Posted by Paul
(here)
Posted by Amzer Zo
(here)
I guess the confusion comes from the use of the term "expansion" as in "oceanic expansion" and which is taken to mean that the earth is "expanding" as in increasing its volume and therefore increasing its total surface area.
If that were the case, subductions would seldom occur and earth's surface would keep "cracking."
However, subductions do occur to accommodate for the accretion of fresh oceanic crust at mid-oceanic ridges. This, therefore, is more in favor of a fairly constant volume for earth as well as earth keeping a fairly constant and nearly fixed total surface area.
To be clear to others reading our discussion, even though I have no new insights or evidence, I remain of the view that the earth's mass is steadily increasing, perhaps doubling over the last 100 million years (give or take a big bunch), and that this expansion mostly shows up in the undersea "rifts", where existing oceanic plates are pushed apart faster than they are pushed below the older continental plates at the ocean's edge.
Well, well, well...
New research indicates that oceanic rifting started almost 4 billion years ago... as indicated by its corresponding volcanic melt product at subductions zones:
From: http://www.redorbit.com/news/science...enland-011913/
Proof Of Ancient Subduction Discovered In Volcanic Rocks
January 19, 2013
Image Caption: Image of southwest Greenland by Jacques Descloitres, MODIS Land Rapid Response Team, courtesy of NASA Visible Earth.
redOrbit Staff & Wire Reports – Your Universe Online
Researchers for the first time have discovered evidence supporting the theory that the processes that act as catalysts for volcanic activity today are similar to those that occurred nearly four billion years ago.
Writing in the journal Geology, Frances Jenner of the Carnegie Institution for Science and colleagues report that 3.8 billion-year-old volcanic rocks recovered from an island in southwestern Greenland support previous geochemical studies that have suggested that subduction-style tectonic activity had been occurring as early as the Eoarchean era.
Those studies had used “similarities between modern subduction zone magmas and those erupted about 3.8 billion years ago,” to argue that subduction-related tectonics had been occurring early on in our planet’s history, the Institution said in a statement Friday.
Previously, scientists had been unable to “locate any suites of volcanic rocks with compositions comparable to modern mid-ocean ridge or oceanic island magmas that were older than 3 billion years and were also free from contamination by continental crust,” they added. However, rocks discovered by Jenner’s team are the “missing piece of the puzzle” because they are comparable to modern-day samples obtained from oceanic islands.
The subduction process takes place at plate boundaries, and begins when the upwelling and melting of the mantle at mid-ocean ridges and the eruption of new seafloor magmas spur on the continued production of oceanic crust, the researchers explain. As that crust moves away from mid-ocean ridges, it cools and becomes denser than the mantle beneath it, ultimately sinking back into the mantle and triggering additional eruptions.
“Volcanic eruptions that are triggered by subduction of oceanic crust are chemically distinct from those erupting at mid-ocean ridges and oceanic island chains, such as Hawaii,” representatives from the Institution said. “The differences between the chemistry of magmas produced at each of these tectonic settings provide ‘geochemical fingerprints’ that can be used to try to identify the types of tectonic activity taking place early in the Earth’s history.”
“The Innersuartuut samples may represent the world’s oldest recognized suite of oceanic island basalts, free from contamination by continental crust,” added Jenner said. “This evidence strengthens previous arguments that subduction of oceanic crust into the mantle has been taking place since at least 3.8 billion years ago.”
Source: redOrbit Staff & Wire Reports - Your Universe Online
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Here is another research which demonstrate the existence of continental material (crust) as far back as 2 billion years ago:
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Scientists spot possible remains of "Rodinia," ancient lost microcontinent
http://www.sott.net/article/258877-S...microcontinent
Rebecca Boyle
PopSci
Tue, 26 Feb 2013 14:59 CST
© United States Antarctic Program/Wikipedia --Supercontinent Rodinia This map of supercontinent Rodinia shows the ancient locations of the continents. "Mauritia" is sandwiched between what is now India and Madagascar.
Tourists vacationing on the sunny isles of Reunion and Mauritius have no idea what secrets those sandy beaches hold. The islands could be hiding the remains of an ancient micro-continent, quietly torn apart between 50 and 100 million years ago, according to a new study. Scientists think they have spotted a fragment of a continent known as Mauritia.
The small strip of continent was once tucked tightly between the lands now known as India and Madagascar, back when those areas were packed into a supercontinent known as Rodinia. (It's the older and less-famous relative of supercontinent Pangaea.) Evidence of this sandwiched continent came from sand grains on Mauritius beaches, according to Trond Torsvik of the University of Oslo in Norway and colleagues.
Rodinia would have existed from the Precambrian era, about 2 billion years ago, to around 85 million years ago when plate tectonics broke it apart. Continental breakup is usually a mantle plume's doing--hot rock from within the Earth softens up tectonic plates, which eventually split. This is how the land masses now known as Madagascar, India, Australia and Antarctica broke up and migrated to their current locations on the planet. As this took place and the Indian Ocean formed, small fragments located on the edges of the rupture zone broke off--this is how the Seychelles came to be. Mini-continent Mauritius just wasn't so lucky, and it slipped beneath the waves, disappearing with time.
© GFZ/Steinberger - Ancient Lost Continent, Newly Discovered: The areas with topography just below the sea surface are now regarded as continental fragments. The colored track west of Reunion is the calculated movement of the Reunion hotspot. The black lines with yellow circles and the red circle indicate the corresponding calculated track on the African plate and the Indian plate, respectively. The numbers in the circles are ages in millions of years.
Much later, volcano eruptions spewed its remnants back to the top of the Earth's crust, Torsvik and colleagues write.
They examined sands in Mauritius that formed from eroded volcanic rocks, dating to about 9 million years ago. But when the team looked at these sand grains, they found ancient zircon minerals that were far more ancient--between 660 and 1,970 million years old. The presence of these ancient zircons points to microcontinent fragments churning up due to more recent volcanic activity, the team says.
The discovery of a possible microcontinent fragment suggests continental leftovers like the Seychelles might be more common than scientists thought. A paper describing the research is published this week in Nature Geoscience.
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