Science 10 – Plate Tectonics Theory
Wegener’s work on continental drift in the early 1900’s was provocative, but lacked
empirical depth. Research in the areas of geologic seafloor mapping and paleo-magnetics
in the 1950’s and 1960’s by Hess and Mason were key in determining a sound
understanding of plate tectonics.
Plate Tectonics Theory - explains the global distribution of geological phenomena such
as ____________________, ______________________, ________________________,
and ____________________________ in terms of the formation, destruction, movement,
and interaction of the earth's ___________.
The crust of Earth is made up of about _____ interlocking moving pieces that meet along
plate boundaries. Most geologic activity tends to occur along these plate boundaries (i.e.
earthquakes, volcanoes, etc.) There are 3 types of plate boundaries: divergent,
convergent and transform.
Divergent (
) – two plates move __________, resulting in upwelling of material
from the mantle to create new seafloor (Mid-Atlantic Ridge & East African Rift Valley)
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect1/30diverge1.gif
Convergent (
) – two plates move ____________ resulting in:
a) An oceanic plate subducting under an overriding continental plate to eventually be
reabsorbed into the mantle. Usually creates trenches and violently explosive volcanoes
(Pacific Ring of Fire)
http://wrgis.wr.usgs.gov/docs/parks/pltec/contvsocn288x157.gif
b) An oceanic plate subducting under an overriding oceanic plate to eventually be
reabsorbed into the mantle. Usually creates trenches and calm volcanoes (Samoa)
http://pubs.usgs.gov/publications/graphics/Fig21oceanocean.gif
c) A continental plate subducting under another continental plate to eventually be
reabsorbed into the mantle. Usually creates mountain ranges (Himalayas)
http://pubs.usgs.gov/publications/graphics/Fig21contcont.gif
Transform (
) – two plates grind ___________________ without the production
or destruction of the lithosphere (San Andreas Fault)
http://web.odu.edu/webroot/instr/sci/tmmathew.nsf/files/transforms.jpg/$FILE/transforms
Note: Shallow earthquakes tend to occur in the crust (ridges, trenches), intermediates in
subduction zones, and deep earthquakes in the mantle.
Cause of Plate Movement – Mantle Convection
Mantle Convection – the rise and fall of warming and cooling mantle due to heat from the
planet’s initial formation and ___________________________
Ridge Push – As molten magma rises at a
mid-ocean ridge it heats the rocks around it.
The heat expelled by the magma expands the
rock at the ridge, pushing them above the
surrounding ocean floor. As time passes,
the heated rocks cool and their density
increases causing _____________ to pull
this dense rock away from the ridge.
Slab Pull - As a crustal plate moves further
from an oceanic ridge, it cools and becomes
increasingly dense. This causes it to sink
beneath the continental crust in a subduction
zone. The weight of this sinking, cooling
plate causes a major pulling action, which
causes the rest of the plate to be pulled
downwards as well.
Evidence Supporting Plate Tectonic Theory
1) ____________________________
Hess, an American scientist in
the navy found ridges, and deep
trenches along the ocean floor
through radar imaging. His work,
along with help from other
scientists inclu. Wilson (Cdn!),
found that the sea floor was
spreading out at ridges, and
being recycled back into the
Earth at trenches.
http://www.jenningsk12.net
2) ____________________________
Every 1-2 million years the poles of the
Earth switch. New rock is constantly
being formed under the ocean in the
Mid-Atlantic ridge. When we look at
rock underneath it appears to have
switched direction every 1-2 million
years, implying oceanic plates are
moving apart.
http://www.riverdell.k12.nj.us/staff/molnar/onotes12.gif
3) ___________________________
A hot-spot is an area of the crust where there
is extensive melting of rock (caused by
radioactivity) near the surface of oceanic crust.
This is seen in Hawaii. It is believed that
there is a chain of volcanic structures
extending from the Hawaiian islands,
to Midway island continuing northward
onto the Aleutian trench. As the Pacific
plate moved over this hot-spot new volcano
structures formed.
http://pubs.usgs.gov/publications/graphics/hot_spot.gif