The document discusses sea floor spreading and the evidence that supports this theory. It describes how molten material rises up from the mantle at mid-ocean ridges, cools and forms new crust that constantly pushes older crust outward. It spreads until the dense oceanic crust sinks back into the mantle at deep ocean trenches in the process of subduction, which balances out the growth at ridges. Key evidence includes magnetic stripes in the crust, drilling samples, and the overall structure of ridges and trenches.

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.