Transcript Sea-floor spreading PowerPoint

Sea-Floor Spreading
Chapter 1, Section 4
Pages 33-39
The Mid-Ocean Ridge
The longest chain of mountains in the
world
 Extends into all of Earth’s Oceans
 Most of the mountains lie under the ocean
 In the mid 1900’s, scientists used sonar to
map it

Evidence for Sea-Floor Spreading

Harry Hess proposed the following in
1960:
 At the mid-ocean ridge, molten
material rises from the mantle and
erupts. The molten material then
spreads out, pushing older rock to
both sides of the ridge.
Evidence 1

Molten Material
 Alvin (A submarine) and its crew found
molten material that had quickly
hardened after erupting under water
Evidence 2
Magnetic Stripes
Reversal of magnetic field causes iron in ocean floor to
line up in the direction of Earth’s magnetic poles
When the poles are reversed, the iron faces the
opposite direction causing “stripes” in the ocean floor
Evidence 3

Drilling Samples
 The Glomar Challenger, a drilling ship in
1968 gathered samples of the ocean
floor
 The age of the rock from the samples
were older the farther away they got
from the Mid-ocean ridge.
Subduction at Deep-Ocean Trenches
A
deep-ocean trench forms where the
oceanic crust bends downwards.
 Subduction is the process by which the
ocean floor sinks beneath a deepocean trench and back into the mantle
Subduction-cntd.
Convection currents push new crust at the
M.O.R. away from the ridge and towards
trenches
 New crust is hot, but it cools and becomes
more dense, getting pulled below the
trench
 At deep-ocean trenches, subduction
allows part of the ocean floor to sink
back into the mantle, over tens of
millions of years

Subduction and Earth’s Oceans
Pacific Ocean- it is shrinking because
subduction is happening slower than new
crust is being added
 Atlantic Ocean- it expanding because new
crust is being faster than subduction
