Skip to comments.Sea floor records ancient Earth
Posted on 03/23/2007 11:06:03 PM PDT by Ernest_at_the_Beach
A sliver of four-billion-year-old sea floor has offered a glimpse into the inner workings of an adolescent Earth.
The baked and twisted rocks, now part of Greenland, show the earliest evidence of plate tectonics, colossal movements of the planet's outer shell.
Until now, researchers were unable to say when the process, which explains how oceans and continents form, began.
The unique find, described in the journal Science, shows the movements started soon after the planet formed.
"Since the plate tectonic paradigm is the framework in which we interpret all modern-day geology, it is important to know how far back in time it operated," said Professor Minik Rosing of the University of Copenhagen and one of the authors of the paper.
Sea floor is not normally preserved for more than 200 million years
Sea floor is not normally preserved for more than 200 million years
Professor John Valley, a geologist at the University of Wisconsin, Madison described the work as "significant" and "exciting".
"If these observations are substantiated it will be a significant line of new evidence indicating that plate tectonics was active and familiar as early as 3.8 billion years ago," he said.
"That really is an important conclusion."
Crack and spread
Plate tectonics is a geological theory used to explain the observed large-scale motions of the Earth's surface.
The relatively thin outer shell of the planet is composed of two layers: the lithosphere and the asthenosphere.
The lithosphere - made up of the outer crust and the top-most layer of the underlying mantle - is broken up into huge plates; seven major plates and several smaller ones.
These float above the asthenosphere and move in relation to one another.
Today, oceanic crust is created at plate boundaries known as mid-ocean ridges, where magma rises from the asthenospehere through cracks in the ocean floor, cools and spreads away.
As it moves away from the spreading centre towards the edges of the oceans it becomes cooler, denser and eventually starts to sink back into the mantle to be recycled.
"Sea floor is not normally preserved for more than 200 million years," said Professor Rosing.
Most is destroyed at subduction zones, such as those found along the edge of the Pacific Ocean, where oceanic crust plunges under the buoyant and long-lived continental crust.
However, in certain circumstances, fragments of the sea floor known as ophiloites are preserved when they are scraped on to the land.
This exceptional process typically occurs when continental crust begins to be sucked into a subduction zone, clogging the system.
"It goes down into the subduction zone until the buoyancy of the continent arrests the process of subduction," explained Eldridge Moores, emeritus professor of geology at the University of California, Davis.
"The continent then pops back up, preserving a little bit of the overriding wedge of oceanic crust and mantle that was on the overriding plate."
Ophiolites are found today in Cyprus and Oman and show a distinctive structure.
At their base, crystalline rocks preserve the top layer of the mantle. Above, "fossilised" magma chambers give way to a layer of stacked vertical pipes, known as sheeted dykes.
These represent the conduits through which magma is extruded onto the sea floor as pillow lavas, bulbous lobes of basaltic rock that form when lava cools quickly in contact with water.
The rocks analysed in Greenland are found in an area known as the Isua Belt, a zone of intensely deformed rocks in the southwest of the island that geologists have pored over for decades.
The ophiolite structure was mapped between outcrops covering 4-5km (2.5-3 miles) and shows the correct sequence of layers found in an ophiolite, except the lowest mantle portion.
"You can actually recognise features that formed in a couple of minutes, 3.8 billion years ago - a quarter of all time - and you can actually go and touch them with your hand," said Professor Rosing.
Crucially, they show well preserved sheeted dykes and pillow lavas, clear evidence to many that these are the ancient remains of sea floor created by processes seen today.
"What this tells you unequivocally is that the process of sea-floor spreading that we observe today appears to be present in one of, if not the, oldest sequence of rocks on Earth," said Professor Moores. "That is a significant milestone."
In particular, it pushes back the oldest known evidence of plate tectonics by at least 1.3 billion years and gives scientists clues to the processes that formed the surface of the Earth today.
Although the structures and processes that led to their formation would be similar to the modern era, they would not be exactly the same.
The young Earth was much hotter than now, and as it shed heat, it put many of the tectonic processes into overdrive.
"If you had plate tectonics you probably would have had more plates, moving faster, and they probably would have been thinner," said Professor Moores.
The rate of recycling of oceanic crust would therefore have been even quicker than today, making the fact that the rocks in Isua are preserved at all even more extraordinary.
"These fragments are extremely rare," said Professor Rosing. "It's just very exciting when you get one of these glimpses when you can look back nearly four billion years in time."
Actually, that conclusion presupposes that there was a Gondwanaland in the first place.Fossil Mantle Plume Under South America"The conduit appears to have remained geographically fixed with respect to the overlying continent despite thousands of kilometers of South American plate motion. This observation runs contrary to a major tenet of plate tectonic theory -- that the motion of lithospheric plates is essentially independent of flow in the upper mantle beneath the plates -- and implies that the upper mantle and the overlying South American continent have remained coupled since the breakup of the Gondwanaland super-continent and opening of the South Atlantic Ocean some 120 million years ago.
by William Corliss
But What About The Hawaiian Volcanic Chain?P.D. Ihinger is challenging the well-entrenched "Hawaiian-volcanic-chain" theory. For example, the Hawaiian volcanoes do not line up exactly. There are dozens of short, overlapping segments rather than a continuous trace across the Pacific basin. On the map, you will also see a sharp dog-leg in the trace. Further, the volcanoes Mauna Loa and Mauna Kea, only 40 kilometers apart, disgorge lavas that are distinctly different.
by William Corliss
Researchers Suggest Answer to Geological PuzzleSMU geologists Rebecca Ghent and Douglas Oliver studied the location of the major hot spots and determined that a disproportionate number occur at latitudes between 20 and 30 degrees north and south of the equator. Their observation became much more significant when the hot spots were weighted according to the amount of volcanic material that they produced. Statistical analysis shows that the likelihood of this distribution arising by chance is less than one percent... Oliver and Ghent said their observation may shed light on other geological phenomena, such as the development of superplumes, which are clusters of mantle plumes arriving together at the Earth's surface. Scientists believe that massive superplume events are responsible for some of the major changes that have occurred on the Earth, such as the breakup of the supercontinent known as Pangea into the present continents.
National Science Foundation
The Earth Is Expanding And We Don't Know Why"The geological and geophysical implications of such Earth expansion are so profound that most geologists and geophysicists shy away from them. In order to fit with the reconstruction that seems to be required, the volume of the Earth was only 51 per cent of its present value, and the surface area 64 per cent of that of the present day, 200 million years ago. Established theory says that the Earth's interior is stable, an inner core of nickel iron surrounded by an outer layer that behaves like a fluid. Perhaps we are completely wrong and the inner core is in some state nobody has yet imagined, a state that is undergoing a transition from a high-density state to a lower density state, and pushing out the crust, the skin of the Earth, as it expands."
by William R. Corliss
Science Frontiers #37
(Owen, Hugh; "The Earth Is Expanding and We Don't Know Why, "New Scientist, p. 27, November 22, 1984.)
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Now that is serious...more so than Global warming...are we causing this expansion with our constant drilling looking for oil?
I think the problem is all the laser beans in use in CD players, computers, and the like.
The Tarim APWP Paradox
Gilder, S -- Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
Cogne, J -- Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
Courtillot, V -- Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
Chen, Y -- Universite d'Orleans, BP6749, Orleans, 45067 France
Gomez, J -- Institut de Physique du Globe de Paris, 4 place Jussieu, Paris cedex 05, IdF 75252 France
Central Asia boasts one of the world's densest regions sampled for paleomagnetic data thanks to numerous sections of well-exposed rocks possessing stable remanent magnetizations. Such is the case for the Tarim craton, which is represented by a quasi-continuous time sequence of paleomagnetic poles since the Permo-Carboniferous. Most of these poles are derived from studies demonstrating positive fold and-or reversal tests, with N equal to or greater than 6 sites or 50 samples. Samples collected from Permo-Carboniferous rocks usually have reverse polarities, and samples collected from Cretaceous rocks usually have normal polarities, consistent with the geomagnetic polarity time scale. Despite the apparent excellent quality of the paleomagnetic data from Tarim, they impose geologically unrealistic tectonic displacements when compared to the Eurasian and-or Indian APWPs. This leads to the Tarim APWP paradox: is there a problem (inclination shallowing, overprinting, etc.) with the plentiful Tarim data, or is the Eurasian APWP not representative of the land east of the Ural Mountains? If the latter is true, then previous tectonic reconstructions must be reconsidered. If the former is true, then when/how can we rely on the paleomagnetic data? We present arguments showing that both scenarios have their pros and cons.
Freeze-fry from the snowball EarthOn Dec. 14, Daniel Schrag of Harvard University] presented recent observations in favor of the snowball Earth at the American Geophysical Unions meeting in San Francisco, which lasted from Dec. 1217...
by Christina Reed
But some scientists still remain skeptical. During a presentation at the AGU meeting, Gregory Jenkins of Pennsylvania State University supported what is called the high obliquity hypothesis over the snowball Earth hypothesis. Jenkins argued that climatic conditions during the Neoproterozoic resulted from a severe tilt of Earth on its axis, possibly caused by the impact of the planetoid that formed the moon. Although a tilted Earth may have cooled the equator and warmed the poles, the theory is still hotly debated as an explanation because it seems to require low-latitude cooling prior to Neoproterozoic glacial events.
Schrag and others defend the snowball Earth theory, saying it brings context to mysteries that have plagued the geological community for decades. It explains how glaciers survived in the tropics, how iron-rich rock emerged in an oxygen-enriched world and how warm-water carbonate rocks found themselves perched atop glacial deposits. It may even explain the explosion of life in the Cambrian.
[P]aleomagnetism specialist Joe Kirschvink of Caltech linked the effect a frozen Earth would have on the hydrological cycle to a buildup of volcanic carbon dioxide. If ice covered the oceans to the equator, it would block off evaporation, dry up the clouds and deny water the chance to erode the land. Carbon dioxide would build up in the atmosphere instead of being washed out by rain and carried back to the oceans as carbonates from land. With enough carbon dioxide, the "snowball Earth" would then become a hothouse, Kirschvink said in a 1992 paper published in the book The Proterozoic Biosphere.
In 1992, Kenneth Caldeira of Lawrence Livermore National Laboratory and James F. Kasting of Pennsylvania State University calculated that the amount of carbon dioxide needed to reverse the snowball effect would be 350 times present-day levels. With a global average of 50°C below zero, Earth would bake under extreme global warming to 4050°C in only a few thousand years.
Scientists Poke Holes in 'Snowball Earth' HypothesisFrank Corsetti of USC, a co-author on the study, said "this is the first real evidence that substantial photosynthesis occurred in the Earth's oceans during the extreme ice age 700 million years ago, which is a challenge for the snowball theory."
National Science Foundation
Press Release 05-173
September 29, 2005
The evidence does not prove large parts of the ocean remained free of sheet ice during the pre-Cambrian glaciation. Although unlikely, Olcott said it is possible one of the tiny "refugia" under the "Snowball Earth" hypothesis allowed such marine life to exist.
But, she said, "finding the one anomalous spot would be quite unlikely," adding that the samples she studied came from an extensive formation of rocks with similar characteristics.
"At what point does an enormous refugium become open ocean?" she asked.
Laser beans ? Is that a new brand from Bush's ?
Welcome to FR! If we manage to bring over everyone from there, we'll add, well, okay, maybe a couple of dozen... ;') Nice beginning to your profile page!
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