Temperature Greatly Influences Volcanic Activity Along Mid-Ocean Ridges
Submerged deep in the ocean is an underwater mountain system known as the mid-ocean ridge that contains the longest mountain range in the world and stands up high above the surrounding sea-floor. A large number of volcanic eruptions occur along mid-ocean ridges each year. The reason for this and the elevated ridge, according to a recent study, is the temperature difference deep within the Earth's mantle, said a press release Thursday . These findings have been published in the journal Science, on April 4.
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The uplifted sea-floor that is the mid-ocean ridge contains a ridge running along its spine formed by plate tectonics. Magma from the mantle of the Earth rises up in the form of lava in this ridge and cools to form a new crust. Layer upon layer of crust is formed resulting in an elevated floor. But the ridges are not of uniform thickness everywhere. In some places, the peaks are submerged deep down the ocean surface while in other places the ridge tops are exposed above the water's surface, like in Iceland.
"These variations in ridge depth require an explanation," said Colleen Dalton, assistant professor of geological sciences at Brown and lead author of this research. "Something is keeping them either sitting high or sitting low."
Temperature of rocks deep below the earth's surface was found to be the reason for the variation in ridges. The scientists analyzed the speeds of seismic waves generated by earthquakes and found that at a depth of 400 kilometers the mantle temperature along the ridges varied as much as 250 degrees Celsius. Ridges with high points showed high temperatures and ridges with low points showed lower temperatures. They also found that volcanic hot spots in Iceland, islands of Ascension, Tristan da Cunha, and others are all above warm spots in Earth's mantle.
"It is clear from our results that what's being erupted at the ridges is controlled by temperature deep in the mantle," Dalton said. "It resolves a long-standing controversy and has not been shown definitively before."
But there is another theory to the variation in ridge elevation. A hot mantle spews up magma to the surface forming ridges. So a hotter mantle would mean more magma. But a hot mantle is not the only way that more magma is produced. Under certain circumstances the solid ultramafic rock that forms the mantle undergoes partial melting and depending on the chemical composition of the rock large quantities of magma may be produced. Examples of these rocks are basalt, granite, etc. Scientists are still puzzled on whether the mid-ocean ridges are formed due to differences in the mantle temperature or chemical composition of the rocks.
Dalton and her colleagues have tried to solve this puzzle by introducing two additional data sets. One was by analyzing the chemical composition of basalt, which differs depending on the temperature and composition of the mantle material from which they're derived. Over 17,000 basalts formed along mid-ocean ridges were analyzed for their chemical composition.
The other data set was seismic wave tomography. During an earthquake, energy is released in seismic waves that travel throughout the Earth's interior. By measuring the velocity of those waves, scientists can gather data about the characteristics of the rocks through which they traveled. "It's like performing a CAT scan of the inside of the Earth," Dalton said.
Seismic velocity depends greatly on the temperature of the rocks in the crust and mantle. Surprisingly, hot areas slow down seismic waves and cold areas speed them up.
Dalton and her colleagues combined the seismic data from hundreds of earthquakes with data on elevation and rock chemistry from the ridges. Correlations among the three data sets revealed that temperature deep in the mantle varied between 1,300 and 1,550 degrees Celsius underneath about 61,000 kilometers of ridge terrain. "It turned out," said Dalton, "that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range."
This research shows that the rise or fall in the ridge elevation depends on rise or fall of the mantle temperature. The coolest point beneath the ridges was found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean. The hottest spot was near Iceland, which is also the ridges' highest elevation point.
This study corroborates the evidence of the Iceland plume, an elevation of anomalously hot rock, thought to lie deep in the mantle below Iceland and which perhaps intersects the mid-ocean ridge. In fact, this study showed that all regions with above-average temperature are located near volcanic hot spots, which points to mantle plumes as the culprit for the excess volume of magma in these areas.The Earth's mantle is in a state of constant flux. It undergoes mantle convection that is the slow creeping motion of Earth's solid mantle caused by convection currents carrying heat from the interior of the Earth to the surface .
"Convection is why we have plate tectonics and earthquakes," Dalton said. "It's also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth."
The chemical composition of the mantel and the variations in its temperature are the key factors behind Earth's convection and this study points to temperature as a primary factor in how convection is expressed on the surface, according to Dalton. "We get consistent and coherent temperature measurements from the mantle from three independent datasets," Dalton said. "All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth."
This is a huge step towards understanding the inner workings of the Earth and can be used to estimate the temperature in the Earth's mantle all over the globe.
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