seafloor spreading hypothesis

the mechanism for continental drift

Seafloor spreading is the mechanism for Wegeners drifting continents. Convection currents within the mantle take the continents on a conveyor-belt ride of oceanic crust that, over millions of years, takes them around the planets surface. The spreading plate takes along any continent that rides on it. Click image to the left or use the URL below. URL:

an essay in geopoetry

Harry Hess was a geology professor and a naval officer who commanded an attack transport ship during WWII. Like other ships, Hesss ship had echo sounders that mapped the seafloor. Hess discovered hundreds of flat-topped mountains in the Pacific that he gave the name guyot. He puzzled at what could have formed mountains that appeared to be eroded at the top but were more than a mile beneath the sea surface. Hess also noticed trenches that were as much as 7 miles deep. Meanwhile, other scientists like Bruce Heezen discovered the underwater mountain range they called the Great Global Rift. Although the rift was mostly in the deep sea, it occasionally came close to land. These scientists thought the rift was a set of breaks in Earths crust. The final piece that was needed was the work of Vine and Matthews, who had discovered the bands of alternating magnetic polarity in the seafloor symmetrically about the rift.

seafloor spreading

The features of the seafloor and the patterns of magnetic polarity symmetrically about the mid-ocean ridges were the pieces that Hess needed. He resurrected Wegeners continental drift hypothesis and also the mantle convection idea of Holmes. Hess wrote that hot magma rose up into the rift valley at the mid-ocean ridges. The lava oozed up and forced the existing seafloor away from the rift in opposite directions. Since magnetite crystals point in the direction of the magnetic north pole as the lava cools, the different stripes of magnetic polarity revealed the different ages of the seafloor. The seafloor at the ridge is from the Brunhes normal; beyond that is basalt from the Matuyama reverse; and beyond that from the Gauss normal. Hess called this idea seafloor spreading. As oceanic crust forms and spreads, moving away from the ridge crest, it pushes the continent away from the ridge axis. If the oceanic crust reaches a deep sea trench, it sinks into the trench and is lost into the mantle. The oldest crust is coldest and lies deepest in the ocean because it is less buoyant than the hot new crust. Hess could also use seafloor spreading to explain the flat topped guyots. He suggested that they were once active volcanoes that were exposed to erosion above sea level. As the seafloor they sat on moved away from the ridge, the crust on which they sat become less buoyant and the guyots moved deeper beneath sea level.

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instructional diagrams

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questions

the evidence that old seafloor is destroyed at deep-sea trenches includes

a) heat flow is high at the trenches.

-->  b) magnetic stripes end at the trenches.

c) the crust is very thin at the trenches.

d) all of the above

why did harry hess call his paper describing plate tectonics an essay in geopoetry?

a) some of the data fit and some needed poetic license to make part of the story.

b) the ideas presented were just a fantasy.

-->  c) the data all fit together so well, that it hardly seemed possible.

d) he wrote the paper in rhyme.

flat-topped underwater mountains are called

-->  a) guyots

b) plateaus

c) mid-ocean ridges

d) mesas

the alternating stripes of normal and reverse magnetism in seafloor basalts on the sides of the mid-ocean ridges led to the idea of

a) magnetic rifting

b) continental drift

c) plate drift

-->  d) seafloor spreading

older crust is hot and more buoyant than younger crust.

a) true

-->  b) false

harry hess suggested that guyots were

a) eroded mountains that remained stationary as sea level rose.

b) eroded beaches that sank below sea level.

-->  c) volcanoes that were exposed to erosion above sea level and then sunk.

d) none of these.

the mechanism for continental drift that wegener never knew about is

a) gases

-->  b) convection currents

c) conveyor belts

d) gravity

harry hess suggested that old oceanic crust was

-->  a) recycled back into the mantle.

b) adhered onto the continents.

c) created at mid-ocean ridges.

d) the source of the flat-topped guyots.

the rock made underwater from cooled lava is basalt.

-->  a) true

b) false

how do continents move?

a) seafloor spreading creates new seafloor, which grows up to be new continents that move away from the ridge.

-->  b) convection currents create seafloor spreading, which pushes the lithospheric plate and

c) eruptions of lava at deep-sea trenches create continents, which move across the seafloor.

d) none of these.

diagram questions

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