Ocean 101U, Spring 1994
Page 1
Plate Tectonics
TOPIC
VISUALS
Review of features we have already seen
Continental margins, ridges & rises, rift valleys & fracture zones,
seamounts, guyots & atolls, trenches, island chains & arcs
Additional evidence that helped scientists figure out the underlying
processes accounting for sea floor structure
1. Wegener: fit of continental margins of continents: S. America &
Africa, N. America & Europe
Also matching geologic formations and fossils
Also fossils not fitting present climatic zones (glaciers in
Sahara, palm trees in Antarctica)
Problem: if continents “drift,” how do they plow through
basaltic sea floor?
OH T148
2. Magnetic polar wandering curves conform to same pattern
Dux 3.23
3. Seismic activity (earthquakes) most frequent near ridges/rises and
trenches
Implies places where segments of crust are maving past each
other
Deep (and shallow) earthquakes at and to one side of
trenches—also strings of volcanoes paralleling trenches
Shallow earthquakes along ridge axes and fracture zones
Pacific Ring of Fire
Dux 3.8
4. Heat flow through crust greatest near ridge/rise crests
Indicates magma is closer to surface
Pillow basalts indicate fresh magma that has cooled rapidly
Dux 3.9
5. Ocean floor basalt (bedrock) gets steadily older with increasing
distance from ridge/rise crests
Maximum age of sea floor is <200 million years despite ages
of billions of years on land—where did the old sea floor go?
Some older sea floor found in deposits now on land, proof
there was a sea floor 200+ million years ago
Dux 3.12
6. Ocean floor has thicker sediment cover & sediment in contact with
basalt bedrock is older with increasing distance from ridge/rise crests
Reinforces findings from age of sea floor basalt that sea floor
steadily increase in age with distance from ridge/rise crests
Dux 3.12
Revised February 17, 2016
Ocean 101U, Spring 1994
Page 2
7. The clincher: Polar reversals at irregular intervals 170 times in last Dux 3.15
76 million years—none since 710 kybp
When magma cools, it takes on prevailing polarity
“Stripes” of alternating polarity flanking ridges/rises matching
reversals from terrestrial volcanoes
8. The smoking gun: Hydrothermal vents discovered in late 1970s,
predicted by theory
Heating of subsurface water in cracks creates low- and hightemperature “hot springs” of mineral-laden water at many sites
A unique environment in every oceanographic aspect—we
will focus on it in biology section.
Important details of plate tectonics story
Continental drift was hypothesized first, then sea-floor spreading was
demonstrated
They’re really the same thing because sea-floor spreading
drives the motion of the earth’s crustal plates which carry the
continents in their drifting. Together we call them plate
tectonics
T149-154
Pangaea existed about 250–200 mybp, then broke up
Don’t worry about pre-Pangaea movements except to know
that there were some.
Don’t worry about post-Pangaea movements except to know
that 1) the Atlantic opened, 2) North America is overriding the
E. Pacific Rise, and 3) India smashed into Asia
It’s believed that plate tectonics has actually had a big effect
on changing the Earth’s climate over millions of years by
blocking ocean currents and changing the arrangement of land
and ocean blocks
Divergent (constructive) plate boundaries
Rift valleys are where new sea floor is created by upwelling of
magma from convection cells in mantle, shallow extensional
earthquakes
Fresh pillow & sheet-flow basalt, covered first by
hydrothermal sediment to varying degree
Accumulates other sediment as it travels away from the crest
Fracture zones are result of transform faults, shallow strike- Dux 3.17
slip earthquakes
Revised February 17, 2016
Ocean 101U, Spring 1994
Page 3
(Half-) spreading rate averages from 1-10 cm/yr—steep-sided
mid-Atlantic ridge 1.5 (=1 mile/100,000 yr), rounded E.
Pacific rise an extreme 18.3 cm/yr (averages)
Mid-plate features
Sea floor gradually gets deeper as it spreads from the
spreading center at a ridge or rise. Cooling and contraction
make the oceanic crust and lithosphere denser, so they
isostatically sink and float lower atop the asthenosphere.
As it spreads from the ridge/rise, sediment levels out the
deepening seafloor to create the abyssal plains. Abyssal hills
are believed to be jagged peaks and fault blocks that originated
in the ridges/rises, but have descended isostatically and been
covered and smoothed with a coat of sediment.
Seamounts, however, are volcanoes that erupt at or away from
the spreading center, and magma comes up through the
existing plate and seafloor. More on this below.
Convergent (destructive) plate boundaries
Trenches are where new seafloor is destroyed by subduction of Dux 3.7
oceanic crust into convection cells in mantle. Deep
compressional earthquakes, also shallower quakes from
stresses on continental crust.
This is where the mistake in the textbook shows up—the
oceanic crust blends into mantle rather than continuing under
continental crust.
You saw or will see in lab that you don’t always observe a
trench at a convergent boundary. Off the NW coast, for
example, the trench is filled with sediment.
Where there is a trench, it takes the place of the continental
rise (i.e., sediments fall into the trench instead of building into
a fan).
Deeper trenches where seafloor is older, colder, denser, farther
from spreading center. Marianas is consuming some of oldest
sea floor on Earth. Trench off NW coast is easily filled with
sediment because sea floor is young, warm, buoyant, and
trench is not very deep.
Volcanic belts result from melting oceanic crust bursting
Dux 3.20
upward to the surface to form mountain ranges (where
subduction is below a continent, giving andesite as in Andes
or St. Helens) or island arcs (where subduction is below
another oceanic plate, giving more basaltic lava as in
Aleutians).
Continental margins
Revised February 17, 2016
Ocean 101U, Spring 1994
Page 4
Leading (active) margins are at convergent plate boundaries,
Dux 3.20
e.g., west coast of South America, with andesitic volcanic
mountain range or island arc, narrow shelf, and trench.
Trailing (passive) margins, e.g. U.S. East Coast, are not at
plate boundaries, can build up broad shelf with lots of
sediment, a rise not a trench
Collision of continents forms high mountain ranges of mostly Dux 3.20
continental crust with some marine sediments, i.e. Himalayas
(also Alps).
More general case: exotic terranes, pieces of both continental Dux Box
3.1
& oceanic crust on moving oceanic plate glommed onto
leading margin of continent—how much of N. American West
Coast was built
Hot spots
“Holes” in the center of a plate allowing magma to come
through—”fountains” in the mantle?
Leave a trail of volcanic activity as plate moves over fixed
spot
Hawaiian chain (max age 25 my) & Emperor seamounts (max
age 75 my)—changed direction 40 million years ago
Iceland—hot spot under Mid-Atlantic Ridge
Yellowstone thought to be a continental hot spot leaving a
track from southern Oregon
Islands built of very pure oceanic crustal material—basalt—
versus mixed material (andesite) at leading continental
margins
More common in Pacific than Atlantic or Indian
Get deeper with age both as a result of cooling & contraction
& isostatic subsidence and because seafloor on which it is
riding is also subsiding as it ravels away from the spreading
center.
Largely accounts for formation of guyots and atolls.
U.S. West Coast
Movement of and accretion of terranes onto N. American plate
has overrun it—in fact has overrun E. Pacific Rise.
San Andreas is a transform fault from a spreading center
oriented E–W south of Baja—edge of S. California is moving
NW
Revised February 17, 2016
T 131
Dux 3.22
Ocean 101U, Spring 1994
Juan de Fuca Ridge with hydrothermal vents is off our coast,
subduction is occuring beneath our coast, andesitic Cascade
volcanoes result (Olympics are oceanic crust terranes)—has
caused extremely large earthquakes in recent times, more due.
Shallow but very powerful earthquake beneath Seattle 1000
years ago recently discovered Seattle fault caused landslides
and a local tsunami. Not a subduction earthquake.
Revised February 17, 2016
Page 5
Dux 3.18