The Origin of Oceanic Trenches. SUMMARY. Deep folds, thousands of miles long and several miles deep, lie on the floor of the western Pacific Ocean, directly opposite the center of the Atlantic Ocean.
SUMMARY • Deep folds, thousands of miles long and several miles deep, lie on the floor of the western Pacific Ocean, directly opposite the center of the Atlantic Ocean. • The plate tectonic theory claims that plates drifting on the earth’s surface dive into the earth and drag down the folds. • Many reasons will be given why this cannot happen.
As the flood increasingly altered earth’s balanced, spherical shape, gravity increasingly tried to squeeze the earth back toward a more spherical shape. • Once a “tipping point” was reached, that portion of the subterranean chamber floor with the most overlying rock removed rose almost 10 miles to become the Atlantic floor. • This caused the Pacific floor to subside and buckle inward, producing folds, called oceanic trenches. • Measurements and discoveries near trenches confirm this subsidence and the absence of diving plates. • Shifts of material throughout the inner earth produced voluminous amounts of magma, much of which rose onto the Pacific floor. • Slight mass imbalances remain, so earthquakes now occur and continents steadily shift—not drift—toward the trench region of the western Pacific.
Imagine standing at the edge of something that reminds you of the Grand Canyon, but this “canyon” is several times deeper. • Its walls are almost as steep as the Grand Canyon’s, but the view across the 60-mile-wide depression is never obstructed by intermediate land forms. • This “canyon” is thousands of miles longer than the Grand Canyon and does not have sharp bends. • Such depressions, called oceanic trenches, are often shaped like long arcs that connect at cusps. • Oceanic trenches would be the leading natural wonders of the world, if water did not hide them. (Average ocean depth is 2.5 miles; the deepest trench is 6.86 miles below sea level.) • Sixteen trenches are concentrated on the western Pacific floor. • What concentrated so many trenches, and why in the Western Pacific?
Drifting vs. Shifting • The distinction between drifting and shifting is subtle but important. • A box drifts on the sea, but a box shifts in the back of a truck. • Drifting is a continuing movement on or in a fluid, often for a great distance, while shifting is a slight, limited, but significant lateral movement on or in a solid. • Drifting is caused by a steady, unyielding, outside force, while shifting is usually caused by gravity and a sudden change in equilibrium. • Drifting requires a continuing energy source, but shifting requires a disturbing event. • The plate tectonic theory says continents steadily drift. • The hydroplate theory says crustal plates drifted rapidly, but briefly, on a layer of escaping, high-pressure water near the end of the flood. • This drifting produced imbalances. • Since then, these and other imbalances caused by the flood sporadically shift continents and everything below.
Surprisingly, trenches contain shallow-water fossils. • Materials [including fossils] which are usually supposed to be deposited only in shallow water have actually been found on the floor of some of the deep trenches. • Why are such unlikely fossils in a remote part of the ocean—a thousand times deeper than one would expect?
Most of the earth’s crust is vertically balanced, like blocks of ice floating in a pan of water. • Large, dense blocks sink in, while lighter blocks “float” higher up. • This is called isostatic equilibrium. • However, oceanic trenches are earth’s most glaring departure from this equilibrium. • That may be an important clue about how trenches formed.
As various authorities have written: • “... trenches are characterized by large negative gravity anomalies. That is, there appears to be a mass deficiency beneath the trenches, and thus something must be holding the trenches down or else they would rise in order to restore isostatic equilibrium.The most striking phenomenon associated with the trenches is a deficiency in gravity ... Measurements of gravity near trenches show pronounced departures from the expected values. These gravity anomalies are among the largest found on earth. It is clear that isostatic equilibrium does not exist near the trenches. The trench-producing forces must be acting ... to pull the crust under the trenches downward!”
In other words, something has pulled, not pushed, trenches down. • The downward pull of gravity in and above trenches is less than expected, even after adjusting for the trench’s shape, so less mass exists under trenches than one would expect. • It is as if something deep inside the earth “sucked” downward the material directly below trenches. • This would reduce the mass below trenches. (If you want to show a slight weight loss, weigh yourself while on a ship sailing over a trench.)
A useful illustration is to think of a slight vacuum, or reduced mass, under trenches. • While the term density deficiency is more descriptive and accurate, most people understand the consequence of a partial vacuum which “nature abhors.” • That is, nature always tries to move material to fill a vacuum. • If one waited long enough, material inside the earth must flow in under trenches to fill this “partial vacuum.” • Today, crustal plates move an inch or so each year toward trenches, so this “partial vacuum” is being filled in modern times. • Later, we will see where the missing mass under trenches went and what created the “partial vacuum.” • Clearly, this filling in has not been going on for long.
A spinning body, such as a figure skater or the earth, spins faster if it suddenly becomes more compact about its spin axis. This skater starts a spin with outstretched arms. Then, as she pulls her arms in near her spin axis, she spins so fast she becomes a blur. Gravity tries to make the earth as compact and round as possible. Earthquakes cause the earth to become more compact and spin slightly faster. Therefore, the farther back in time we look, the less compact we should find the earth—at least until we arrive at the time the out-of-balance condition arose. Because earthquakes can occur deep within the earth, the out-of-balance condition affected the entire earth and, as you will see, formed trenches. Spin
A technique called seismic tomography has detected slight density increases under continents. • The technique uses earthquake waves to see inside the earth, just as a CAT scan uses x-rays from many angles to see inside your body. • Each earthquake radiates waves through the earth. • Seismometers located throughout the world receive these waves. • Knowing the precise time of arrival and the time of an earthquake, each wave’s velocity along a specific path can be calculated. • After many earthquakes and knowing the velocities along tens of thousands of different paths, a computer can estimate the wave speed at every point inside the earth. • Higher than normal speed implies either colder or denser rock at that point. • Earthquake waves travel faster under continents. • Some increases in speed are too great to be caused entirely by colder temperatures.
Almost 90% of all earthquake energy is released under trenches. • Earthquakes often occur near sloping planes, called Benioff zones, that intersect a trench. • These earthquake zones enter the mantle at 35°–60° angles below the horizontal and extend to depths of about 420 miles.
A fault is a long, deep fracture in the ground along which the opposite sides have slipped relative to each other. • During an earthquake, opposite sides of a fault “unlock” and rapid sliding begins. • If the side of a fault nearest a distant seismometer moves toward the seismometer, a compression wave will be detected first. • If that side moves away from the seismometer, a tension wave will be detected first. • By examining the first wave to reach many seismometers, one can deduce the orientation of the fault plane and whether the earthquake was triggered by compression or tension. • Earthquakes near trenches are almost always due to horizontal tension failures at right angles to the trench axis. • Measurements also show that microearthquakes on the ocean floor tend to occur at low tide.
A prominent feature on all ocean floors is the Mid-Oceanic Ridge. One characteristic of the ridge figures prominently in the two competing theories for how trenches formed. As explained in the preceding chapter, the ridge is cracked in a strange pattern. Some cracks are nearly perpendicular to the ridge axis, while other cracks are parallel to it. Their shapes and orientation are best explained by the stretching of the ridge. What would stretch the ridge in two perpendicular directions? (These cracks are easily seen along the Mid-Oceanic Ridge in this picture.
More than 20,000 submarine volcanoes, called seamounts, litter the Pacific floor. • Some rise almost as high from the surrounding seafloor as Mount Everest rises above sea level. • Strangely, the Atlantic has few seamounts. • If one plate dives (subducts) beneath another, why aren’t seamounts and soft sediments scraped off the top of the descending plate?
About 2,000 flat-topped seamounts, called tablemounts, are 3,000–6,000 feet below sea level. • Evidently, as these volcanoes tried to grow above sea level, wave action planed off their tops. • Either sea level was once much lower, or ocean floors were higher, or both. • Each possibility raises new and difficult questions.
Enormous amounts of melted basalt, called flood basalts, have spilled out on the earth’s surface, especially in the Pacific. • They will help us test theories of trench formation. • A typical “spill” could cover the eastern United States to the height of the Appalachian Mountains—from Atlanta to New York City and from the Appalachian Mountains to the Atlantic Ocean. • More than a dozen of these convulsions have occurred at different places on earth, dwarfing in volume the total magma used to form all volcanic cones.
Theories Attempting to Explain the Origin of Oceanic Trenches • Two broad theories include an explanation for how oceanic trenches formed. • Each explanation will be described as its advocates would. • Then we will test these conflicting explanations against physical observations and requirements.
(A) Before the flood, the weight of rock and water, pushing down on the subterranean chamber’s floor, balanced the floor’s upward pressure. The rupture destroyed that equilibrium. Directly below the rupture, the imbalance grew as escaping, high-velocity water and crumbling, unsupportable walls widened the globe-encircling rupture hundreds of miles. Eventually, the imbalance overwhelmed the strength of the floor. First, the Mid-Atlantic Ridge buckled, or sprang, upward. As Europe and Africa slid eastward and the Americas slid westward (based on today’s directions), weight was removed from the rising floor, lifting it faster and accelerating the hydroplates even more. Pressure under the floor, represented by the large black arrows, naturally decreased as the floor rose. (B) Friction melted much of the inner earth as mass shifted toward the rising Atlantic. The melt lubricated the shifts, allowed gravitational settling, formed the earth’s inner and outer core, and increased earth’s spin rate. The solid floors of the Pacific and Indian Oceans subsided considerably as material shifted inside the earth toward the Atlantic, and as magma (flood basalts) spilled out. Where land subsided the most, directly opposite the rising Atlantic, the crust buckled downward forming trenches. Gravity is still smoothing out these imbalances—shifting (not drifting) material, including continents, toward trenches. Hydroplate Explanation for Trenches
The Hydroplate Theory • At the end of the flood phase, crumbling walls and erosion from escaping high-velocity water had widened the globe-encircling rupture to an average of about 800 miles. • Exposed at the bottom of this wide, water-filled gap was the subterranean chamber floor, about 10 miles below the earth’s surface. • Before the rupture, the gigantic pressure immediately under the floor corresponded to the weight of almost 10 miles of rock and 3/4 mile of water that pressed down on the floor. • Afterward, with 10 miles of rock suddenly gone, only the strength of the chamber floor and 10 miles of water on top of it resisted this upward pressure. • Consequently, as the rupture widened, the Mid-Oceanic Ridge suddenly buckled up.
The continental-drift phase began with hydroplates sliding “downhill” on a layer of water, away from the rising Mid-Atlantic Ridge. • This removed more weight from the rising portion of the subterranean chamber floor, causing it to rise even faster and accelerate the hydroplates even more.
As that part of the chamber floor rose to become the Atlantic floor, it stretched horizontally in all directions, as a balloon stretches when its radius increases. • This stretching produced cracks parallel and perpendicular to the Mid-Oceanic Ridge. • Because this began in what is now the Atlantic, the Mid-Atlantic Ridge and its cracks are the most prominent of the oceanic ridge system.
Obviously, the great confining pressure in the mantle and core did not allow deep voids to open up under the rising Atlantic floor. • So even deeper material was “sucked” upward. • Throughout the inner earth, material shifted toward the rising Atlantic floor, forming a broader, but shallower, depression on the opposite side of the earth—what is now the Pacific and Indian Oceans. • Just as the Atlantic floor stretched horizontally as it rose, the western Pacific floor compressed horizontally as it subsided. • Subsidence in the Pacific and Indian Oceans began a startling 20–25 minutes after the Atlantic floor began its rise, the time it takes stresses and strains from a seismic wave to pass through the earth. • Both movements contributed to the “downhill” slide of hydroplates.
Centered on the Pacific and Indian Oceans is the trench region of the western Pacific. • As material beneath the western Pacific was “sucked” down, it buckled downward in places forming trenches. • The Atlantic Ocean (centered at 21.5°W longitude and 10°S latitude) is almost exactly opposite this trench region (centered at 159°E longitude and 10°N latitude).
A simple, classic experiment illustrates some aspects of this event. • A cup of water is poured into an empty 1-gallon can. The can is heated from below until steam flows out the opening in the top. The heat is turned off, and the cap is quickly screwed on the top of the can, trapping hot steam in the metal can. As the steam cools, a partial vacuum forms inside the can. The can’s walls buckle in, forming wrinkles in the metal—“miniature trenches.”
The upper 5 miles of the earth’s crust is hard and brittle. • Below the top 5 miles, the large confining pressure will deform rock if pressure differences are great enough. • Consequently, as the western Pacific floor subsided (sank), it buckled into “downward creases,” forming trenches. • The hard crust and deformable mantle frequently produced deformations with an “arc and cusp” shape. • The brittle crust cracked and slid in many places, especially along paths called Benioff zones.
Notice that the trench axis will generally not be a straight line. Sediments (green) hide the top of a fault plane that should rise above the floor a few hundred feet at most. Other sediments (not shown) and flood basalts (dark gray) cover most of the western Pacific floor. The three large black arrows show the direction of the rising Atlantic and the forces that downwarped the mantle and hydroplate. Earthquakes occur on the many faults produced, especially in Benioff zones and at low tides. Most volcanoes are not above Benioff zones, but are in the center of the western Pacific where downwarping was greatest. Trench Cross Section Based on Hydroplate Theory
Deformations throughout the earth slid countless pieces of highly compressed rock over, along, and through each other, generating extreme friction—and, therefore, heat. • To appreciate the heat generated, slide a brick one foot along a sidewalk. The brick and sidewalk will warm slightly. Sliding a brick an inch but with a mile of rock squarely on top would melt part of the brick and sidewalk. Earth’s radius is almost 4,000 miles. Place a few thousand of those miles of rock on top of the brick and slide it only one thousandth of an inch. The heat generated would melt the entire brick and much of the sidewalk below. • Small movements deep inside the solid earth would melt huge volumes of minerals, especially those with lower melting temperatures.
Much of this magma (liquid rock) flowed up onto the deepening granite hydroplate in the western Pacific floor. (Researchers have begun to detect this granite under the floors of the Pacific and Indian Oceans.) • The more magma that flowed up into this basin, the more the Pacific hydroplate sank. • Hydroplates sliding downhill, away from the rising Atlantic floor, slid toward this vast and very deep pool of magma. • Other magma gushed out on the continents as flood basalts. • Some magma, unable to escape fast enough, is trapped in magma chambers. • The rest constitutes the earth’s liquid outer core.
Let’s suppose the inner earth initially had a more uniform mixture of minerals throughout. • Melting, as described above, would cause denser minerals to settle and lighter minerals to rise, a process called gravitational settling. • This would generate more heat and produce more melting and gravitational settling—followed by more heating, melting, and settling. • After many such cycles, the earth’s core would form with the densest minerals settling to form the solid inner core and the melt rising to form the liquid outer core.
This frictional heating, internal melting, and gravitational settling of the denser components would have increased earth’s rotational speed. • Today, the earth spins 365.256 times each year, but there are historical reasons for believing a year once had 360 days.
We see here in this picture that skaters spin faster as they become more compact. Likewise, as denser minerals settled through the magma toward the center of the earth, the inner core spun faster than the outer earth and the melt moved upward. The inner core is still spinning faster (0.4° per year), because the liquid outer core allows slippage between the faster inner core and the slower outer earth. Other evidence supports these dramatic events.
Gravity is the basic driving mechanism that formed trenches and slowly shifts the crust. • Gravity always tries to make the earth more spherical. • If you suddenly removed a bucket of water from a swimming pool (or even a 10-mile-thick layer of rock lying above what is now the Atlantic floor), gravity would act to smooth out the irregularity. • Because massive volumes of rock inside the earth do not flow as fast as water in a swimming pool, pressure deficiencies, which we might think of as slight partial vacuums, still exist under trenches. • Today—especially at low tide—mantle material flows very slightly in under trenches to reduce these “partial vacuums.” • This stretches the crust above, produces extensional earthquakes near trenches, shifts plates toward trenches, and makes the earth measurably rounder.
Both the hydroplate theory and the plate tectonic theory are explained as their advocates would explain the theories. One should critically question every detail of both theories, and not accept either until the evidence has been weighed.
The Plate Tectonic Theory • The earth’s crust is broken into rigid plates, 30–60 miles thick, each with an area roughly the size of a continent. Some plates carry portions of oceans and continents. Plates move relative to each other over the earth’s surface, an inch or so per year.
Internal heat circulates the mantle causing continental-size plates to drift over the earth’s surface. Consequently, material rises at oceanic ridges (forcing the seafloor to spread), so plates must subduct at oceanic trenches, allowing layered sediments (shown in yellow) to collect. According to plate tectonics, earthquakes occur where subducting plates slide (Benioff zones) and at other plate boundaries. This theory says subducting plates also melt rock, and the magma rises to form volcanoes. [Actually, most volcanoes are not above Benioff zones. If this theory is correct, the yellow sediments hide a cliff face that is at least 30 miles high and the trench axis should be a straight line. W.B.] Plate Tectonic Explanation for Trenches
Heat is the basic driving mechanism that formed trenches and moves plates. • Just as hot water circulates in a pan on a stove, hot rock circulates slowly inside the earth’s mantle. • Radioactive decay warms some parts of the mantle more than others. • The warmer rock expands, becomes less dense (more buoyant), and slowly rises, as a cork rises when submerged in water. • Sometimes, plumes of hot rock rising from the outer core break through the earth’s crust as flood basalts. • Conversely, relatively cold rock descends. • Rising and descending rock inside the mantle forms circulation cells (convection cells) which drag plates forward. • Currents within the mantle rise at oceanic ridges, create new crust, and produce seafloor spreading.
Because new crust forms at oceanic ridges, old crust must be consumed somewhere. • This happens when two plates converge. • The older plate, having had more time to cool, is denser. • Therefore, it sinks below the younger plate and subducts into the mantle, forming a trench. • A cold, sinking edge will pull the rest of the plate and enhance circulation in the mantle. • Earthquakes occur under trenches when subducting plates slip along Benioff zones. • At great depths, subducting plates melt, releasing magma which migrates up to the earth’s surface to form volcanoes. • Of course, such slow processes would require hundreds of millions of years to produce what we see today.
Final Thoughts • Thomas Crowder Chamberlin, former president of the University of Wisconsin and the first head of the Geology Department at the University of Chicago, published a famous paper in which he warned researchers not to let one hypothesis dominate their thinking. • Instead, they should always have or seek multiple working hypotheses, especially in fields, such as geology, where much remains to be learned. • Chamberlin stated that testing competing hypotheses or theories sharpens one’s analytical skills, develops thoroughness, reduces biases, and helps students and teachers learn to discriminate and think independently rather than simply memorize and conform.
Chamberlin said the dangers of teaching only one explanation are especially great in the earth sciences. • The explanation for oceanic trenches is an example. • The plate tectonic theory dominates the earth sciences. • A recent survey of scientists selected it as the most significant theory of the 20th century. • Undoubtedly, Darwin’s theory of organic evolution would be voted as the most significant theory of the 19th century. • Both dominate, despite their growing scientific problems, because schools and the media ignore competing explanations. • Chamberlin warned about the comfort of conformity.
The subject of “trenches” offers students and teachers a great opportunity. • More information can be added as student interest, time, and ability permit. • Relevant topics could include: fossils, volcanoes, earthquakes, gravity anomalies, flood basalts, seismic tomography, arcs, cusps, tides, the core-mantle boundary, and many others. • Students can examine and compare the evidence and tentatively decide which is the stronger theory. • Teachers and parents have a simple, satisfying task: provide information, ask questions, challenge answers, and allow students the excitement of discovery.
PREDICTION 7: A 10-mile-thick granite layer (a hydroplate) will be found a few miles under the western Pacific floor. PREDICTION 8: Fossils of land animals, not just shallow-water plant fossils, will be found in and near trenches.
PREDICTION 9: Precise measurements of the center of the western Pacific floor will show it is rising relative to the center of the earth, because plates are still shifting. PREDICTION 10: When greater precision is achieved in measuring the inner core’s rotational speed, it will be found to be slowing relative to the rest of the earth.
PREDICTION 11: A well-designed blind test will not support McDougall’s age sequences for seven Hawaiian volcanoes.
During a 1964 earthquake in Niigata, Japan, the ground turned to a dense liquidlike substance, causing this empty concrete tank to float up from just below ground level. This was the first time geologists identified the phenomenon of liquefaction, which had undoubtedly occurred in other large earthquakes. Liquefaction has even lifted empty tanks up through asphalt pavement and raised pipelines and logs out of the ground. In other words, buried objects that are less dense than surrounding soil rise buoyantly when that soil liquefies. What causes liquefaction? What would happen to buried animals and plants in temporarily liquefied sediments? Floating Tank
During the above earthquake, building number 3 sank in and tipped 22 degrees as the ground partially liquefied. Another building, seen at the red arrow, tipped almost 70 degrees, so much that its roof is nearly vertical. Sinking Buildings
SUMMARY • Liquefaction—associated with quicksand, earthquakes, and wave action—played a major role in rapidly sorting sediments, plants, and animals during the flood. • Indeed, the worldwide presence of sorted fossils and sedimentary layers shows that a gigantic global flood occurred. • Massive liquefaction also left other diagnostic features such as cross-bedded sandstone, plumes, and mounds.
Sedimentary rocks are distinguished by sharply-defined layers, called strata. • Fossils almost always lie within such layers. • Fossils and strata, seen globally, have many unusual characteristics. • A little-known and poorly-understood phenomenon called liquefaction (lik-wuh-FAK-shun) explains these characteristics. • It also explains why we do not see fossils and strata forming on a large scale today.