Geological Sciences 107 Our Dynamic Planet

PLANET EARTH IS A DYNAMIC EVOLVING SYSTEM - FROM 4.6 BILLION YEARS AGO TO NOW STRUCTURE & COMPOSITION REFLECT ONGOING EVOLUTION EVOLUTION DEPENDS ON HOW HEAT- "THE GEOLOGICAL LIFEBLOOD OF PLANETS" - TRANSFERRED OUT OF COOLING EARTH BY THERMAL CONVECTION (Hot stuff rises) THERMAL CONVECTION CAUSES PLATE TECTONICS: PLATES OF EARTH'S SURFACE MOVE RELATIVE TO EACH OTHER AT A FEW mm/yr (about speed fingernails grow) PLATE MOTIONS CAUSE EARTHQUAKES, VOLCANOS, MOUNTAIN BUILDING AT PLATE BOUNDARIES PLATE TECTONICS MAKES EARTH WHAT IT IS - DIFFERENT FROM NEIGHBOR PLANETS (Mars, Venus, & Earth: 3 bears principle) PLATE MOTIONS ARE CRUCIAL FOR THE ORIGIN OF LIFE, ITS SURVIVAL, AND OUR CLIMATE PLATE MOTIONS PROVIDE RESOURCES AS WELL AS HAZARDS TO SOCIETY "CIVILIZATION EXISTS BY GEOLOGICAL CONSENT"

EARTH- A LAYERED PLANET DUE TO ITS EVOLUTION THE PLATES (= LITHOSPHERE) ARE THE COLD, STRONG OUTER BOUNDARY LAYER ("thin scum": 100 / 6371 km) MOVING OVER WEAKER ASTHENOSPHERE

Earth’s surface divided up into about a dozen major lithospheric plates

Three Types of Plate Boundaries (Margins)

NORMAL FAULTING Wasatch Fault Salt Lake City, Utah Region under extension

REVERSE FAULTING Region under compression

STRIKE SLIP FAULTING Offset fence showing 3.5 m of left- lateral strike-slip motion on San Andreas fault in 1906 San Francisco earthquake

Western North America: Three Major Plate Boundaries

CONVERGENT (SUBDUCTION)-

Kirby et al., NISQUALLY, WASHINGTON EARTHQUAKE (M 6.8, $2B damage) Within subducting Juan de Fuca plate Intraslab earthquakes now recognized hazard

CASCADE VOLCANOES INDICATE JUAN DE FUCA PLATE SUBDUCTION BENEATH NORTH AMERICA Mt Saint Helens 1980 eruption USGS Mt. St. Helens: May 18, 1980

Mount St. Helens as part of the new dome collapses. USDA Forest Service photograph by taken moments after a 3.2 magnitude earthquake triggered the event on July 18, 2005

TRANSFORM- San Andreas Fault System: Motion Between North American & Pacific Plates

San Andreas Lake- San Francisco Bay Area

1906 SAN FRANCISCO EARTHQUAKE (Magnitude 7.8) ~ 4 m of slip on 450 km of San Andreas Fault ~2500 deaths, ~28,000 buildings destroyed (most by fire)

San Andreas Fault- Carrizo Plane, California

San Andreas Fault- Palmdale, California

DIVERGENT: Seafloor Spreading in the Gulf of California

Plate Boundaries in Western North America Evolve Over Time

Western North America Tectonics- The Movie

ICELAND - Part of the Mid-Atlantic Ridge - Made of Recently Erupted Basalt - Formed in past 15 million years Thingvellir Eurasian plate North American plate 20 mm/yr

1973 Eruption on the island of Heimaey, Iceland

East Africa Rift - New Spreading Center Forms by Rifting Continental Crust Africa is splitting up into Nubia (West Africa) and Somalia (East Africa)

2001 Eruption near Goma, Congo Along the East African Rift

DECEMBER 2004 INDIAN OCEAN EARTHQUAKE AND TSUNAMI

INDIAN PLATE MOVES NORTH COLLIDING WITH EURASIA

COMPLEX PLATE BOUNDARY ZONE IN SOUTHEAST ASIA Northward motion of India deforms large region Many small plates (microplates) and blocks Molnar & Tapponier, 1977

India subducts beneath Burma microplate Earthquakes occur at plate interface along the Sumatra arc (Sunda trench) Largest & most destructive earthquakes results from many years of accumulated motion

BETWEEN EARTHQUAKES: India subducts beneath Burma at about 20 mm/yr Fault interface is locked EARTHQUAKE: Fault interface slips, overriding plate rebounds, releasing accumulated motion and generating tsunami HOW OFTEN: Fault slipped ~ 10 m --> mm / 20 mm/yr = 500 yr Faults aren’t exactly periodic - intervals vary Crucial for assessing hazards Stein & Wysession, 2003 INDIA BURMA Tsunami generated SUMATRA TRENCH

Earthquakes rupture a patch along fault's surface. Generally speaking, the larger the rupture patch, the larger the earthquake magnitude. Initial estimates from the aftershock distribution show the magnitude 9.0 Sumatra- Andaman Islands Earthquake ruptured a patch of fault roughly the size of California, and modeling seismic waves show that most of the slip occurred in the southern 400 km of the patch. For comparison, a magnitude 5 earthquake would rupture a patch roughly the size of New York City's Central Park.

MAGNITUDE 9 EARTHQUAKES ARE INFREQUENT Stein & Wysession after IRIS

TSUNAMI - water wave generated by earthquakes, volcanic eruptions & landslides NY Times

TSUNAMI GENERATED ALONG FAULT, WHERE SEA FLOOR DISPLACED, AND SPREADS OUTWARD Red - up motion, blue down Hyndeman and Wang, 1993

TSUNAMI SPEED IN DEEP WATER of depth d c = (gd) 1/2 g = 9.8 m/s 2 d = 4000 m c = 200 m/s = 720 km/hr = 450 m/hr Tsunami generated along fault, where sea floor displaced, and spreads outward Reached Sri Lanka in 2 hrs, India in 2-3

Because seismic waves travel much faster (km/s) than tsunamis, rapid analysis of seismograms can identify earthquakes likely to cause major tsunamis and predict when waves will arrive TSUNAMI WARNING Deep ocean buoys can measure wave heights, verify tsunami and reduce false alarms

HOWEVER, HARD TO PREDICT EARTHQUAKES recurrence is highly variable M>7 mean 132 yr  105 yr Estimated probability in 30 yrs 7-51% Sieh et al., 1989 Extend earthquake history with geologic records - paleoseismology

Plate boundary volcanism produces atmospheric gases (carbon dioxide CO 2 ; water H 2 O) needed to support life and keep planet warm enough for life ("greenhouse" ) May explain how life evolved on earth (at midocean ridge hot springs) Plate tectonics raises continents above sea level Plate tectonics produces mineral resources including fossil fuels PLATE TECTONICS IS DESTRUCTIVE TO HUMAN SOCIETY BUT CRUCIAL FOR HUMAN LIFE Press & Siever

“CIVILIZATION EXISTS BY GEOLOGICAL CONSENT - SUBJECT TO CHANGE WITHOUT NOTICE” The same geologic processes that make our planet habitable also make it dangerous

TWO GUIDING PRINCIPLES FOR STUDYING THE EARTH UNIFORMITARIANISM > past history of Earth explained by what is happening now > "the present is the key to the past" SCIENTIFIC METHOD- Logical, orderly form of inquiry > OBSERVE PHENOMENA- Evidence; measurements and data > HYPOTHESIS FORMULATION- Explain observations by developing hypothesis (INITIALLY UNPROVEN IDEA) > TEST- WITH EXPERIMENTS &/OR OBSERVATIONS > THEORY (ACCEPTED) -If enough testing accept as theory (statement designed to explain phenomenon or set of rules/principles designed for study) Assume all governed by scientific laws … IF NEW OBSERVATIONS, EXPERIMENTS, OR IDEAS THEN….

How to study geological processes with the scientific method?- Example 1 1. Observe- Franklin observed cold winter in Europe after major volcanic eruption in Iceland 2. Hypothesis- Franklin’s hypothesis: large eruption put dust into atmosphere and decrease amount of sunlight reaching surface, so colder temperatures. 3. Test- More data from eruptions and better understanding sunlight/atmospheric interactions 4. Theory- Dust from eruptions decrease sunlight reaching surface and result in colder temperatures for up to a few years

How to study geological processes with the scientific method?- Example 2 1. Observe- Shorelines, rocks, fossils indicate similar origins on continents now very far apart 2. Hypothesis- Continental drift? Continents were once joined & now split 3. Test- Seafloor spreading & other data now confirm it 4. Theory- Plate Tectonics Theory!

How to study geological processes with the scientific method?- Example 3 1. Observe- Earthquakes occur; Tidal forces 2. Hypothesis- Browning (1990): Tidal forces trigger earthquakes. Predict big earthquake in New Madrid 3. Test- Scientists: NO correlation between tidal forces & past big earthquakes for Browning model. Predicted New Madrid Earthquake did not occur. 4. Theory-None! Why was it a big media story?

Earth’s Plate Tectonics Future- The Movie