SEA FLOOR SPREADING
3.2
WHAT ARE MID-OCEAN RIDGES(MOR)?
• Baseball seams - curved along
the ocean floors
• Seams form mountain
ranges: MOR
• MORs form long chains of
mountains that rise up
from the ocean floor
WHAT ARE MORS?
• Mapped MORs using sonar
• Sounds waves that measure
distance
• Steep sided valleys slip the
tops of some MORs
• Longest mountain ranges on
earth
• Rocky Mountains
• Andes Mountains
WHAT IS SEA FLOOR SPREADING(SFS)?
• MORs continually add new material to the
ocean floor = sea floor spreading
• Sea floor spreading begins at MORs – crack
in the crust
• New molten material:
• Rises up
• Erupts
• Cools off
• Hardens to form a solid strip of rock
WHAT IS SFS?
• SFS adds more crust to
the ocean floor.
• At the same time, older
strips of rock move
outward from either side
of the ridge.
SFS EVIDENCE
• Evidence from Ocean Material
• Pillow rocks = only form when molten material harden
quickly – underwater eruption
• Evidence from Magnetic Stripes
• Magnetic minerals align with the poles as the rock cools
• Form unseen magnetic stripes
• Patterns mirror each other on opposite sides of the ridge
• Evidence from Drilling Samples
• Further from the ridge = older the rock
• Youngest rock is at the center of the ridges
• Rock’s age shows SFS is happening
WHAT HAPPENS AT DEEP-OCEAN
TRENCHES(DOT)?
• Ocean floor keep getting
wider without stopping?
• Ocean floor plunges into
deep underwater canyons
= DOT
• In a process taking tens of
millions of years, part of
the ocean floor sinks back
into the mantle at DOTs.
PROCESS OF SUBDUCTION
Washcloth analogy
More dense = sinks – same with ocean floor
New crust = hot, older = cools
As it cools, it becomes more dense
Collide with continental crust = dense crust is
pulled down into the mantle
• Subduction: process by which the ocean floor
sinks beneath a DOT and back into the mantle
again
• SFS and subduction = giant conveyor belt
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SUBDUCTION AND EARTH’S OCEANS
• SFS and subduction = change the size and shape
of oceans
• Ocean floor is new every 200 million years
• New rock – MOR to a DOT
• Size of oceans = speed of this process
• Many DOTs = ocean may shrink
• Atlantic ocean is expanding – why?