Plate Tectonics

What best describes the earthquakes associated with convergent (subduction) zone boundaries? A. They range in depths B. They range in magnitudes (strength) C. They are mostly deep D. Choices A & B E. Choices B & C

D. Choices A & B

Which of the following is not an example of a divergent plate boundary? A. Iceland B. Red Sea (Africa-Saudi Arabia) C. Mid-Atlantic ridge D. Hawaii

D. Hawaii Hawaii is a hot spot.

Transform boundary -Type of earthquakes?

Earthquakes are shallow but they can be powerful (not as large as in convergent plate boundaries though).

Convergent

moves toward Ocean - continent (subduction type) Continent - continent (collision type)

Convergent:

subduction zones where oceanic crust is being melted • These zones are associated with coastal mountain ranges,volcanoes, and earthquakes (e.g. Pacific NW, Chile, Indonesia)

Scientific Milestones

• Convection Currents ('40s) • Paleomagnetism & bathymetry ('50s)

Divergent:

• Divergent: Oceanic ridges (submerged mountain ranges) are areas where new crust is being formed.

Mid-ocean ridges

• Most extensive chain of mountains on earth • More than 90% lie in the deep ocean. • Wraps around the globe for more than 40,000 mi and covers about 23% of the Earth's surface. • About 70% of the Earth's surface was produced in a mid-ocean ridge.

Divergent

moves away

Transform

moves side by side

Scientific theory of plate tectonics:

"the lithosphere of the earth is divided into a small number of plates which float on or travel independently over the mantle and much of earth's seismic activity occurs at the boundaries of these plates"

Which best describes mid-ocean ridges? A. They are made of basaltic lava with deposits of valuable minerals. B. They are ridges because the plates are converging there. C. They are places where felsic lava erupts to form composite volcanoes. D. They are plate boundaries with very strong, deep earthquakes.

A. They are made of basaltic lava with deposits of valuable minerals.

Which type of stress is dominant at convergent plate boundaries? A. tensional stress B. compressive stress C. shear stress

B. compressive stress—thrust faults

How does the age of ocean lithosphere compare to continental lithosphere? A. It is the same age. B. It is much older. C. It is much younger.

C. It is much younger.

Which change would occur with the evolution from subduction to collision? A. There would be deeper earthquakes. B. Earthquakes would not happen anymore. C. The lithosphere would get thinner. D. Volcanoes would become inactive.

D. Volcanoes would become inactive.

Earthquakes at divergent boundaries?

Due to tensional stress, earthquakes tend to be weak and shallow. At the midocean ridges, the lithosphere is being created and it is very thin.

Divergent Boundary: Sea Floor Spreading

Mid-Atlantic Ridge • New oceanic crust formed at spreading centers

"Ring of fire":

Subduction zone volcanoes along the west coast of the Americas

Deepest place on Earth?

The trenches are the deepest places—subduction-type convergent boundary

Tranform:

Two plates are sliding past each other laterally. • These zones are associated with earthquakes (e.g. San Andreas Fault).

Divergent Boundaries

What kind of faults? NORMAL What kind of igneous rocks? MAFIC What kinds of earthquakes? SHALLOW

Paleomagnetism & Bathymetry

World War II: Surface boats searching for submarines

Subduction-zone volcanoes:

composite with high viscosity lavas, explosive eruptions, and felsic to intermediate compositions

Continental Drift (1915) based on

geography and fossil evidence

(convection currents/paleomagnetism/bathymetry)

provided more data/proof that the continental plates move around the surface of the earth.

Transform Boundary: San Andreas

• No active volcanoes near Bay Area, but EARTHQUAKES! So why? • Transform (Strike-slip fault) plate boundary • Lateral movement of plates • One day Los Angeles (Pacific plate) will be right next to SanFrancisco (North American plate).

Convergent Boundary -Collision Type

• Regional Metamorphism (Folding) • Thrust faulting • Mountain building (orogeny) (Himalayas (India/Asia continents); parts of the Mediterranean (Africa/Eurasia continents) • Shallow (generally weaker) earthquakes

Divergent Plate boundaries ...are characterized by ...include features

• Shallow and weaker earthquakes • Normal faults • Mid-ocean ridges or riftvalleys • Basaltic lava • Young, thin crust • Black smokers -hydrothermal vents • Pillow basalts • Hydrothermal metamorphism

Convergent Boundary -Subduction Type

• Subduction of Ocean Crust • Earthquakes range greatly in size and depth, and may form tsunamis • Thrust faults from compressive stresses • Forms trenches and volcanic arcs

Convergent Boundary: Subduction

• Subduction zones are areas where crust are pushed into the mantel • Scientific explanation for coastal ranges, volcanoes, and earthquakes

Historical Background

• The Origins of Continents and Oceans by Alfred Wegner (1915) • Present continents once formed a single land mass (Pangaea) • Theory of Continental Drift