

DIVERGENT VOLCANISM

• Formed where plates move apart• New ocean floor is produced here• Pillow lava forms at ocean ridges• Characterized by non-explosive eruptions• About 2/3 of Earth’s volcanoes occur at divergent

boundaries• Ex: Icelandic volcanoes uncharacteristically explosive due to hot spot

HOT SPOTS• Unusually hot regions of Earth’s mantle where high-

temperature plumes (columns) of magma rise to the surface

• Usually form far from plate boundaries• Stationary – plate moves over spot

• Ex: Hawaiian Islands

Bend in chain change in direction of plate

movement!

VOLCANO ANATOMY

• Conduit: tube-like structure that magma travels through to surface

• Vent: opening that lava comes out of

• Crater: bowl-shaped depression around the vent – Usually less than 1 km (0.6 mi) in diameter

TYPES OF VOLCANOES• Appearance determined by:• Type of

material that forms volcano

• Type of eruptions

• 3 types:1. Shield2. Cinder cone3. Composite

1.

2.

3.

SHIELD VOLCANOES

• Broad, gently sloping sides• Non-explosive, quiet eruptions• Made of layers of basaltic lava• Largest type (width wise)

• Ex: Mauna Loa (Hawaii)

CINDER CONE

• Steep sides, cone shaped• Mostly basaltic lava• Explosive eruptions • Smallest type (most less than 500 m or 0.3 mi high)• Often on or near larger volcanoes

• Ex: Lassen Volcanic Park (California)

• Large, cone shaped, concave slopes• Much larger than cinder cones

• Made of layers of rock (harden lava) from explosion and lava flows

• Violent eruptions (with periods of quiet ones)• Ex: Mt, Augustine (Alaska), Mt. St. Helens

(Washington)

COMPOSITE VOLCANO

MT. VESUVIUS, ITALY • Catastrophic eruption of A.D. 79 destroyed the

cities of Pompeii and Herculaneum and killed at least 16,000 people

• Buried 10’ deep with lava & ash• Thermal energy570 degrees F (300 degrees C)

• most died instantly of extreme heat, with many casualties shocked into a sort of instant rigor mortis.

• Most died instantly of extreme heat, with many casualties shocked into a sort of instant rigor mortis.