Transcript B. The sea floor spreads apart at divergent boundaries 1. Rift Valley

Objectives:
• TLW…identify different plate boundaries.
• TLW…explain what happens when plates move
apart.
• TLW… determine how to measure the direction
and speed of plates.
• TLW…model the magnetic reversal of Earth’s
poles in an experiment.
Main Ideas:
Supporting Details:
A. Tectonic plates have
different boundaries
• A plate boundary is where the edges of
two plates meet.
• Scientist has discover three
boundaries which are:
a) divergent boundary
b) convergent boundary
c) transform boundary
1. divergent boundary
• Occurs where plates move apart and
are found in the ocean.
2. convergent boundary
•Occurs where plates push together
3. transform boundary
•Occurs where plates scrape past
each other
Main Ideas:
B. The sea floor spreads
apart at divergent
boundaries
Supporting Details:
• divergent boundaries are also called
spreading centers.
•Mid-ocean ridge mark sites where the
ocean floor is spreading
1. Rift Valley
C. Mid-Ocean Ridges and
Rift Valleys
• ridges continue to widen, a gap
•Mid-ocean ridges are the longest chain
of mountains on Earth
•World’s longest ridge, the Mid-Atlantic
Ridge, runs the length of the Atlantic
Ocean
Main Ideas:
D. Sea-Floor Rock and
Magnetic Reversals
Supporting Details:
•Scientist found a surprising discovery
about the Earth’s magnetic field when
studying the Sea floor rock.
•Earth’s magnetic field affects the entire
planet and the Earth’s geographic and
magnetic poles are not in the same
place.
•Earth’s magnetic poles switch places
every so often. North becomes South
and the South becomes North
1. Magnetic reversal
•The poles switching in directions
•These changes are cause by changes
in the Earth’s magnetic field
Main Ideas:
Supporting Details:
• no one knows why these changes happens;
however, the Earth’s geographic poles never
change places.
•Sea-floor rock record every magnetic reversal
•Magnetic reversals line up like stripes in the rock
•Scientists had further evidence of plate
movement.
•Most recent reversal happened only about
760,000 years ago
•The oldest reversal happened more than 150
million years ago.
Main Ideas:
E. Continents split apart
at divergent
boundaries
Supporting Details:
• Continents also spread apart at a divergent
boundary.
•This is when volcanoes form because the rift
valley grows wider, the continent begins to split
apart.
•As the rift valley continues to widen, the thinned
valley floor sinks lower and lower until it is below
sea level.
•Water from a nearby ocean or river may fill the
valley and form a sea or a lake.
F. Hot spots can be used
to track plate
movements
• hot spots are heated rock rises in plumes or
thin columns, from the mantle
Main Ideas:
Supporting Details:
• Volcanoes often develop above the plume.
•Most hot spots occur far from plate boundaries,
they offer away to measure plate movement
•Hot spots generally stays in one place while the
tectonic plate above it keeps moving
•At the hot spot, the heat from the plume partly
melts some of the rock in the tectonic plate above
it.
•Plates stays over the hot spot long enough, the
rock above it will melt and in time a volcano will
form at the surface of the plate.
•Plate moves slowly over a hot spot
Main Ideas:
Supporting Details:
• when the plate moves on, it carries the first
volcano away from the hot spot.
•Many hot spots provide a fixed point that
scientists can use to measure the speed and
direction of plate movements.