Continuation of Earth’s process part 1

Sea- floor Spreading Harry Hees (1960s) proposed that molten material in the mantle rises to the surface at a mid-ocean ridge, turns and flows sideways, carrying the sea floor away from the ridge in both directions the sea floor that is carried away from the ridge is forced down into the mantle at the edge of the oceans forming trenches

Evidences for Sea-floor Spreading: Age of Rocks Scientists in the ship Glomar Challenger collected rock samples from the sea floor They found no rocks older than 200 M years They also found the youngest rocks near the mid-ocean ridges

Evidences for Sea-floor Spreading: Geomagnetic Clues Iron materials in rocks (basalt) align themselves according to the magnetic orientation of the earth Scientists found that rocks showed many magnetic reversals

The magnetic alignment of the rocks reversed back and forth in strips parallel to the mid-ocean ridge This showed that new rocks are being formed at the mid ocean ridges

Rocks forming at the ridge crest record the magnetism existing at the time they solidify. Sea floor increases in age and is more deeply buried by sediment away from the ridge because sediments have had a longer time to collect. Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating magnetic anomaly stripes of the sea floor and measuring their distance from the ridge crest. Continents are moved by the expanding sea floor.

The Plate Tectonics Theory Using the theory of continental drift and sea-floor spreading, scientists proposed that sections of the sea-floor and continents move around in relation to one another The earth’s crust and upper mantle are broken into sections or plates and these plates move around on the mantle

The Plate Tectonics Theory Plates are composed of the lithosphere (crust and a part of the upper mantle) and float on a denser plastic-like layer (asthenosphere) Interactions between plates include: Move towards each other and collide Pull apart Move past each other Result of their movements are seen in plate boundaries

Types of Plate Boundaries Divergent boundaries (also called spreading centers) are the place where two plates move apart.

Types of Plate Boundaries Convergent boundaries form where two plates move together.

Types of Plate Boundaries Transform fault boundaries are margins where two plates grind past each other without the production or destruction of the lithosphere.

The San Andreas fault in southern California is a transform fault that connects the sea-floor spreading ridge of the Gulf of California with the spreading ridge off Oregon and Washington.

Causes of Plate Tectonics Movement of plates is due to convection current Thermal convection of the “plastic” rocks of the asthenosphere drags along the overlying lithospheric plates.