Transcript Document

You may wonder…
Why are the continents
moving?
Seafloor Spreading
The oceans are widening along the midocean ridges.
Volcanoes located along ocean ridges erupt,
creating new ocean floor.
Key concepts
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.
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