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
Main Idea: Oceanic crust forms at ocean ridges and becomes
part of the seafloor
Mapping the Ocean Floor
Until mid-1900s, many thought ocean floors
were flat and ocean crust unchanging & older
than continental crust
 Advances in technology during 40s & 50s showed these
widely accepted ideas to be false
Magnetometer: a device that can detect small changes in
magnetic fields
 Originally developed to detect magnetic fields generated by
the steel hulls of submarines
 Modifications were made to measure the magnetic field
strength of ocean-floor rocks
Other technology
Developments in sonar technology
 Enabled scientists to measure water depth and map the
topography of ocean floor
Ocean-Floor Topography
• Using maps from sonar & magnetometer data
 Scientists discovered vast mountain chains called ocean ridges that
run along ocean floors around Earth much like seams on a baseball
•
•
more than 80,000 km long and up to 3 km above ocean floor!
Volcanoes quite common along these ridges
 Sonar also revealed deep-sea trenches
•
•
Narrow, elongated depression in seafloor
Deepest trench, Marianas Trench, in Pacific Ocean that is more than 11 km
deep…..Mount Everest stands 9 km!
 How could they have formed?
 What’s the source of volcanism associated w/ these mountains?
 What forces could depress Earth’s crust to such depths?
Ocean Rocks & Sediments
Ages of rock that make up seafloor vary, but
these variations are predictable!
 Age of oceanic crust consistently increases with distance from
ridge
What was weird was that scientists also discovered even the
oldest parts of seafloor are geologically young—around 180
million years
• Why are ocean-floor rocks so young compared to continental
rocks? Some of which are at least 3.8 billion years old
Ocean Rocks & Sediments
Ocean floor sediments are typically a few
hundred meters thick
 This seems odd when you consider that large areas of
continents are blanketed with sedimentary rocks that are as
much as 20 km thick….
Scientists found that thickness of sediments increases
with distance from the ocean ridge
Magnetism
Earth’s magnetic field
 Generated by the flow of molten iron in outer core
 Causes compass needle to point North
Magnetic Reversal
 Happens when flow in outer core changes
 Have occurred many times in Earth’s history
Magnetic Polarity Time Scale
Paleomagnetism: study of the history of Earth’s
magnetic field
How can we study Earth’s magnetic field past?
 When lava solidifies, iron-bearing minerals such as magnetite
crystallize. As they crystallize, they behave like tiny
compasses and align with Earth’s magnetic field
 So if we sift through such minerals, we are able to tell the
polarity of the Earth at the time of crystallization
 This phenomenon allowed scientists to construct a magnetic
polarity time scale
Magnetic Symmetry
• Regions of normal & reverse polarity form a series of
strips across the ocean floor parallel to ocean ridges
 Originally hypothesized because we knew oceanic crust is mostly basaltic
rock, containing large amounts of iron-bearing minerals. Therefore, rocks
on the ocean floor should show a record of magnetic reversals
• Ages & widths of the strips match from one side of
the ridges to the other
Magnetic Symmetry
• By matching magnetic patterns on seafloor with the known
pattern of magnetic reversals on land, we could determine the
age of ocean floor from magnetic recording and create
isochron maps of the ocean floor
Isochron: imaginary line on a map that shows points that have
the same age—that is, they formed at the same time
Seafloor Spreading
Theory that explains how new ocean crust is
formed at ocean ridges and destroyed at deepsea trenches
http://earthguide.ucsd.edu/eoc/teachers/t_tectonics/p_seafloor
spreading.html
 Magma is forced upward toward surface of the crust along ocean ridge.
As the 2 sides of the ridge spread apart, the rising magma fills the gap that
is created. When the magma cools and solidifies, a small amount of new
ocean floor is added to Earth’s surface
 As spreading along ocean ridge continues, more magma is forced upward
and solidifies
 Cycle of spreading and intrusion of magma continues the formation of
ocean floor, which slowly moves away from the ridge
Seafloor Spreading
Links to Wegener
• Seafloor spreading was the missing link
 Continents are not passing through ocean crust. In
fact, continents are more like passengers that ride
along while ocean crust slowly moves away from
ocean ridges!
https://www.youtube.com/watch?v=6CsTTmvX6mc