Earth Science Standard 3.a - Students know

features of the ocean floor (magnetic

patterns, age, and sea-floor topography)

provide evidence of plate tectonics.

Seafloor Spreading

• Matching Coastlines• Similar Rock Types• Similar Fossils• Climatic Change

Continental Drift explained some things….

Seafloor Spreading explained a few more

things….

• Proof for how the plates moved (Hess)• new oceanic crust is created at the ridges and

recycled at the trenches• Evidence found at the bottom of the oceans

Seafloor Spreading

Ridge

Trench

• Mid-Ocean ridge – underwater mountain system where the crust is lifted and pulled apart

• Trenches – narrow, steep sided depression that forms as one type of crust slides underneath another

• Continental slope – transition from continental crust to oceanic crust

• Continental shelf – submerged border of a continent

Seafloor Topography

At ridges, the crust is split apart and magma rises up!

At trenches, the crust is recycled as oceanic crust is subducted into the hot

mantle!

• Ocean ridges are typically found in the ocean• Trenches are found near the coastlines

(continents)

1. Magma rises up to the ocean floor causing ocean crust to be pushed apart (RIDGE PUSH)

2. The magma rushes up and forms new crust. 3. Old crust pushed toward the trenches

(continents).4. At the trenches, cooler material pulls oceanic

crust down into the mantle (SLAB PULL)5. This subducted crust (crust that is pulled

underneath another type of crust) is then recycled in the mantle.

Process of Seafloor Spreading

What causes the crust to separate at the ridges and to be pulled down at the trenches?

• Convection Currents in the mantle – Hot material rises and cooler material sinks

Earth’s continents are moving! In what

directions can they move?

http://education.sdsc.edu/optiputer/flash/convection.htm

1. Age of Rock• Rock at ridges = Youngest• Rock at Trenches = Oldest• Rock gets older in a predictable way

Seafloor Spreading Evidence

Age of the Sea Floor

2. Paleomagnetism• When rock with iron/magnetite forms (cools),

it traps a record of Earth’s magnetic field• Earth’s magnetic field has reversed many times

since its creation

• Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”.

• They hold a record of reversals in Earth’s magnetic field.

• Each mid-ocean ridge is lined by mirror-image patterns of magnetic field reversals.

Normal Polarity (+) = Today’s Polarity (N)

Reversed Polarity (-) = South as North

How is this evidence?• It shows that new seafloor is constantly being

created. • As new seafloor is formed, it slowly pushes the

older seafloor towards the continents (trenches) where crust is recycled into the mantle or crumbled up to become part of the continents.

It provided the MECHANISMWegener was missing!

Draw and Color Diagram• Include Convection Currents!• Label the youngest rocks on the seafloor and the

oldest!