Subduction

Subduction - Plate Tectonics III 1. One proof of seafloor...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
Plate Tectonics III 1. One proof of seafloor spreading is the subsidence of volcanic islands to form atolls or guyots, first noted by Charles Darwin. 2. Another proof of seafloor spreading is the symmetrical pattern of magnetic anomaly bands. Seafloor is magnetized by the Earth's field when it forms, but this field has reversed many times, leaving a distinctive symmetric pattern of anomaly bands that can be detected and used to date the seafloor. 3. Transform faults are active plate boundaries with motion parallel to their length. Many connect spreading rise segments. Inactive former transform traces called fracture zones create a parallel fabric across the seafloor. 4. Subduction zones consume old, dense oceanic lithosphere, which recycles into hot asthenosphere under its own weight. Subduction is marked by trenches, accreted sediment wedges, great earthquakes which cause tsunamis, and volcanic arcs.
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
1 c:\work\classes\ess1\subducti.doc PLATE TECTONICS III New hypotheses arise unpredictably from a combination of scholarship, observation, and intuition. Most are wrong. A strong hint that you're on the right track is when your hypothesis to solve problem A also solves problem B (previously thought to be unrelated). 1. One proof of seafloor spreading is the subsidence of volcanic islands to form atolls or guyots, first noted by Charles Darwin. Most of the small mid-ocean islands began as basaltic volcanic seamounts, which grew above sea level to become active volcanic islands. When volcanism stops, rain- and wave-erosion wears away the part of the cone above sea level, leaving the submerged part. Meanwhile, coral grows and forms reefs along the coast (if it is warm enough). Coral reefs: -Are composed of crystals of calcium carbonate, CaCO 3 . -This calcium carbonate was originally in solution in seawater. -Were deposited by coral animals, which build colonies ("coral heads") of ~30,000 individuals (alive only in the outer most layer). -The coral animal strains seawater for food, so it needs clean water (far from muddy rivers of the continents). -Its symbiotic algae photosynthesize, so they need sunlight, which restricts reef-building to shallow water. -Calcium carbonate is only stable in warm water, which restricts reef-
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 6

Subduction - Plate Tectonics III 1. One proof of seafloor...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online