Download 4. Seafloor Spreading Notes

Earth Science Standard
3.a - Students know
features of the ocean
floor (magnetic
patterns, age, and seafloor topography)
provide evidence of
plate tectonics.
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Matching Coastlines
Similar Rock Types
Similar Fossils
Climatic Change
• Proof for how the plates moved (Hess)
• new oceanic crust is created at the ridges and
recycled at the trenches
• Evidence found at the bottom of the oceans
Ridge
Trench
• Mid-Ocean ridge – underwater mountain
system where the crust is lifted and pulled apart
• Trenches – narrow, steep sided depression that
forms as one type of crust slides underneath
another
• Continental slope – transition from continental
crust to oceanic crust
• Continental shelf – submerged border of a
continent
• Ocean ridges are typically found in the ocean
• Trenches are found near the coastlines
(continents)
1. Magma rises up to the ocean floor causing
ocean crust to be pushed apart (RIDGE PUSH)
2. The magma rushes up and forms new crust.
3. Old crust pushed toward the trenches
(continents).
4. At the trenches, cooler material pulls oceanic
crust down into the mantle (SLAB PULL)
5. This subducted crust (crust that is pulled
underneath another type of crust) is then
recycled in the mantle.
• Convection Currents in the mantle – Hot material
rises and cooler material sinks
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Age of Rock
Rock at ridges = Youngest
Rock at Trenches = Oldest
Rock gets older in a predictable way
Age of the Sea Floor
2. Paleomagnetism
• When rock with iron/magnetite forms (cools),
it traps a record of Earth’s magnetic field
• Earth’s magnetic field has reversed many times
since its creation
• Scientists discovered
that the rock that makes
up the ocean floor lies in
a pattern of magnetized
“stripes”.
• They hold a record of
reversals in Earth’s
magnetic field.
• Each mid-ocean ridge is
lined by mirror-image
patterns of magnetic
field reversals.
Normal Polarity (+) = Today’s Polarity (N)
Reversed Polarity (-) = South as North
How is this evidence?
• It shows that new seafloor is constantly being
created.
• As new seafloor is formed, it slowly pushes the
older seafloor towards the continents (trenches)
where crust is recycled into the mantle or
crumbled up to become part of the continents.
• Include Convection Currents!
• Label the youngest rocks on the seafloor and the
oldest!