Download Seafloor Spreading

Section 17.2
Seafloor Spreading
Objectives
Summarize the evidence that led to the
discovery of seafloor spreading.
Explain the significance of magnetic patterns
on the seafloor.
Explain the process of seafloor spreading.
Section 17.2
Seafloor Spreading
Mapping the Ocean Floor
Until the mid-1900s, many scientists thought
that the ocean floors were essentially flat
and that oceanic crust was unchanging
and was much older than continental crust.
Advances in technology during the 1940s
and 1950s showed that all of these widely
accepted ideas were incorrect.
Section 17.2
Seafloor Spreading
How do we know what is on the ocean
floor?
We have mapped the magnetic fields and the
topography of the ocean floor using sonar and
magnetometers.
Section 17.2
Seafloor Spreading
Tools to Map the Ocean Floor
1. magnetometer
can detect small changes in magnetic
fields.generated by ocean floor rocks.
2. sonar
can measure water depth and map
the topography of the ocean floor.
Section 17.2
Seafloor Spreading
Ocean-Floor Topography
These maps showed underwater mountain
chains (ridges) and deep-sea trenches.
Section 17.2
Seafloor Spreading
Ocean-Floor Topography
The deepest trench, the Marianas Trench, is
more than 11 km deep. Mount Everest, the
world’s tallest mountain, stands at 9 km
above sea level, and could fit inside the
Marianas Trench with six Empire State
buildings stacked on top.
Section 17.2
Seafloor Spreading
What evidence did these tools give us?
Concept 1. The age of oceanic crust
increases with distance from a ridge.
Proof: The sediment is thicker the closer
you are to the continents.
Section 17.2
Seafloor Spreading
What evidence did these tools give us?
Concept 2: Magnetic north has changed
over time.
Proof: The magnetic record is in the
rocks! The study of this is called
Paleomagnetism.
Section 17.2
Seafloor Spreading
Magnetism
Earth has a magnetic field generated by the
flow of molten iron in the outer core. This field
is what causes a compass needle to point to
the North.
A magnetic reversal happens when the flow
in the outer core changes, and Earth’s
magnetic field changes direction.
Section 17.2
Seafloor Spreading
Paleomagnetism
What happens?
1. Lava comes up through the mid-ocean
ridge.
2. Lava cools and the iron-bearing
minerals crystallize.
3. As they crystallize, these minerals
behave like tiny compasses and align
with Earth’s magnetic field.
Section 17.2
Seafloor Spreading
Magnetism
What does this look like on the ocean floor?
Section 17.2
Seafloor Spreading
Magnetic symmetry
By matching the magnetic patterns on the
seafloor with the known pattern of magnetic
reversals on land, scientists were able to
determine the age of the ocean floor from
magnetic recording and to create maps of
the ocean floor.
Section 17.2
Seafloor Spreading
Seafloor Spreading
Seafloor spreading is the theory that
explains how new ocean crust is formed
at ocean ridges and destroyed at deep-sea
trenches.
Section 17.2
Seafloor Spreading
How Seafloor Spreading Occurs
1. Magma is hotter and less dense than
surrounding mantle material
2. It is forced toward the surface of the crust
along an ocean ridge.
3. the two sides of the ridge spread apart, the
rising magma fills the gap that is created.
4. When the magma solidifies, new ocean floor
is added to Earth’s surface.
Section 17.2
Seafloor Spreading
Result of Seafloor Spreading
The cycle of spreading and the intrusion of
magma continues the formation of ocean floor,
The newest crust is by the mid-ocean ridge.
The oldest crust is by the continents.
Activity
• Get a partner – sit in two desks next to each
other.
• Practice pulling the two pieces of paper out
of the “ridge” at the same SLOW speed.
• Get one colored marker.
Answer the questions based on the
activity and your notes.