Unit 2 Introduction to Earth Systems In Class Notes #3

Unit 2 Introduction to Earth Systems In Class Notes #3

Magnetism, Earthquakes, Volcanoes

Earth’s Magnetic Field

• Liquid outer core consists of iron

– Produces dynamo

– Produces magnetic field

• Dipolar magnetic field

– Bar magnet

• Reversals of magnetic field over time

• Produces magnetic polarities in rock –

Rock magnetization

Seafloor Spreading and Magnetic Anomalies

Driving Forces of Plate Tectonics

• Plate movement due to Earth's attempt to lose internal heat

• Conduction

• *Convection

Ridge Push

• Not really a "push,"

instead it is gravity- driven sliding

• Plate slide down sides of mid-ocean ridge

• Gravity gives the push, not magmas at ridge

• Ridge push is probably the main force on

plates

Slab Pull

• Subduction driven by density

• Denser (older) lithosphere sinks back into mantle

pulling plate behind it

• Problem: plates are not under

extensional stress

• Flow of material

associated with hot spot plumes drives plates

• Columns of rising material vs.

convection cells

• Chaotic convection in mantle (plumes of rising material)

http://www.howstuffworks.com/earthquake6.htm

EARTHQUAKES

Earthquakes

Most destructive forces on Earth. But it is buildings and other human structures

that cause injury and death, not the earthquake itself

1988 - Soviet Armenia: magnitude 6.9, 25,000 people died 1985 - Mexico City: magnitude 8.1, 9500 people

1989 - Loma Prieta, CA: magnitude 7.1, 40 people died 1995 - Kobe, Japan: magnitude 7, ~6000 people died

30,000

earthquakes occur worldwide annually that are strong enough to

be felt

Typically only 75 of them are considered to be

significant

• Vibration of earth produced by rapid release of energy (seismic waves) with radiate in all

directions from the source (focus)

• Like ripples from dropping a stone in a pond, energy dissipates with distance

• Earthquakes don't occur randomly. Occur on faults or fractures within the earth

• Explained by plate tectonics.

Most occur on plate boundaries

• Sometimes in plate interiors if enough stress is built up

Elastic Rebound Theory

• Forces bend rock on either side of fault

• Rock strains ever so slowly

• Weakest point breaks

• Break sends out shock waves, which migrate

outwards from he original break, causing shaking

• Stress is released

• Aftershocks are adjustments to that change in stress

• Always less strong than the main shock, but they may cause more damage to weakened structures

Duration of Shaking

• 1989 Loma Prieta: 15 seconds

• 1960 San Francisco: 40 seconds

• 1962 Alaska: 4 minutes!!!!

Types of Waves

Earthquake waves = seismic waves. Recorded on seismometers on seismographs.

Types of Waves

• Surface waves - travel on Earth's surface, away from epicenter.

– Very slow waves. Cause a lot of damage, rolling feeling at end of earthquake

• Body Waves - travel through Earth's interior, spread outward from focus

Body Waves

P waves:

• Pressure or compressional waves. Vibrate parallel to direction of wave travel like a slinky.

• Fast travel: 4-7 km/sec (15,000 mph)

• P is primary, or first wave to arrive at recording station

S waves:

• Shear waves. Vibrates perpendicular to direction of wave travel. Like snapping a rope

• Slower than P wave: 2-5 km/sec (11,000 mph)

• So S is secondary, or second wave to arrive at recording station

Locating the Epicenter of an Earthquake

• P, S and surface waves all start out at same time.

• The further you are away from the quake, the longer the time span between arrival of P and S wave.

• The distance of the seismometer to the

earthquake can be determined by the time between the arrival of P wave and arrival of S waves.

• Can tell the distance, but not the direction.

• Therefore, multiple sites must be used to find epicenter.

http://georoc.mpch-mainz.gwdg.de/volcano.gif

VOLCANOES

• Located at plate boundaries & EQ zones (why ?)

• Result in surface pyroclastic and extrusive rocks

• Pyroclastic : particles thrown into air during eruption - settle to form ash, tuff & agglomerate

• Magma extruded to surface to form extrusive igneous rocks (lava), e.g rhyolite, andesite & basalt (type

depends on acidity)

– Acidic : viscous, flows poorly

– Basic : more fluid - flows on very gentle slopes over vast areas, e.g basalt plains of Victoria

Volcanoes

Volcanoes

• Basic parts of a volcano

• Crater (depression at the summit of a volcano, connected by a vent or pipe to the magma

chamber below)

• Caldera (crater more than 1 km in diameter,

formed at the summit of a volcano when lava is drained from an underground magma chamber, causing the summit of the volcano to be

unsupported, and to collapse)

Types of Volcanoes

1. Shield volcanoes - Hawaii

• Docile lava outpouring. Only minor pyroclastic material

• Lava forms broad dome with central crater

• Slope is 2-10 degrees, like flattened shield

• Very long lived, very large, massive amounts of lava (pahoehoe and aa)

Example: Kilauea

Mauna Loa, on the Big Island of Hawaii, is the largest active volcano in the world. It last erupted in 1984. Mauna Loa erupted 14 times in the 20th Century, and 37 times since 1832. Mauna Loa is the most massive mountain on Earth, rising to an elevation of 13,677 feet above sea level, or 31,677 feet above the sea floor. Its volume is 10,000 miles³.

2. Cinder cones

• Erupt pyroclastic material

• Steep slopes (30 to 40 degrees)

• Not very long lived.

• Typically small, less than 1000 feet tall

• Often parasitic on larger volcanoes

Examples: Paricutin in Mexico, Sunset crater in Arizona

3. Composite volcanoes

• Erupt lava and pyroclastic material

• Intermediate slopes because lava acts like protective coating on pyroclastic layers

• Built up over long periods of time

• Most picturesque, but most violent

Examples: Mt. Vesuvius, Mt. Shasta, Mt. Fuji, Yellowstone

Eruptions from Volcanoes

• May produce ejecta, (lava rock or ash), molten lava, and/or toxic gases.

• Gases: H₂O, CO₂, SO₂, HCl

• Occur when pressure within magma chamber forces molten magma up through a conduit and out a vent.

• Benefits: new landforms, minerals, and nutrients from broken down lava

Eruption of Mount Saint Helens, May 18, 1980

• The eruption of Mount Saint Helens was the most destructive in the history of the United States

• Mount Saint Helens is located in southwest

Washington in the Cascade Range, a mountain range dominated by periodically active volcanic peaksIn

• Images include pre-eruption activity and post-

eruption effects such as the blast area, mud flows, ash fall, and altered terrain

• http://www.ngdc.noaa.gov/seg/hazard/slideset/31/31_thumbs.ht ml