1. Seafloor Spreading
Plates on the Move

2. Introduction
Most scientists believe that Earth’s crust is broken into about 20 pieces called plates.
Each plate is the thickness of the crust and the rigid upper mantle.
The plates move slowly, floating on the mushy, flowing mantle below them.
Some of the plates move up to two inches (five cent.) a year.

3. Where two plates meet---
The surface of some plates is mostly ocean, while that of others is made up of entire continents and parts of oceans.
Where two plates meet---
They can spread apart
Come together
Or slide past each other

4. Interactions Between Plates
These interactions between plates and the intense pressure, friction, and crust melting…are responsible for much of our planet’s volcanic and earthquake activity.
They also build mountains and recycle Earth’s crust.

5. Basic Steps: Color each piece -Letters A, E, & H -color red (Magma)
-Letters B & C –color yellow (Mid-ocean Ridges)
-Letters D & G-color brown (subduction zones) arrows any color but brown.
-Letter F – color blue & brown (Volcanic Arcs & ocean)
-Letter I – color blue & brown ( Ex. Of Mount St. Helens)

6. Students are looking at parts of four plates.
Each plate is numbered with arrows showing its directions of movement.
Most of each plate’s surface is ocean.
Look at the edges of plates 2 & 3.
The plates are spreading apart.
This opens giant cracks called rifts in the seafloor.

7. DIRECTIONS:
Paste piece A on the model/it shows that hot magma rises and slowly oozes out of the rifts as lava. As the lava cools, it builds underwater mountains. Label piece A MAGMA

8. Paste piece B & C in their places
Paste piece B & C in their places. As plates 2 & 3 move apart, they act like giant conveyor belts moving the mountains away from the cracks. More mountains form in their place. In this way, underwater mountain ranges called ridges build up. Label B & C Ridges.

9. Plates 1 & 2 are moving toward each other
Plates 1 & 2 are moving toward each other. Where they meet, the edge of 2 bends and dives under 1. This forms a trench. Label D TRENCH.
As the edge of plate 2 dives into Earth’s crust, it grinds and scrapes against plate 1. If the edges lock together, pressure can build, causing an earthquake.

10. High heat inside the crust melts the edge of plate 2 into magma
High heat inside the crust melts the edge of plate 2 into magma. Some of the magma may rise and break through the seafloor. Paste piece E in its place. Label MAGMA.
Magma that erupts from the seafloor can build up a line of island volcanoes called a volcanic arc. Paste piece F in place. Label F VOLCANIC ARC. Japan, Philippines, and the Aleutian Islands off Alaska are parts of volcanic arcs.

11. Look at plates 3 & 4---notice how they are moving toward each other
Look at plates 3 & 4---notice how they are moving toward each other. At the edge of 3 there is an ocean. At the edge of 4 there is a continent. Paste piece G in place. This is a trench that forms when 3 dives under 4. Label it TRENCH.

12. Paste I in place. This is a line of volcanoes on land built from lava during eruptions. Label VOLCANOES. Mount St. Helens formed this way. Also, many earthquakes on land occur where the ocean edge of one plate dives under the continent edge of another.
Paste piece H in place. This is magma rising from the melting edge of 3. Piece H is pasted on top on piece I.

13. Label Trenches Subduction zones Convergent boundaries
Divergent boundaries
Lithosphere
Asthenosphere
Convection current

14. Sea Floor Spreading
Plate tectonics is a theory that the Earth’s Crust is broken into sections (plates) that move around the Asthenosphere. The Lithosphere is made up of the crust and part of the upper mantle. Plates can move together as Convergent Boundaries. Convection Currents are the energy in the mantle that forces magma up to the crust. Volcanoes and earthquakes are a result of movement of these currents. Magma rises up and pushes old magma to the side, causing Sea-floor spreading. When one plate moves under another, the plate creates a Subduction Zone.