Download Late 20th Century Tests of the Continental Drift Hypothesis

Late 20th Century Tests of the
Continental Drift Hypothesis
5 – Characteristics of the Ocean Trenches
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What to look for:
• Like ridges and transform faults, the trenches are places where
lithospheric plates move against each other. Benioff zones of
earthquake foci tell us this.
• The Benioff zones indicate that the movements are both deeper and
occur in wider bands than at other plate margins, with one plate
dipping beneath the other and going deeper with distance.
• Progressively deeper melting of the subducting plate means the
magma generated must move farther through the mantle and crust
to reach the surface. This allows more time for differentiation, so the
volcanoes become more felsic farther inland from the trench.
• There is a narrow, intense, negative gravity anomaly immediately
above the trench. This indicates that something stronger than local
gravity pulls the subducting plate downward there.
If crust is forming at the ridges, and if Earth
is not expanding (it’s not), then crust must
be being destroyed somewhere at the
same rate it is forming.
Where?
The other edges of the plates are equally easy to find
in the same way as the ridges and transform faults.
Notice how much broader these bands of epicenters are. They are all beside trenches
FLASHBACK – remember that in the Atlantic (and Indian) Ocean the ridge is half-way between two
continents that it originally rifted. Not surprisingly, the ocean basin is symmetric. Immediately beside the
ridge/rift lie the abyssal plains – basaltic oceanic crust that has moved away from the ridge, cooled, and
subsided as far as it can. Depths on the abyssal plains run fairly consistently 4-5 km below seal level.
Except for the ridge, there is generally little volcanic or seismic activity in such an ocean. The edges of the
facing continents are at the shelf break, where the shallow waters (~200m) over continental crust (the shelf)
end. The shelf and adjacent coastal plain are geologically identical – the position of the shoreline marks
where one ends and the other begins. The slope is the original rifted edge of the continent, modified by later
deposition, erosion, and mass wasting. The continental rise is the accumulation of sediment that results from
the erosion and slumping (plus some sediment from the land). It thins outward as it covers the edges of the
abyssal plains.
Coastal
Plain
Continental Shelf
Shelf
Break
M.A.R.
Rift
Sea Level
Abyssal Plains
Continental Rises
Continental Slopes
Mirror image of
the other shelf
Continental Margins such as these are called “Atlantic”, “stable”,
or “trailing”, depending on the context of the sentence. “Atlantic”
obviously refers to the typical location where they were first
studied. “Stable” refers to the fact that that there are no
volcanoes or seismic zones on them. “Trailing” means they are
on the side of the continent following behind as it drifts away from
the ridge.
Sea Level
Continental margins around the Pacific, on the other hand, are patently unstable,
and are so called. There is more than one type of unstable margin, however.
In rare cases (such as the west
coast of the USA from southern
California to southern Oregon)
the active margin of the
continent lies on a transform
fault. Movement on the fault
creates earthquakes, but no
magma is generated and no
volcanic activity occurs. These
are simply called transform
continental margins.
The Pacific Basin is referred to as the “ring of fire” because it is almost surrounded
by active plate margins. The next slide shows a cross-section from A (Korea) to A’
(Bolivia). The dashed part of the abyssal plain will be omitted to save space.
Notice that the ridge/rift and abyssal plain part of the basin is identical to a basin with stable
margins. The processes at the ridge create this regardless of what happens at the far end of
the drift system. The continental margins, however, have a much different shape.
Instead of a rise/slope/shelf, these margins have a trench/arc configuration, sometimes with a
separate marine “back-arc basin” between the arc and the mainland. With the back-arc basin
the margin is a “Japanese-type” margin. Without one it is and “Andean” margin.
JAPANESE-TYPE MARGIN
Korea
(Asia)
Sea of Japan Japan
Japan
Trench
(Fujiyama)
ANDEAN MARGIN
Abyssal Plain
(note omission of
a long segment)
East Pacific Rise Peru-Chile
Altiplano
(ridge/rift)
Trench Western
Eastern
Andes
Andes
Sea Level
Volcanic Arc
(on mainland –
no back-arc
basin!)
Volcanic Arc
Continental
Mainland
Back-arc
basin
Trench (to ~10 km deep)
Trench
DEEPER MOVEMENTS
CREATE THE
BENIOFF ZONE
OF EARTHQUAKE FOCI
Pacific Ocean
From M.A.R.
ANDES VOLCANIC ARC
From East Pacific Rise
X
X
X
X
X
X
X
X
X
X
X
X
X
This plate is
subducting
beneath this one
X
X
X
X
X
X
X
X
X
X
X
X
X
MANTLE
As the next slide will explain, composition of magma and volcanic rocks changes
across the arch from the trench toward the continental center. In the Andes, the
western range erupts andesitic material (the rock is named for the mountains)
and the eastern range erupts rhyolitic material.
Western Andes
Altiplano
Eastern Andes
Sea Level
Peru-Chile
Trench
More felsic
(more quartz,
more k-feldspar,
less biotite,
less Ca in
plagioclase)
The same change across the arc occurs in other
unstable margins, though it might not go as far as
rhyolite.
Western Andes
Altiplano
Eastern Andes
Andesite
Rhyolite
Diorite
Granite
Pacific Ocean
From M.A.R.
From East Pacific Rise
Shallower melting allows
less time to differentiate
on the way to the surface
Deeper melting allows more
time to differentiate on the
way to the surface
MANTLE
MANTLE
The force of gravity is not the same everywhere on Earth. Because its strength depends
upon the total mass to which a falling object is attracted, places with more massive
(dense) bedrock will have a slight but measurably higher strength of gravity. A gravimeter
is a device for measuring its strength, and depends on the period of a very precise
pendulum. As with the magnetic field we can predict the value if we know what the
bedrock should be. We then go measure and see if our prediction is right. Usually it is,
but sometimes the measured value is higher (positive gravity anomaly) and sometimes
lower (negative gravity anomaly). Figure out why the buried (hidden) igneous rock in the
figure causes a positive anomaly. (Sedimentary rock usually has a density of 2.5 or less;
igneous rock ranges from 3.5 to 4).
© Georgia Geological Survey
Here is an actual gravity map of
Georgia. The red areas are
positive anomalies (higher than
normal for “average” continental
rocks) Notice that there are a lot
of them along the Fall Line and in
the south-central Coastal Plain.
These are probably caused by the
mafic intrusions and extrusions in
an ancient rift basin.
The two roundish ones near Tifton
and Douglas might be extinct,
buried volcanoes like Mts
Kilimanjaro and Kenya in the East
African Rift.
We can do some calculations to determine why the anomaly is here. We begin by imagining what
might be missing to cause the mass deficit. Presumably it is some sort of rock that should be
where the trench is – that is, that has the same volume as the trench. The only rock that would
be dense enough to make the anomaly go away if we filled the trench with it is peridotite – the
stuff of the mantle.
Apparently the trench is a place where something stronger than local gravity is pulling
downward on the oceanic plate that is subducting and displacing the asthenosphere in both
directions – landward and seaward of the trench.
If this pull were to disappear the mantle material would move back under the trench and force the
plate back to its original level – abyssal plain level. The negative anomaly and both the wide
positive ones would go away.
Take-home message:
• Like ridges and transform faults, the trenches are places where
lithospheric plates move against each other. Benioff zones of
earthquake foci tell us this.
• The Benioff zones indicate that the movements are both deeper and
occur in wider bands than at other plate margins, with one plate
dipping beneath the other and going deeper with distance.
• Progressively deeper melting of the subducting plate means the
magma generated must move farther through the mantle and crust
to reach the surface. This allows more time for differentiation, so the
volcanoes become more felsic farther inland from the trench.
• There is a narrow, intense, negative gravity anomaly immediately
above the trench. This indicates that something stronger than local
gravity pulls the subducting plate downward there.