You may wonder…

Why are the continents moving?

Seafloor Spreading

The oceans are widening along the mid-ocean ridges.

Volcanoes located along ocean ridges erupt, creating new ocean floor.

Plates—continental crust, oceanic crust

Features—faults, trenches, mid-ocean ridges, folded mountains, hot spots, volcanoes

Related actions —earthquakes, volcanic activity, seafloor spreading, mountain building, convection in mantle.

Key concepts

What are the plates made of?

• Ocean plates are made of basalt.

• Continental plates are made of granite.

Divergent Plate Boundaries

Two land or ocean plates move apart in opposite directions. Magma flows to the surface between

them creating new crust.

Mid-Atlantic Ridge = Divergent Boundary

Iceland – a continent directly over the Mid-Atlantic Ridge

Subduction Zones

• An ocean plate and a continental plate hit head-on. The ocean plate subducts under the continent forming a trench. The subducting plate melts. Magma rises to the surface creating a string of volcanic mountains parallel to the shoreline.

Sierra Nevada Mountains

Subduction zones form chains of volcanic mountains along the shoreline.

Andes Mountains

Subduction zones form chains of volcanic mountains along the shoreline.

Island Arcs

Two ocean plates hit head-on. One ocean plate is forced to subduct under the other forming an ocean trench. The

subducting plate melts. Magma rises to the surface forming a string of volcanic islands parallel to the trench.

The Aleutian Islands

Ocean Trenches

http://www.nhusd.k12.ca.us/ALVE/wow/Ocean/seafloor.gif

http://www.thirteen.org/savageearth/hellscrust/assets/images/ringoffire.jpg