Late 20th Century Tests of the Continental Drift Hypothesis

5 – Characteristics of the Ocean Trenches

Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States.

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

Sea Level

M.A.R. Rift

Abyssal Plains Continental Rises

Continental Slopes

Coastal Continental Shelf Plain Shelf Break

Mirror image of the other shelf

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.

Sea Level

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.

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.

Korea Sea of Japan Japan Abyssal Plain East Pacific Rise Peru-Chile Altiplano (Asia) Japan Trench (note omission of (ridge/rift) Trench Western Eastern (Fujiyama) a long segment) Andes Andes

Sea Level

Continental Mainland

Volcanic Arc

Back-arc basin Trench (to ~10 km deep) Trench

Volcanic Arc (on mainland – no back-arc basin!)

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 ANDEAN MARGIN

From East Pacific Rise

From M.A.R.

MANTLE

Pacific Ocean

DEEPER MOVEMENTS CREATE THE

BENIOFF ZONE OF EARTHQUAKE FOCI

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ANDES VOLCANIC ARC

This plate is subducting beneath this one

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.

More felsic (more quartz, more k-feldspar, less biotite, less Ca in plagioclase)

Sea Level

Peru-Chile Trench

Western Andes Altiplano Eastern Andes

The same change across the arc occurs in other unstable margins, though it might not go as far as rhyolite.

From East Pacific Rise

From M.A.R.

MANTLE MANTLE

Western Andes Altiplano Eastern Andes

Andesite Diorite

Rhyolite Granite

Deeper melting allows more time to differentiate on the

way to the surface

Shallower melting allows less time to differentiate

on the way to the surface

Pacific Ocean

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).

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.

© Georgia Geological Survey

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.