Above: Magnified image of a ringwoodite crystal with a diameter of approximately 150 micrometres (Wikimedia Commons/Jasperox)
Oceans cover 71% of the Earth’s surface. Ranging in size and depth, they have been used by humans for thousands of years as a means of travel. But the water in the oceans has also travelled vast distances, including time spent deep below the surface of the Earth. As part of this journey, large amounts of water are trapped inside the mineral ringwoodite.
Did you know? Ringwoodite, a mineral key to the global water cycle, has been found in meteorites. Ringwoodite forms when the mineral peridot encounters the high temperatures and pressures present in the Earth’s mantle. The first direct evidence to demonstrate ringwoodite’s role in the global water cycle comes from a sample of the mineral found inside a diamond from Brazil that was carried to the surface during a volcanic eruption.
Since the surface of the Earth is made up of rocky plates which sit on top of the mantle, the peridot found in the Brazilian diamond was probably on one of these plates located at the bottom of the ocean. When the plate moved, the peridot would have been sent deep below the Earth’s surface, along with water from the ocean. Once the peridot made it to the mantle's transition zone, between 410 to 660 kilometres below the Earth’s surface, high temperature and pressure conditions would have transformed it into a denser mineral: ringwoodite. Eventually a diamond formed around the ringwoodite.
Did you know? Plate tectonics is the theory that describes how the Earth’s surface is made up of moving plates. Most researchers believe that these plates started moving about 3 billion years ago. Ringwoodite has the ability to absorb water, but not in liquid, solid or gas form. The same high temperature and pressure conditions that transform peridot into ringwoodite cause the water molecules to split, creating hydroxyl radicals (OH). These hydroxyl radicals are then absorbed in the pores of the ringwoodite, trapping the broken-down water inside the molecular structure of the mineral.
The ringwoodite trapped in the Brazillian diamond carried an amount of water equivalent to 1.5% of its total weight. Based on the total amount of ringwoodite estimated to be in the mantle, there could be at least as much water located deep below the surface of the planet as there is in the oceans.
If water was not cycled below the surface, the interior of the Earth would be a dry desert. In fact, huge amounts of water are brought to the surface by volcanoes, so the water in the mantle needs to be constantly replenished. Along with bringing water back to the surface, volcanic eruptions also release gases into the atmosphere and are an important part of the Earth’s rock cycle.
Did you know? The Earth’s mantle is made up of three zones: the upper mantle, the transition zone, and the lower mantle. Only the transition zone appears to contain significant quantities of water. Magma is molten rock, also called lava when it reaches the Earth’s surface following a volcanic eruption. The lava that eventually appears on the surface contains many minerals aside from ringwoodite. It also bubbles because of the gas produced below the surface. The presence of water deep below the Earth’s surface can dictate where volcanoes form, since water is an important factor in creating magma.
To better understand how magma forms in the transition zone, researchers from Northwestern University and the University of New Mexico analyzed seismic data related to earthquakes and volcanic activity. They created samples of ringwoodite and exposed them to the high temperatures and pressures present in the transition zone. The ringwoodite partially melted, releasing some of the its water. In turn, the water decreased the melting point of the rock and allowed magma to form.
Even though the water in the mantle isn’t a readily usable resource, its discovery helps in understanding how the planet cycles its water. It also provides a better understanding of how tectonic plates move and volcanoes form, illustrating how water, gases, and minerals are continuously being cycled between the surface and the interior of the planet.
Aqua Facts (2011)
Oceanic Institute, Hawai’i Pacific University
A list of interesting facts about the oceans.
New evidence for oceans of water deep in the Earth (2014)
Megan Fellman, Northwestern University
An article describing research into the presence of water in the Earth’s mantle.
Deep Earth has oceans' worth of water, $10 diamond reveals (2014)
Emily Chung, CBC News: Technology & Science
News report on research into the presence of water in the Earth’s mantle.
The Rock Cycle
Geological Society of London
An illustration of “how surface and deep Earth processes produce the rocks we stand on, and use to build our homes”, with a links to related resources.