Table of Contents
Chapter 7: Plate Tectonics
Section 2: Seafloor Spreading
Seafloor Spreading
A. Mapping the Ocean Floor
1. If you were to lower a rope from a boat until
it reached the seafloor, you could record the
depth of the ocean at that particular point.
2. This is exactly how it was done until
German scientists discovered how to use
sound waves to locate submarines during
World War I.
Seafloor Spreading
A. Mapping the Ocean Floor
3. Sound waves echo off the ocean bottom—the
longer the sound waves take to return to the
ship, the deeper the water is.
4. Using sound waves,
researchers discovered
an underwater system
of ridges, or
mountains, and valleys
like those found on the
continents.
Seafloor Spreading
A. Mapping the Ocean Floor
5. In some of these ridges are long rift valleys.
These rift valleys are like rips in the ocean
floor. Volcanic eruptions and earthquakes
occur in the rift valleys from time to time.
6. Underwater volcanic eruptions create
underwater mountains. When these
mountains push out of the water, they create
island.
7. In the Atlantic, the Pacific, and in other
oceans around the world, a system of ridges,
called the mid-ocean ridges, is present.
Seafloor Spreading
B The Seafloor Moves
1. In the early 1960s, Princeton University
scientist Harry Hess suggested an explanation
on the ocean floor movement. His nowfamous theory is known as seafloor
spreading.
2. Hess proposed that hot, less dense material
below Earth’s crust rises toward the surface
at the mid-ocean ridges. Then, it flows
sideways, carrying the seafloor away from
the ridge in both directions.
Seafloor Spreading
B. The Seafloor Moves
3. As the seafloor spreads apart at a mid-ocean
ridge, new seafloor is created.
4. The older
seafloor
moves
away from
the ridge
in
opposite
directions.
Seafloor Spreading
C. Evidence for Spreading
1. In 1968, scientists aboard the research ship
Glomar Challenger began gathering
information about the rocks on the seafloor.
2. Scientists found that the youngest rocks are
located at the mid-ocean ridges. The ages of
rocks become increasingly older in samples
obtained farther from the ridges, adding to
the evidence for seafloor spreading.
Seafloor Spreading
C. Evidence for Spreading
3. As molten material is forced upward along the
ridges, it brings heat and chemicals that support
exotic life-forms in deep, ocean water.
4. The giant clams,
mussels, and tube
worms, get heat and
chemicals from magma
pouring out of rifts in
mid-ocean ridges.
Seafloor Spreading
D. Magnetic Clues
1. Earth’s magnetic field has a north pole and
south pole.
2. Invisible lines of magnetic force leave
Earth near the south pole and enter the
north pole. At this time, Earth’s magnetic
field travels from south to north.
3. However, the lines of magnetic force
traveled in the opposite direction
sometimes.
Seafloor Spreading
D. Magnetic Clues
4.These direction changes are called magnetic
reversals . During a magnetic reversal, the
lines of magnetic force run the opposite way.
5. Scientists have determined that Earth’s
magnetic field has reversed itself many
times in the past.
6.The reversals are recorded in rocks forming
along mid-ocean ridges.
Seafloor Spreading
E. Magnetic Time Scale
1. Whenever Earth’s magnetic field reverses,
newly forming iron minerals will record the
magnetic reversal.
2. Using a sensing device called a
magnetometer (mag nuh TAH muh tur) to
detect magnetic fields, scientists found that
rocks on the ocean floor show many periods
of magnetic reversal.
Seafloor Spreading
E. Magnetic Time Scale
3. A strong magnetic
reading is recorded
when the polarity of a
rock is the same as
the polarity of Earth’s
magnetic field.
4. Look at the picture. Normal polarities in rocks
show up as large peaks. After the magnetic
reversal, the magnetometer records a weak
reading.
Seafloor Spreading
Magnetic Time Scale
5. Over time, the reversals are shown in strips
parallel to mid-ocean ridges. Changes in Earth’s
magnetic field can be seen on both sides of midocean ridge.
6.This discovery provided strong support that
seafloor spreading was indeed occurring.
7. This helped explain how the crust could
move—something that the continental drift
hypothesis could not do.
Section Check
2
Question 1
What is seafloor spreading?
Section Check
2
Question 2
What method of mapping the ocean floor was
developed by scientists in the 1940s and 1950s?
Section Check
2
Question 3
What was the significance of the discovery that
the magnetic alignment in rocks on the ocean
floor reverses back and forth in strips parallel to
mid-ocean ridges?
Section Check
2
Q1. Answer
Seafloor spreading is the process in which hot,
less dense material below Earth’s crust rises
toward the surface at the mid-ocean ridges.
This material flows sideways and carries the
seafloor away from the ridge in both directions.
Section Check
2
Q 2. Answer
In the 1940s and 1950s, scientists began to use
sound waves echoing off the ocean bottom to
map large areas of the ocean floor in detail.
Section Check
2
Q3. Answer
The magnetic reversals
showed that new rock
was being formed at the
mid-ocean ridges, and
helped explain how the
crust could move.
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