Seafloor Spreading

Topography Review:Ocean Floor Models

Activity Instructions1. Make marks 1 cm apart up the side of the box starting

at the bottom using the marking pencil.2. Label the elevations 0 cm, 1 cm, 2 cm, etc.3. Carefully fill the container to the 1 cm line with water.4. Use the marking pencil to draw a line on the plastic

model following points of contact with the water exactly.

5. Fill the box to the 2 cm line and repeat step #4. Continue to do this until the model is completely submerged.

6. Put the clear lid on top of the container and carefully trace the lines from the model on to the lid labeling each line with its elevation (0 cm, 1 cm, 2 cm, etc.).

Analysis QuestionsCopy the questions on your paper and answer them.

1. Describe the general shape of the contour lines.

2. What general statement can be made about the closeness of the contour lines and the steepness of the slope?

3. Where is the volcano the steepest?

4. Create a line model similar to what is on the lid and label the steep and non-steep areas.

Studying the Sea FloorSonar Mapping Activity (complete first few

locations as a class)Harry Hess Reading/ Discussion

SS Sapona Shipwreck

The 17 positions on the map shown here give the length of time that a sonar pulse took to travel to the ocean floor and back to the sonar array onboard your boat. Use the formula discussed in the article to determine the depth at each location. Transfer the data to the Ocean floor worksheet to create an ocean floor profile.

Harry Hess Reading Questions1. What is echolocation and why did organisms evolve to use

echolocation?

2. Why was Wegner’s idea about Continental Drift rejected by the scientific community?

3. What did Harry Hess find in the Pacific Ocean?

4. What happens at a mid-ocean ridge?

5. Harry Hess’s discovery led to a new theory that supported continental drift. What is the name of this theory?

Harry Hess & the Sea Floor

Hess’s Next DiscoveryThe idea of continental drift had been

developed by Alfred Wegner, but Wegner could not explain WHY the continents move.

Hess’s discovery of the vast underwater mountain ranges called mid-ocean ridges were fundamental to explaining the mechanism by which the continents “drifted”Hess used sonar to discover the under-water

mountain ranges, which extend for over 80,000 km under Earth’s oceans

Geologic puzzles solved by HessIf the oceans have existed for at least 4 billion

years, why has so little sediment accumulated on the ocean floor?

Why are the rocks of the ocean floor significantly younger than rocks found on continents?

Why are fossils found in ocean sediments no more than 180 million years old?

How do the continents move?

Journey to the Ridge ActivityIn this activity we will be aboard a ship

traveling from the East Coast of the United States to the country of Morocco in Africa.

As we go we will be collecting rock samples and using radiometric dating to determine the age of the rocks

We will then graph our data as a function of distance from the East Coast of the U.S.

Mid-Ocean RidgesMid-Ocean Ridges

Undersea mountain chainIn 1968, the Glomar Challenger took rock

samples and discovered that the rock samples got older as they moved away from the MOR What does this mean?

Continents = ~3.8 byo Ocean = ~190 myo

Seafloor Spreading Demonstration

Sea Floor SpreadingHarry Hess (1960) – proposed idea of sea-floor

spreadingThe process by which new oceanic crust forms

along a mid-ocean ridge and older oceanic crust moves away from the ridge

Molten rock from inside Earth rises through cracks in ridges, cools, and forms new oceanic crust This process occurs over and over again, with new crust

being created along the mid-ocean ridge and the older crust being pushed away from the mid-ocean ridge

Age v. LocationThe closer the oceanic crust is to a

mid-ocean ridge,

the YOUNGER it is

Ocean crust that is farther away

from a mid-ocean ridge is OLDER

This begged the question: If the ocean floor is moving, why not the

continents?

Moving ContinentsContinents move as the seafloor spreads

along a mid-ocean ridge Continents do not move THROUGH the solid

mantle of the seafloor

Ocean TrenchesThe sea floor has been spreading for millions

of years, but it is not getting bigger… WHY?Ocean Trenches

Huge, deep canyons in the seafloorAt an ocean trench, dense oceanic crust is

sinking into the mantle Older crust is destroyed/ recycled (by

melting) at ocean trenches

Density of Earth’s Layers Activity

Google Earth Ridges & Trenches

PaleomagnetismAs magma cools to

form rock, iron-rich minerals align with Earth’s magnetic field.Like a compass

Magnetic ReversalsNorth was not always North

Scientists have found that the magnetic field of the Earth has flipped or reversed many times

Normal Polarity vs. Reversed PolarityWhen these rocks are placed in chronological

order, we see alternating bands on either side of the mid-ocean ridge.