I can: describe what
happens during Sea
Floor Spreading
DO NOW: What is a Midocean Ridge?
Vocabulary
•Sonar
•Sea-floor spreading
•Deep-ocean trench
•Subduction
•Mid-ocean ridge
60 years ago, in the
1950’s, Sonar was
invented.
Sea-Floor Spreading
Sonar - a device
that bounces
sound waves off
under-water
objects and then
records the
echoes of these
sound waves. The
time it takes for
the echo to arrive
indicates the
distance to the
object.
Bats use it!
So do dolphins!
Sonar brought Wegner’s
theory back to life!
Mid-Ocean Ridge
Mid-Ocean Ridge – the longest chain of
mountains in the world---these are
divergent plate boundaries (seafloor
moves apart.
Harry Hess :
American Geologist
No! Harry Hess, not
Harry Potter!
Sea-Floor Spreading
• Harry
Hess
•1960
• An American
geologist who
studied mid-ocean
ridges.
• He suggested that
the ocean floors
move like conveyor
belts, carrying the
continents along
with them.
Sea floor spreads as if there are 2
conveyor belts on either side
I can: Explain the evidence
for sea floor spreading
DO NOW:
1. What is subduction?
2. What is a deep-ocean trench?
Subduction is a process where the
ocean floor sinks back into the mantle at
a deep ocean trench.
Sea floor spreads as if there are 2
conveyor belts on either side
VERY ACTIVE
Pillow Rock forms when
magma cools in water. This is
evidence of volcanic activity!
Think-Pair-Share:
How can we know for sure that the sea
floor is actually spreading?
Sea Floor Spreading Evidence
1.
2.
Age of rocks at the mid-ocean ridge
Magnetic clues in rocks
Evidence 1: Age of Rocks
• Scientists have collected rock samples at the midocean ridge
• Youngest rocks located at the ridges.
• Rocks are increasingly older as you move farther
from the ridge
Older
Older
rocks
rocks
Magma
Evidence 1: Age of Rocks
How is this Possible?
Propose a mechanism (the way in which this
happens)
Older
rocks
Older
rocks
Magma
I can: Explain the evidence
for sea floor spreading
DO NOW:
1. What information from rocks at
the mid-ocean ridge gives evidence
for sea-floor spreading?
2. How is this possible? Describe the
mechanism.
Sea Floor Spreading: Mechanism
1. Magma rises up to the ridge
2. Rock layers near the ridge are pushed
away
3. Molten rock cools down creating new sea
floor
Older
Older
rocks
rocks
Magma
This shows you the age of the sea floor. The red
areas are new rocks and the blue areas are the
oldest oceanic rock.
DO NOW:
1. What information from rocks at
the mid-ocean ridge gives evidence
for sea-floor spreading?
2. How is this possible? Describe the
mechanism.
Evidence 2: Magnetic Clues
A magnet has a north end and a south end with
a magnetic field—Invisible lines of magnetic
force
Evidence 2: Magnetic Clues
-The Earth is a giant magnet
-Compass needle points to the north pole
(following the magnetic field)
-Magnetic field protects Earth from solar
wind (high energy particles form the sun)
Magnetosphere
-magnetic field
that protects the
earth
Solar wind
-high energy
particles from the
sun that flow into
the space
Evidence 2: Magnetic Clues
-Earths magnetic force field can reverse (Magnetic
reversal)
-Happens every 200-300 thousand years
Normal magnetic polarity
South to north
Reversed magnetic polarity
North to South
North
North
South
South
Evidence 2: Magnetic Clues
- Molten rock contains magnetic minerals and
iron that line up with Earth’s magnetic field
- Earth’s magnetic field is recorded in the rock
when it cools down
Evidence 2: Magnetic Clues
What would the magnetic field recorded in
these rocks look like if the sea-floor is
spreading?
-We know that the Earth’s magnetic field reverses
every 200-300 thousand years
-This magnetic field is recorded in the rocks as
they move and the sea-floor spreads
Draw a diagram of the sea floor to show this.
What would the magnetic field
recorded in these rocks look like if the
sea-floor is spreading?
Normal magnetic
polarity
Reversed magnetic
polarity
Evidence 2: Magnetic Clues
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
Evidence
• Age of rocks at the
mid-ocean ridge
• Magnetic stripes
If new crust is being added to
Earth’s surface, is Earth
getting bigger?
Subduction is a process
where the ocean floor sinks back
into the mantle at a deep ocean
trench.