Notes of Sea floor speading

1. Sea-Floor Spreading

2. Mapping the Mid-Ocean
Ridge
 The East Pacific Rise is just one part of the mid-
ocean ridge, the longest chain of mountains in the
world.
 The mid-ocean ridge curves like the seams of a
baseball along the sea floor, extending into all of
Earth’s oceans.
 Most of the mountains in the mid-ocean ridge lie
hidden under hundreds of meters of water.

3.  A steep-sided valley almost twice as deep at the
Grand Canyon, splits the top of the mid-ocean ridge
for most of its length.
 Scientists used sonar to map the mid-ocean ridge
and made them curious as to how it got there.

4. Evidence for Sea-Floor
Spreading
 Harry Hess, an American geologist, was one of the
scientists who studied the mid-ocean ridge.
 He studied the mid-ocean ridge and began to think
about the ocean floor in relation to the problem of
continental drift.
 When looking at the ocean floor, he realized that
maybe Alfred Wegener’s theory of continental drift
was in fact, true!

5.  Hess suggested in 1960 that the ocean floors move
like conveyor belts, carrying continents along with
them.
 This movement begins at the mid-ocean ridge,
which forms along a crack in the oceanic crust.
 At the mid-ocean ridge, molten material rises from
the mantle and erupts.

6.  The molten material then spreads out, pushing
older rock to both sides of the ridge.
 As the molten material cools, it forms a strip of solid
rock in the center of the ridge and then more molten
material flows into the crack.
 This material splits apart the strip of solid rock that
formed before, pushing it aside.

7.  Hess called the process that continually adds new
material to the ocean floor sea floor spreading.
 http://emvc.geol.ucsb.edu/forteachers/flashmovie
s/Spreading.swf


8.  Several types of evidence from the oceans
supported Hess’s theory of sea floor spreading:
1. Evidence from molten material
2. Evidence from magnetic stripes
3. Evidence from drilling samples

9. Subduction at Deep-
Ocean Trenches
 Hess wondered how can the ocean floor keep
getting wider and wider without the Earth getting
larger?
 The ocean floor plunges into the deep under water
canyons called deep-ocean trenches.
 A deep-ocean trench forms where the oceanic crust
bends downward.

11.  Where there are deep-ocean trenches, a process
called subduction takes place.
 Subduction is the process by which the ocean floor
sinks beneath a deep-ocean trench and back into the
mantle to be recycled.
 Convection currents under the lithosphere push
new crust that forms at the mid-ocean ridge away
from the ridge and toward a deep-ocean trench.

12.  New oceanic crust is hot but as it moves away from
the mid-ocean ridge it cools and becomes denser.
 Eventually gravity pulls this older, denser oceanic
crust down beneath the trench.
 At deep-ocean trenches, subduction allows part of
the ocean floor to sink back into the mantle under
the continental crust.

13. Check These Out!!
 http://www.classzone.com/books/earth_science/ter
c/content/visualizations/es0902/es0902page01.cfm?
chapter_no=visualization
 http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/
p_subduction.html

14. Review
1. What is the role of the mid ocean ridge in sea-floor
spreading?
2. What is the evidence for se-floor spreading?
3. Draw a diagram of the processes of sea-floor
spreading and subduction. Label the trench, mid-
ocean ridge, oceanic crust, continental crust and
indicate with arrows the direction of sea-floor
spreading and subduction.