I can: describe what happens during Sea

Floor SpreadingDO NOW: What is a Mid-ocean 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 SpreadingSea-Floor SpreadingSonar - 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 dolphinsSo do dolphins!

Sonar brought WegnerSonar brought Wegner’’s s theory back to life!theory back to life!

Mid-Ocean RidgeMid-Ocean Ridge

Mid-Ocean RidgeMid-Ocean Ridge – the longest – the longest chain of mountains in the world---chain of mountains in the world---these are these are divergent divergent plate plate boundaries (seafloor moves apart.boundaries (seafloor moves apart.

Harry Hess : Harry Hess : American GeologistAmerican Geologist

No! Harry HessHarry 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 spreadingDO 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 EvidenceSea Floor Spreading Evidence

1. Age of rocks at the mid-ocean ridge2. Magnetic clues in rocks

Evidence 1: Age of Rocks• Scientists have collected rock samples at the mid-

ocean ridge• Youngest rocks located at the ridges.• Rocks are increasingly older as you move farther

from the ridge Older rocks

Older rocks

Magma

How is this Possible? Propose a mechanism (the way in which this happens)

Older rocks

Older rocks

Magma

Evidence 1: Age of Rocks

I can: Explain the evidence for sea floor spreadingDO 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.

1. Magma rises up to the ridge2. Rock layers near the ridge are pushed

away3. Molten rock cools down creating new sea

floorOlder rocks

Older rocks

Magma

Sea Floor Spreading: Mechanism

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.

This shows you the age of the sea floor. The This shows you the age of the sea floor. The red red areas are new rocksareas are new rocks and the and the blue areas are the blue areas are the oldest oceanic rockoldest oceanic rock..

Evidence 2: Magnetic Clues

A magnet has a north end and a south end with a magnetic field—Invisible lines of magnetic force

-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)

Evidence 2: Magnetic Clues

Magnetosphere-magnetic field that protects the earth

Solar wind-high energy particles from the sun that flow into the space

-Earths magnetic force field can reverse (Magnetic reversal)

-Happens every 200-300 thousand years

North North

SouthSouth

Normal magnetic polaritySouth to north

Reversed magnetic polarityNorth to 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.

Evidence 2: Magnetic Clues

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 Iceland is spreading open on the Mid-open on the Mid-Atlantic Ridge!Atlantic Ridge!

Can you see where the Mid-Atlantic Ridge bisects Iceland? This is a rift valley!

Sea Floor SpreadingSea Floor SpreadingEvidenceEvidence

•Age of rocks at the mid-ocean ridge

•Magnetic stripes

If new crust is being added to If new crust is being added to EarthEarth’’s surface, is Earth s surface, is Earth

getting bigger?getting bigger?

Subduction is a process where the ocean floor sinks back into the mantle at a deep ocean

trench.