Level 1 — Absolute Beginner
Scientists study very old rocks in Australia. The rocks are about 3.1 billion years old. They come from a place called the Pilbara Craton. It is one of the oldest parts of Earth's crust.
The rocks show something surprising. Water went far below the surface a long time ago. The water helped the rock melt. Melted rock under the ground is called magma. The magma fed volcanoes.
Today, water goes down when one big plate slides under another plate. But the young Earth was very hot. Its crust was soft. Plates probably could not slide like that.
The team has an idea. They call it dripduction. Wet pieces of crust became heavy. They sank down into hot rock in blobs, like drips. So Earth moved water down very early. The study is in the journal Nature Communications.
- rocks
- Hard pieces of stone.
- crust
- The hard outer layer of the Earth.
- magma
- Hot melted rock under the ground.
- volcanoes
- Mountains that send out hot rock and ash.
- melt
- To change from hard to liquid because of heat.
- heavy
- Weighing a lot.
- sank
- Went down, past tense of sink.
- study
- A piece of careful scientific work.
Level 2 — Elementary
An international team of scientists has studied volcanic rocks from Western Australia. The rocks are about 3.1 billion years old and come from the Pilbara Craton, one of the best preserved pieces of very ancient crust on the planet. The team was led by the geochemist Eric Vandenburg of the University of Adelaide.
The rocks carry chemical signs that water once travelled far below the surface. Water matters because even a small amount of it mixed into hot rock makes the rock melt at a lower temperature. The melted rock, called magma, then rose and fed volcanoes.
The team calculates that the mantle under that ancient corner of Australia held roughly as much water as the mantle under today's arc volcanoes. Arc volcanoes sit above places where one tectonic plate slides under another. That is a puzzle, because the early Earth was much hotter and its crust was softer, so plates probably could not slide under one another the way they do today.
The researchers suggest a process they call dripduction. Pieces of crust that were rich in water became heavy and sank into the mantle in blobs instead of as rigid slabs. The buried water then lowered the melting point of the rock around it. If the team is right, Earth was moving water between its surface and its interior about a billion years earlier than many models assumed. The study appears in the journal Nature Communications.
- volcanic
- Made by or connected with a volcano.
- ancient
- Extremely old.
- mantle
- The thick layer of hot rock between Earth's crust and its core.
- magma
- Molten rock below the surface.
- tectonic plate
- One of the huge moving pieces that make up Earth's outer shell.
- temperature
- How hot or cold something is.
- blobs
- Soft round lumps of something.
- interior
- The inside part of something.
Level 3 — Intermediate
Volcanic rocks about 3.1 billion years old, collected from the Pilbara Craton in Western Australia, have given geologists an unexpected look at how the young Earth handled water. Writing in Nature Communications, an international team led by the geochemist Eric Vandenburg of the University of Adelaide reports chemical signatures showing that water travelled far below the surface and helped melt the rock that fed ancient volcanoes.
The quantity is what makes the result striking. The team calculates that the mantle beneath that corner of ancient Australia held roughly as much water as the mantle beneath today's arc volcanoes, the chains that rise above subduction zones, where one tectonic plate sinks under another. Water lowers the melting point of hot rock, which is why those modern volcanoes are so active, and the Pilbara rocks suggest that the same chemistry was already at work three billion years ago.
That creates a problem. The early Earth was considerably hotter, and its crust was softer and more buoyant, so rigid plates almost certainly could not slide beneath one another in the way they do now. Vandenburg and his colleagues therefore propose an alternative they call dripduction: pieces of crust that had soaked up water became dense enough to sink, dripping down into the mantle in blobs rather than descending as intact slabs, and carrying their water with them.
If the interpretation holds, Earth began recycling water between its surface and its interior roughly a billion years earlier than many models assume, long before modern plate tectonics was fully running. The caution is the usual one for a result of this kind. This is a single study of a single region, and dripduction remains the team's proposed explanation rather than a settled account of how the early planet worked.
- geochemist
- A scientist who studies the chemistry of rocks, water and the Earth.
- signatures
- Distinctive patterns that reveal how something formed.
- subduction
- The modern process in which one tectonic plate sinks beneath another.
- buoyant
- Tending to float or stay up rather than sink.
- dense
- Heavy for its size.
- slabs
- Thick flat pieces of something solid.
- recycling
- Moving material around a system so that it is used again.
- interpretation
- The meaning a person gives to a set of facts.
Level 4 — Advanced
The Pilbara Craton in Western Australia is a rarity: a stretch of continental crust old enough, and undisturbed enough, to preserve a chemical record of the planet's adolescence. A new analysis of volcanic rocks there, roughly 3.1 billion years old, argues that the record contains something the standard account did not expect to find so early, namely evidence that surface water was being carried far into the Earth and delivered to the zone where rock melts and volcanoes are fed.
The work, published in Nature Communications by an international team led by the geochemist Eric Vandenburg of the University of Adelaide, rests on an inference rather than a direct observation. Water leaves fingerprints in the chemistry of the magma it helps to produce, because even modest amounts of it depress the melting point of hot mantle rock. Reading those fingerprints backwards, the authors estimate that the mantle beneath this ancient terrain was about as wet as the mantle beneath modern arc volcanoes, the chains that stand above subduction zones, where one tectonic plate descends beneath another.
The awkwardness lies in the mechanism. On a hotter early Earth, with thinner and more buoyant crust, the rigid, sustained descent of a plate that defines modern subduction is difficult to arrange. Vandenburg and his co-authors therefore propose what they term dripduction: hydrated crust becoming dense enough to founder under its own weight and sink in discrete blobs, carrying its water into the mantle without any of the machinery of a subduction zone, and lowering the melting point of the rock it settles among. The consequence, if the reading is correct, is that the exchange of water between surface and interior predates full plate tectonics by something close to a billion years.
Two cautions are worth keeping in view. The conclusion rests on one region, however well preserved, and the mechanism is an explanation offered to fit the chemistry rather than a process anyone has observed. What the study does establish more firmly is the constraint itself. Any account of the Archean Earth now has to accommodate a mantle that was already carrying a substantial load of water, and to say how that water got there.
- craton
- An old, stable core of a continent.
- inference
- A conclusion reached from evidence rather than from direct observation.
- depress
- To push a value, such as a melting point, lower.
- terrain
- A stretch of land, considered in terms of its physical features.
- hydrated
- Containing water that has been chemically taken in.
- founder
- To sink or collapse under its own weight.
- discrete
- Separate and individually distinct.
- Archean
- The eon of Earth's history from about 4 to 2.5 billion years ago.
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