Volcanoes and Plate Tectonics

1. Volcanoes and Plate Tectonics Chapter 7

2. Basic Earth Science Truth • HEAT = ENERGY • DENSITY = MASS/VOLUME • Adding HEAT to a substance causes atoms to move faster, with more energy • This causes its volume to expand • If VOLUME increases while MASS remains constant, DENSITY will _______________.

3. Hot stuff rises! Cold stuff sinks! • Therefore, an area of heated substance will become less dense than adjacent, cooler material. • Less dense: it will tend to rise, while more dense areas will sink • Convection of outer core: • magnetosphere • Convection of asthenosphere: • plate tectonics • Rising magma: Volcanism

4. Phases of Matter • Different substances have • different atomic structures • different melting/boiling points http://www.miamisci.org/af/sln/phases/ • States/phases of MATTER: • Lowest energy: SOLID PHASE • Intermediate energy: LIQUID PHASE • Highest energy: GAS PHASE

5. Roles of Temp/Pressure • Temperature: adds energy, encourages atoms to a “higher-energy state” • Pressure: contains the atoms – inhibits them from expanding or moving to a higher energy state • This is why the inner core is solid, even though it’s hotter than the outer core: the higher pressure keeps it from melting

6. Formation of Magma • Magma = liquid rock • Forms when: • Pressure drops, temp doesn’t drop: this happens at subduction zones, mid-ocean ridges, and hot spots • Addition of fluids such as water: this happens at subduction zones

7. Some terms, in context • Any activity including movement of MAGMA toward or on earth’s surface is VOLCANISM • When MAGMA erupts onto the earth’s surface it is called LAVA. It flows through a VENT. • The structure formed by the vent and volcanic material erupted is a VOLCANO

8. Major Volcanic Zone # 1 • Subduction zones, due to water introduced into the asthenosphere from subducting oceanic plate: Pacific Ring of Fire: Aleutian Islands, Japan • Forms OCEAN TRENCH and • Cont-Ocean: __________________? • Ocean-Ocean: ____________________?

9. Major Volcanic Zone # 2 • Mid-Ocean Ridges: greatest amount of volcanism, but b/c it occurs mostly deep in the ocean, we don’t notice it (Iceland is exception, mid-Atlantic Ridge above SL) • Divergent boundaries: as plates move apart, magma rises up through RIFTS to form new crust. • If on land (as in Iceland) these are called FISSURES

10. Major Volcanic Zone # 3 • Hot Spots: Hawaiian Islands • As plates move over hot spot, volcanism takes place • This is why the oldest Hawaiian island is to the northwest and youngest to the southeast • Unique because NOT at plate boundary • We’re not sure why hot spots occur where they do.

11. Types of lava • MAFIC: dark in color, high in Fe, Mg • Oceanic crust origin • High density • Thin, ropey: PAHOEHOE pa-HOY-hoy • Thick, blocky (not the “ch” word) AA AH-ah • FELSIC: light in color, high in Si, feldspars • Generally continental crust origin • Low density • Lava comps also range between the two

12. More about types of lava • MAFIC lava: • Hot, thin, flows easily • Little water or gases • “quiet” eruptions • Forms SHIELD volcanoes with a gentle slope

13. More about types of lava • FELSIC lava: • Cooler, thicker than mafic • Large amounts of H2O and CO2 • As magma moves toward surface, lower pressure allows gas bubbles to form • Gas pressure causes explosions – “noisy” eruptions • Source is water from subducted plate • Lava erupts as liquid, also as particles

14. PYROCLASTICS or TEPHRA • Rock fragments ejected from a volcano • Blocks of cooled magma • Result of explosion which throws lava into the air, cooling as individual particles < 0.25 mm diameter: VOLCANIC DUST Can circulate in atmosphere long enough to circle the earth, block sun’s rays/heat, causing global cooling. Krakatau caused “The Year that Summer Didn’t come” 1883. New England states had snow in JULY.

15. Tephra Travels • How far depends on size of particles – smallest travels furthest. < 2mm diameter: VOLCANIC ASH During/after an eruption Ash deposited by the inch or even by the FOOT. HOT ash covered the city of Pompeii in 79 A.D. during an eruption of Mt. Vesuvious. Subsequent rains hardened the ash, entombing the occupants of the city for nearly 2,000 years.

16. How ash floats in the air • Tephra result from explosion of lava. The drops of lava are sprayed into the air, and harden. As a result, they are filled with air pockets, as you can see from this microscopic photo

17. Larger Tephra stay close to home • And the bigger the tephra, the steeper the slope of the cone they create: • CINDER CONES: slopes up to 40 degrees, but only a few hundred meters high • STRATOVOLCANO or COMPOSITE CONES: combination: tephra layers interspersed with layers of lava. • This type can result in very high volcanic mountains: the Cascades, Mt. Fuji being a few

18. CRATER? CALDERA? • CRATERS are the funnel-shaped pit at the top of the volcano. • If the magma chamber below the crater empties, sometimes the crater collapses in and forms the larger CALDERA • Can also form as result of explosion which blows off the top of the mountain: Krakatoa, or Mt. St. Helens being examples.

19. Predicting Eruptions • Earthquakes indicate magma movement • Fracturing/temp increase of rocks • Development of a BULGE may indicated magma pushing up from below • Change in comp of gases emitted • ERUPTION HISTORY is useful, but often sketchy or non-existent

20. ET VOLCANOES Many planets/moons active in the past; • MARS • Olympus Mons on Mars: shield volcano 28 km high (nearly 3x Mt. Everest), caldera 70 km wide • Viking landing craft detected seismic activity: marsquakes? • Basalt flows on our Moon • While many of moon’s craters from meteor impact, also possible that impact could cause heat (from explosion) and fracturing which would produce magma and allow it to flood onto the surface

21. Io’s Number One! • One of the four Galilean moons of Jupiter, Io, is so tidally affected by Jupiter’s immense gravity that it is the most volcanically active object in our solar system. • Volcanoes stand 52,000 feet high!

22. Io is the size of our Moon • Several thousand metric tons/year • Primarily erupts sulfur • Plumes of sulfuric magma hundreds of km high