2.2 What additional observations led to the theory of plate tectonics?

magnetic field description

• has N and S poles (Earth’s geographic and magnetic N pole are not in the same place)

• lines wrap around the planet

• protects life from solar storms

magnetic dip

degree to which a magnetic particle points into Earth

• magnetic field caused a dip needle to align parallel with lines of magnetic force and change orientation with increasing latitude, leading to the determination of latitude

igneous rocks

solidify from molten magma underground or after volcanic eruptions at the surface that produce lava

• almost all contain magnetite, a naturally magnetic iron material

• particles of magnetite in magma align themselves with Earth’s magnetic field because magma and lava are fluid.

what do igneous rocks tell us about earth’s magnetic field?

• once molten material is cooled to a certain temperature, internal magnetite particles are frozen into positions, thereby recording the angle of Earth’s magnetic field at that place and time

• tiny compass needles that record the strength and orientation of Earth’s magnetic field.

why does Earth’s magnetic field switch polarity? (N and S switch)

• Earth’s rotation causes the electrically conducting liquid iron outer core to generate a self-sustaining magnetic field

• every so often, the flow of liquid iron is disturbed locally and twists part of the magnetic field in the opposite direction, weakening it

• the pattern of switching polarity is highly irregular

magnetic anomalies

stripes on the ocean-floor of north-south stripes in the oceanic crust

sea floor spreading

• new ocean crust was created at ridges, split apart, moved away from the ridges, and disappeared back into the deep Earth at trenches

mid-ocean ridge

a continuous underwater mountain range that winds through every ocean basin in the world and resembles the seam on a baseball

• entirely volcanic in origin

• new ocean floor forms at the crest of the mid-ocean ridge

• the mid-ocean ridge is a spreading center

• thinner, younger crust

deep-ocean trenches

deepest parts of the ocean floor, resembling a trough

• largest earthquakes in the world happen here

• ocean floor is destroyed here

• thicker, older crust

subduction

when a plate bends downward and slowly plunges back into Earth’s interior

where is the ocean floor the youngest/oldest?

• youngest closer to MOR

• older closer to trenches

why is continental crust older than oceanic crust?

sea floor spreading doesn’t happen, bc of low density of continental rocks, they don’t get recycled and remain at earth’s surface for long periods of time

heat flow

heat moving to surface from Earth’s interior

• heat-flow much higher at MOR

• heat-flow much lower at trenches

forces driving plate motion

1. slab pull = generated by the pull of the weight of a plate as it sinks underneath an overlying plate

2. slab suction = created as a subducting plate drags against the viscous mantle and causes the mantle to flow toward the subduction zone, thereby sucking in nearby plates

Why does a map of worldwide earthquakes closely match the locations of worldwide plate boundaries?

almost all earthquakes happen at plate boundaries