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Seafloor Spreading
Fill in your student handout
Review of continental drift
Continents had once
been joined to form a
single supercontinent
Continental puzzles
Matching fossils
• different landmasses
• Tropical fossils found in Arctic
Rock Types
• Mountains separated by ocean
• Coal fields
Ancient Climates
• Glacial deposits in present mild
Could NOT explain what caused
the landmasses to move or how
they moved (mechanism).
Seafloor Spreading
Mapping the Ocean floor
 New
technology (~1950s)
 Sonar -uses sound waves to
calculate the distances to an object
– mid-ocean ridges stretch along the center
of much of the Earth’s ocean floor
– Deep-ocean trenches: along the edge of
some ocean basins (deepest parts; Mariana
Trench is 11 km deep)
Mid-Ocean Ridges
• longest feature on Earth, winds more than 70,000 km
through all the major ocean basins
• 2-3 km above ocean basins
•1,000 to 4,000 km wide
Iceland: mid-ocean above the
Process of Seafloor spreading at
mid-ocean ridge
Hypothesis of seafloor spreading: Hess
Rift or split in the crust
Molten rock forced up into the rift (its
density is less than surrounding rock)
Flows sideways, carrying seafloor away
from the ridge in both directions
4. Magma cools and becomes solid – new
5. New sea floor moves away from the ridge
*cools, contracts and becomes denser
6. denser, colder seafloor sinks helping to form
the ridge
7. Subduction: old ocean floor sinks beneath
the trench and returns to the mantle
Evidence for
seafloor spreading
1. Mid-ocean ridges
2. Age of the seafloor
drilling rig drilled into the seafloor to
obtain rock samples (1968 Glomar
– youngest rocks located at the mid-ocean
– ages of rocks become increasing older
farther from the ridges
Age of sea floor as measured by fossils
- Older as one moves away from ridges
- Youngest rock is next to the ridge
3. Magnetic field reversals
Using a magnetometer (towed behind a
– detects magnetic fields in iron-bearing
minerals found in the rocks of the seafloor
DISCOVERY: many periods of magnetic
reversals in strips parallel to the midocean ridges.
Magnetic field reversal
• Proves the Earth magnetic field reverses itself
every 27000 years
Why are magnetic reversals
 showed
that new rock was being
formed at the mid-ocean ridges
 explained
how the crust move
Missing link
Seafloor spreading was the missing link
Wegener needed to complete his model of
continental drift.
do the continents move?
(we will soon learn what causes the continents to
Pillow Lava rocks