A new view into the mantle beneath the East Pacific Rise reveals how mid-ocean ridges work.
April 19, 2012 --
Forty years ago this week, the crew of Apollo 16 captured this image of Earth rising above the lunar landscape. The Apollo missions enabled humanity to see for the first time our planet as it appears from space. As Apollo 13 commander Jim Lovell once said: “When I was orbiting the moon and could put my thumb up to the window and completely cover the Earth, I felt a real sense of my own insignificance. Everything I'd ever known could be hidden behind my thumb.” As we approach Earth Day on April 22, we look at the efforts of people throughout the ages to explore, understand and portray our world and its place in the Universe.
Trustees of the British Museum (image rotated
Babylonia Believed to be the earliest known representation of Earth, this stone tablet from Babylon shows the world as a disc, surrounded by a ring of water called the "Bitter River." The world is dominated by the area surrounding Babylon itself, and the Euphrates River bisects most of the inner circle. Unearthed in southern Iraq in the late 1800s, the tablet is housed in the British Museum.
Sixteenth-century interpretation of Ptolemy's
Celestial Spheres In his 2nd century treatise, the "Almagest," Claudius Ptolemy proposed an explanation for the apparent movement of stars and planets, in which Earth was central and immovable, and surrounded by, at progressively greater distances, the Moon, Mercury, Venus, the Sun, Mars, Jupiter, Saturn and a sphere of ‘fixed stars.’ This geocentric view of the cosmos did not meet its first real challenge until Copernicus proposed that the planets revolved around the Sun, and Galileo used his telescope to observe the phases of Venus.
Library of Congress, via the History Blog
Flat Earth The Greek philosopher Aristotle determined that Earth was spherical and not flat almost 2,500 years ago. The notion of a flat earth retained at least a few die-hard devotees for a surprisingly long time. For example, this 1893 map by Orlando Ferguson, recently acquired by the Library of Congress, cites “Scripture that condemns the globe theory” and promotes a book that “knocks the globe theory clean out.”
ANALYSIS: What if Earth Were a Cube?
De Costa, B.F. (September 1879). "The Lenox G
Lenox Globe It is popularly believed that ancient cartographers filled in unknown and unexplored areas of the world with the phrase ‘Here be dragons’. In fact, only one known ancient map – the so-called Lenox Globe, which is believed to date to around 1510 - displays the phrase ‘HC SVNT DRACONES’, from the Latin “hic sunt dracones.” (The phrase is written near the equator on the eastern cost of Asia.) Some nineteenth-century writers, however, believed that it referred, not to dragons, but to the ‘Dagroians’, a people who “feasted upon the dead and picked their bones.”
PHOTOS: Sea Monsters Real & Imagined
Image Database of the Kano Collection, Tohoku
Terra Australis Incognita In this copy of a 1602 map that was created on behalf of China’s Wanli emperor by Italian Matteo Ricci and collaborators, the familiar outlines of most of the world’s continents are coming into shape, although obviously many details remain unfinished. To the map’s makers, however, the likes of Australia, New Zealand and Antarctica are not even figments of the imagination, replaced instead by an enormous southern landmass. The notion of an unknown southern land – a terra australis incognita - was first mooted by Aristotle in 322 BCE; not until 1820 did Fabian von Bellingshausen become the first man to see the Antarctic continent.
South Pole For centuries, gaps in maps were filled by explorers who set out across land and sea, often at immense personal risk. The true nature of “Terra Australis” had long been established by the time Robert Falcon Scott and comrades stood at the South Pole on Jan. 17, 1912; but existing knowledge could not diminish the terrible toll the conditions exacted on the men. “Great God!” wrote Scott in his journal, “this is an awful place.” All five members of Scott’s polar team died before they could reach their base camp.
PHOTOS: Forgotten Discoveries of Scott's Antarctica
Moscow at night Time and technology have enabled us to explore, not just across the surface of the globe or even beneath its waves, but from on high. Here, Moscow is seen at night from the International Space Station, flying at an altitude of approximately 240 miles on March 28, 2012. A solar array panel for the space station is on the left side of the frame. The Aurora Borealis, airglow and daybreak frame the horizon.
Pale Blue Dot In contrast to earlier suppositions about our place in the firmaments, we know now that our globe is not at the center of the cosmos, and that other celestial bodies are not attached to interlaced spheres that rotate around us. We are but one world among many, in one solar system among many, in one galaxy among many. In this image, taken by the Voyager I spacecraft from a distance of 4 billion miles, Earth is but a speck – a pale blue dot – in the cosmic night.
NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring
Blue Marble If satellite images of Earth now seem almost routine, they never lose their ability to enthrall. This picture of the western hemisphere was captured on January 25 by NASA’s latest Earth observation satellite, Suomi NPP. By February 1, it had registered over 3 million views on Flickr – testament to the beauty and fascination of our Blue Marble.
PHOTOS: Earth's Blue Marble Beauty
One of the Earth's best-ever baby pictures reveals how crust forms at the biggest volcanic feature on the planet.
The detailed look at molten magma beneath a mid-ocean ridge, one of the giant undersea cracks that ring the globe like seams on a baseball, sheds light on the driving forces behind plate tectonics. The results of the study are published yesterday (March 27) in the journal Nature.
Most of the Earth (70 percent) is covered by oceanic crust, mainly basalt, formed from lava that burbles out ofmid-ocean ridges. The ridges run across some 40,000 miles (65,000 kilometers) of the seafloor. They mark where crust pulls apart, leaving space for hotter mantle rock underneath to rise up and melt.
But the particulars of this process have been fuzzy. Geoscientists lacked clear images of structures beneath the mid-ocean ridges, which would reveal how magma moves to the surface.
"The upper mantle melting region is a deep and difficult target," said Kerry Key, lead study author and a seismologist at the Scripps Institution of Oceanography in San Diego.
Key and his co-authors peered into this mysterious zone beneath the northern East Pacific Rise, a fast-spreading mid-ocean ridge near Costa Rica.
Plates pull apart, make new crust
Their new image is akin to a sonogram of the Earth, but instead of sound waves, the researchers used a technique called electromagnetic imaging, which looks for subtle variations in Earth's naturally occurring electric and magnetic fields. The variations reveal different layers and liquid beneath the surface.
Key discovered a symmetrical, narrow melt zone beneath the East Pacific Rise. This implies the mantle is simply filling space created by spreading plates, he said. If the rising mantle were pushing the plates apart, there would likely be evidence of localized convection, such as broader, asymmetrical melting.
The study supports one of the dominant theories (the passive flow model) of how mid-ocean ridges work, the researchers said. Earth's crust is like a giant conveyor belt, with plates spreading apart at mid-ocean ridges and diving into the mantle for recycling at subduction zones, Key explained. The plates ride on giant convection cells in the mantle, but mid-ocean ridges aren't linked to these massive swirls. Instead, the ridges' localized melting comes from the space created by slip-sliding tectonic plates, geologists think. However, there's ongoing debate as to whether the driving force is pull at subduction zones — the passive flow model — or push from magma coming up at ridges. [Infographic: Tallest Mountain to Deepest Ocean Trench]
"Our data looks just like the passive flow model," Key told OurAmazingPlanet. "It agrees with what everybody thinks should be going on, but we haven't had a good image before. It looks like something somebody would have drawn in a textbook based on what we thought was going on."
How the mantle melts
The results also confirm models of mantle melting based on rocks scraped off the seafloor at mid-ocean ridges. Sometimes, pieces of the mantle are carried up to the surface with erupting lava, giving geologists a glimpse into this inaccessible part of the Earth.
The first gooey mantle rocks to melt have a high concentration of impurities, such as carbon dioxide and then water, Key said. Finally, between a depth of 37 miles (60 km) and the surface, the melt really gets going, with about 10 percent of the mantle transformed to liquid rock. Just below the surface, a vertical channel to the east of the ridge connects the magma reservoir to the fissures and volcanoes at the surface.
"This really helps to fill out the picture of how ridges work and how the melt gets from where it's formed to the surface," said Don Forsyth, a marine geophysicist at Brown University, who was not involved in the study.
However, Forsyth would like to see additional surveys along the north-south ridge axis to confirm there's no mantle convection. "I think they have strong supporting evidence for passive upwelling, but the symmetry by itself doesn't necessarily prove that it's passive," he told OurAmazingPlanet.
More from LiveScience:
50 Amazing Facts About Earth
Have There Always Been Continents?
How to Journey to the Center of the Earth
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