April 5, 2014
Heating Things Up: What Drives Elevation Along Mid-Ocean Ridges?
April Flowers for redOrbit.com - Your Universe Online
The contours of the Earth's crust are influenced by the high temperatures deep within the Earth's mantle, according to a new study published in Science. A team of researchers, led by Brown University, demonstrated that those temperature differences control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor.
Forming at the boundaries of tectonic plates, mid-ocean ridges circle the globe like seams on a baseball. Magma from deep within the Earth rises up to fill in the cracks between the plates as they move apart, creating fresh crust on the ocean floor as it cools. This new crust is thicker in some places than others, forming ridges with widely varying elevations. In some parts of the world, these ridges are deep in the ocean, miles beneath the surface. In other places such as Iceland, the ridge tops are exposed above the ocean's surface.
"These variations in ridge depth require an explanation," said Colleen Dalton, assistant professor of geological sciences at Brown. "Something is keeping them either sitting high or sitting low."
The research team discovered that the "something" was the temperature of the rocks deep below the Earth's surface.
At depths extending below 250 miles, the team was able to show that mantle temperatures along the ridges vary by as much as 250 degrees Celsius by analyzing the speeds of seismic waves generated by earthquakes. They found that, in general, higher points on the ridges are associated with higher temperatures, while lower points are associated with cooler temperatures. One unsurprising finding of this study is that volcanic hot spots along the ridges — such as volcanoes near Iceland, and the islands of Ascension and Tristan da Cunha — all sit above warm spots in the mantle.
"It is clear from our results that what's being erupted at the ridges is controlled by temperature deep in the mantle," Dalton told Brown University's Kevin Stacey. "It resolves a long-standing controversy and has not been shown definitively before."
The mid-ocean ridges function as a window to the interior of the planet for geologists by providing clues about the properties of the mantle below.
A thicker crust is suggested by a higher ridge elevation, indicating that a larger volume of magma erupted at the surface. The new study explains that this excess magma could have been caused by very hot temperatures in the mantle. The fact that hot mantle material is not the only way to produce excess magma, however, presents a challenge to this theory. The amount of melt is also controlled by the chemical composition of the mantle. Some rock compositions melt at lower temperatures, allowing for a larger volume of molten rock. Because of this, it has been unclear for the last several decades whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.
Dalton's team introduced two additional data sets to help them distinguish between these two possible scenarios.
One data set was the chemistry of basalts, the rock that forms from the solidification of magma at the mid-ocean ridge. Basalt compositions can vary greatly depending on the temperature and composition of the mantle material from which they're derived. To create this data set, the researchers analyzed almost 17,000 basalts formed along mid-ocean ridges worldwide.
Seismic wave tomography made up the second data set. During earthquakes, seismic waves pulse through the rock of the crust and the mantle. Scientists measure the velocity of those waves to gather data about the characteristics of the rocks through which they passed. "It's like performing a CAT scan of the inside of the Earth," Dalton added. Temperature has a great effect on seismic wave speeds, with waves propagating more quickly in cooler rocks than in hotter ones.
By comparing the seismic data from hundreds of earthquakes to data on elevation and rock chemistry from the ridges, the team found correlations which revealed that temperatures deep in the mantle varied between 1,300 and 1,550 degrees Celsius underneath about 38,000 miles 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."
The results demonstrated that as mantle temperatures fall, so too do ridge elevations. The hottest point beneath the ridges was found to be near Iceland — also the site of the ridges' highest elevation — while the lowest temperatures were found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean.
There has been a long-standing debate in the scientific community about whether a mantle plume — a vertical jet of hot rock originating from deep in the Earth — intersects the mid-ocean ridge in Iceland. The findings of this study provide strong support for this theory, as well as for mantle plumes being the culprit for the excess magma volume in all regions with above-average temperatures near volcanic hot spots.
The Earth's mantle does not sit still, despite being made of solid rock. It is constantly undergoing convection, where material from the depths of the Earth churns towards the surface and back again.
"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."
There are two main factors in the mechanism of convection: variations in the composition of the mantle and variations in its temperature. Dalton says that their findings point to temperature as a primary factor in how convection is expressed on the surface.
"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."
Dalton says that the findings will be useful for future research using seismic waves because the temperature readings as indicated by seismology were backed up by the other datasets. This allows them to be used to calibrate seismic readings for places where geochemical samples aren't available, allowing scientists to estimate temperature deep in the Earth's mantle all over the globe.
Image 2 (below): Scientists have found that temperature deep in Earth's mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth's crust. Credit: Dalton Lab / Brown University