Sea Floor Spreading
The Mid-ocean Ridge
Vocabulary
•Mid-ocean ridge
•Sonar
•Sea-floor spreading
•Deep-ocean trench
•Subduction
60 years ago, in the
1950’s, Sonar was
invented.
Bats use it!
So do dolphins!
So Does
Aquaman!
How did sonar bring Wegner’s
theory back to life?
Sonar made it possible
for us to observe the
ocean floor.
Scientists observed a large
mountain chain running
down the center of the
Atlantic ocean.
Scientists found that the
mountain range connected with
others and encircled the globe!
Mapping the Mid-ocean
Ridges
• Ridges connect and curve around
the globe like a baseball
• Ridges lie hundreds of meters below
the surface of the ocean
• Most ridges are underwater
• Section of ridge that is on land is
East African Rift Valley
The discovery and study of the midocean ridges led to the theory
of sea floor spreading
Sea-Floor Spreading
• Harry
Hess
No! Harry Hess, not
Harry Potter!
• An American
geologist who
studied mid-ocean
ridges.
• He suggested that
the ocean floors
move like conveyor
belts, carrying the
continents along
with them.
• Seafloor spreading- new ocean crust is
formed at ocean ridges and destroyed at
deep sea trenches
– Magma forced toward crust
– Fills gaps and hardens
– Forms new ocean floor
• Divergent boundary– Moving apart
– Most are found on seafloor
– Form ocean ridges
– When on continent, form rift valley
Pillow Lava forms when
magma cools in water. This is
evidence of volcanic activity!
• Magnetic reversal- change in earth’s
magnetic field
– Same as present field- normal polarity
– Opposite to present- reversed
This shows you the age of the sea floor. The
red areas are new rocks and the blue areas
are the oldest oceanic rock.
Iceland is spreading
open on the MidAtlantic Ridge!
Can you see where the Mid-Atlantic Ridge bisects Iceland?
This is a rift valley!
Sea Floor Spreading
• Sonar
• Mid-Ocean Ridge
• Ocean is spreading open!!!
•
•
•
•
Pillow Lava
Symmetrical Magnetic Stripes
Symmetrical Age Patterns
Newest Rock at the Ridge
If new crust is being added to
Earth’s surface, is Earth
getting bigger?
• Convergent boundary–
–
–
–
–
Moving toward each other
Classified according to type of crust involved
Oceanic crust mostly basalt
Continental crust granite and sedimentary rock
Oceanic-oceanic- subduction occurs (one plate
descends) creates deep sea trench, forms arc of
volcanic islands: Mariana trench and islands
Subduction is a process
where the ocean floor sinks back
into the mantle at a deep ocean
trench.
• Ridge push- forces in mantle cause
asthenoshere to rise
– Pushes oceanic plate toward trench
– Could create drag on lithosphere
• Slab pull- sinking region of mantle
convection sucks oceanic plate downward