Name _______________________________ Date _______________________ Hour ____________
Sea-floor Spreading
In the 1960's scientists finally found what was missing in Wegener's theory. Geologist Harry
Hess finally found evidence that Wegener was right when mountains were discovered at the
bottom of the ocean. Hess proposed his theory of sea-floor spreading as the answer to the
mystery of continental drift.
Discovery of Underwater Mountains
 The mid-ocean ridge is found throughout the oceans of the earth. This is a 50,000 mile
chain of volcanic mountains in the middle of the ocean.
 These mountains have a deep crack that runs through them. It is called a rift valley.
 Sea-floor spreading happens here as molten magma from the mantle oozes up and
forms new ocean floor spreading the plates apart. It is a slow, regular process. There are
no explosive bursts like volcanic eruptions on land.
Evidence for Sea-floor Spreading
 The mid-ocean ridges occur along plate boundaries that are separating (diverging).
 Rocks that form from molten rock, called "Pillow Lava" were discovered along the midocean ridge by the submersible submarine named Alvin.
 Core samples drilled and collected from the Atlantic ocean floor showed that the
sediment was thinner near the mid-ocean ridges and the age of the rock was younger
meaning the ocean floor was younger at the mid-ocean ridge.
Core Sample Model
Name _______________________________ Date _______________________ Hour ____________
 Those core samples also showed a record of magnetic reversals. The rocks particles
were aligned differently as you moved out away from the mid-ocean ridge on either side
showing a pattern of changes.
1. Scientists used to believe that the ocean floor was flat with nothing but sediments but
now they know that there is a large chain of mountains that extend throughout the oceans
known as _________________________________.
2. Harry Hess proposed that new material rises from the mantle and is added to the ocean
floor along the mid-ocean ridge. He called his theory _______________________.
3. The evidence that supported Hess's ideas came from the:
discovery of _____________ _________ "__________ _________" erupting at the ridge
record of ______________ __________________ in the rocks on the ocean floor and
proof that the ocean rocks get ____________ as you move farther away from the midocean ridge.
4. How did the theory of Sea-floor spreading help to revive the theory of Continental Drift?
5. If new oceanic crust is being formed at the mid-ocean ridge then the Earth should be
getting larger but it does not because there are also areas called deep-ocean __________
where the ocean floor is sinking back into the mantle and being melted and recycled.
6. The process of the oceanic plates sinking back down into the mantle and melting is called
____________________________.
Name _______________________________ Date _______________________ Hour ____________
ANSWER KEY
1. Scientists used to believe that the ocean floor was flat with
nothing but sediments but now they know that there is a large chain
of mountains that extend throughout the oceans known as
MID-OCEAN RIDGE.
2. Harry Hess proposed that new material rises from the mantle and
is added to the ocean floor along the mid-ocean ridge. He called his
theory SEA-FLOOR SPREADING.
3. The evidence that supported Hess's ideas came from the:
discovery of MOLTEN ROCK "PILLOW LAVA" erupting at the ridge
record of MAGNETIC REVERSALS in the rocks on the ocean floor and
proof that the ocean rocks get OLDER as you move farther away
from the mid-ocean ridge.
4. How did the theory of Sea-floor spreading help to revive the
theory of Continental Drift?
SEA-FLOOR SPREADING GAVE EVIDENCE FOR THE MOVEMENT OF
THE CONTINENTS. ONLY THE EVIDENCE NOW SHOWED THE
MOVEMENT OF THE ENTIRE CRUST OF THE EARTH.
5. If new oceanic crust is being formed at the mid-ocean ridge then
the Earth should be getting larger but it does not because there are
also areas called deep-ocean TRENCHES where the ocean floor is
sinking back into the mantle and being melted and recycled.
6. The process of the oceanic plates sinking back down into the
mantle and melting is called SUBDUCTION.