Notes
Earth's layers
- By composition: crust, mantle, core (iron and nickel). By properties: lithosphere (rigid), asthenosphere (weak, flowing), mesosphere, liquid outer core, solid inner core.
- Oceanic crust is thin and dense (basalt); continental crust is thick and less dense (granite-like).
Evidence and theory
- Wegener proposed continental drift with evidence of matching coastlines, fossils and rocks, but had no mechanism.
- Seafloor spreading: new crust forms at mid-ocean ridges. Magnetic stripes are symmetrical and rock age increases away from the ridge.
- Plate tectonics: lithospheric plates move a few cm per year, driven by mantle convection and slab pull.
Boundaries
- Divergent (apart): ridges and rifts. Convergent (together): subduction, trenches, volcanoes, mountains. Transform (sliding): strike-slip faults.
- Hot spots form island chains away from boundaries.
- Speed $\displaystyle = \frac{\text{distance}}{\text{time}}$.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Plates are pieces of the lithosphere.
The youngest rocks are at the ridge.
Plates moving apart form ridges and rifts.
Density decides which plate subducts.
The missing mechanism was the main problem.
The plate carries volcanoes away from the plume.
Free response
Write your answer first, then open the scoring guide.
Free-response question
The seafloor on either side of a mid-ocean ridge is spreading. A seafloor sample 120 km from the ridge crest on the east side has an age of 6 million years. The Hawaiian-type island chain nearby has islands that are 400 km apart in a line and 4 million years apart in age.
(a) Calculate the average spreading rate of the seafloor in cm/yr. (1 km = 100,000 cm)
Show the answer and scoring guide
$120 \text{ km} = 1.2\times10^7$ cm. $\displaystyle \frac{1.2\times10^7}{6\times10^6} = 2$ cm/yr.
Check off the points you earned:
(b) Calculate the speed of the plate carrying the island chain in cm/yr.
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$400 \text{ km} = 4\times10^7$ cm. $\displaystyle \frac{4\times10^7}{4\times10^6} = 10$ cm/yr.
Check off the points you earned:
(c) Describe two pieces of evidence, other than age, that support seafloor spreading.
Show the answer and scoring guide
Symmetrical magnetic stripes on both sides of the ridge record field reversals; the ridge is elevated with a central rift where new crust forms; ocean sediment is thinner near the ridge than far away.
Check off the points you earned: