Ocean 101U, Fall 1993
Review of Major Evidence for Plate Tectonics
Evidence that revealed the underlying processes accounting for sea floor structure
1. Wegener & continental drift
Fit of continental margins: S. America & Africa, N. America & Europe
Matching geologic formations and fossils on opposite sides of oceans
Fossils not fitting present climatic zones (glaciers in Sahara, palm trees in Antarctica)
2. Magnetic polar wandering curves: North Pole seems to be in 2 places at the same time?
3. Seismic activity (earthquakes) most frequent near plate boundaries (ridges/rises, trenches,
continental collisions)—e.g., Pacific Ring of Fire
Implies places where segments of crust are moving toward, away from, or alongside each
other
Deep (and shallow) compressional earthquakes at and to one side of trenches—also strings of
volcanoes paralleling trenches
Shallow extensional earthquakes along ridge axes and fracture zones
4. Heat flow through crust greatest near ridge/rise crests
Indicates fresh magma is closer to surface
Pillow & sheet flow basalts indicate fresh magma (lava) that has cooled rapidly
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
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
7. The clincher: Polar reversals at irregular intervals 170 times in last 76 million years—none
since 710,000 years ago
When magma cools, it takes on prevailing polarity of Earth's magnetic field
Symmetrical “stripes” of alternating polarity flanking ridges/rises match times of reversals
from terrestrial volcanoes
8. The smoking gun: Hydrothermal vents discovered in late 1970s, predicted by theory
Revised February 15, 2016
Ocean 101U, Fall 1993
Heating of subsurface water in cracks in fresh basaltic crust 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 additional details of plate tectonics
Divergent (constructive) plate boundaries
Fresh pillow & sheet-flow basalt, covered first by hydrothermal sediment to varying degree, accumulate other sediments as
they travel away from the ridge crest
Transform faults occur where plates move side-by-side in opposite directions, producing shallow strike-slip earthquakes.
Fracture zones are longer cracks created by (& including) transform faults.
(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
Mid-plate features
Sea floor gradually gets deeper as it spreads from the ridge or rise and ages. Cooling and contraction make the oceanic crust
and lithosphere denser, so they isostatically sink and float lower atop the asthenosphere.
As the deepening seafloor spreads from the ridge/rise, sediment levels it out to create the abyssal plains. Abyssal hills are
believed to be peaks and fault blocks that originated in the ridges/rises, but have descended isostatically and been covered &
smoothed by sediment.
Seamounts, however, are volcanoes that erupt at or away from the spreading center.
Hot spots
Iceland—hot spot under Mid-Atlantic Ridge. Yellowstone thought to be a continental hot spot leaving a track from
southern Oregon
Islands built of pure oceanic crustal material—basalt—used to account for guyots & atolls
Deepen with age both as a result of cooling, contraction, & isostatic subsidence and because seafloor on which it is riding is
also subsiding as it travels away from the spreading center.
Convergent (destructive) plate boundaries
Mistake in Duxbury figures—oceanic crust blends into mantle rather than continuing under continental crust.
Where there is a trench, it takes the place of the continental rise (i.e., sediments fall into the trench instead of building a
rise). Off the NW coast, however, the trench is sediment-filled .
Deeper trenches occur 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.
Subduction of oceanic crust below a continent gives explosive andesite volcanoes, as in Andes, St. Helens, island arcs.
Continental margins
Leading (active) margins are at convergent plate boundaries, e.g., west coast of South America, with 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,
with a rise not a trench
U.S. West Coast
Movement of and accretion of terranes onto N. American plate combined with westward motion of N. AMerican Plate 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 15, 2016
Ocean 101U, Fall 1993
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 powerful earthquake beneath Seattle 1000 years ago—recently discovered Seattle fault caused landslides and a
local tsunami. Not a subduction earthquake.
Revised February 15, 2016