Ancient Rocks

Ancient Rocks Auyuittuq National Park, Baffin Island, Nunavut, Canada.
Photo Credit: Rick Derevan via CC by-NC 2.0 (https://creativecommons.org/licenses/by-nc/2.0/) via Flickr.

Ancient minerals are providing new information on the formation of our planet.


Relics from Earth’s birth. I’m Bob Hirshon and this is Science Update.

Scientists have discovered material in rock deposits that’s nearly unchanged since the formation of our planet, four-and-a-half billion years ago. In the journal Science, University of Quebec geochemist Hanika Rizo says that their mere existence is rewriting the book on Earth’s distant past.

It is difficult to understand knowing that very early we had this giant impact that formed the moon, and this giant impact we thought for a very long time should have erased everything that was formed before in the Earth.

Now they realize that that moon-forming collision must have been with a much smaller object. The chemical composition of the mineral also reveals evidence that the early Earth was rocked by between five and fifteen other collisions that helped shape the core we see today. I’m Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

Scientists continually strive to understand how our planet was formed and how life developed on it. According to our current knowledge, Earth formed about 4.5 billion years ago. During that time, convection in the planet's interior caused deep portions of the mantle to rise upwards, melt, and then separate once again by density. The melts, since they were less dense than the unmelted rock, rose to form Earth's crust, while the denser residues of the melting sank back downward, altering the mantle's chemical composition in the process.

The discovery by Hanika Rizo and her colleagues, however, may give scientists new insights into the Earth's earliest days, helping them better understand how the planet and perhaps even the moon formed. In addition to illuminating the young Earth, the research could potentially reshape our understanding of the internal workings of the planet, including how the mantle mixes.

Rizo and the research team found a geochemical signature of material left over from the early melting events that accompanied Earth's formation in relatively young rocks from a vast underwater plain about the size of Alaska and nearly 20 miles (30 km) thick known as the Ontong Java Plateau, which is located in the Pacific Ocean north of the Solomon Islands. It's believed the plateau was formed 120 million years ago, possibly by the initial phase of a mantle plume center called the Louisville hotspot. They also found the geochemical signature in rocks from Baffin Island, off the coast of Northern Canada, which was formed 60 million years ago.

The researchers figured this out by measuring the variations of the abundance of the isotope of tungsten in these rocks (isotopes are versions of an element in which the number of neutrons in each atom differs from the number of protons). They measured tungsten because it contains one isotope of mass 182 that is created when an isotope of the element hafnium undergoes radioactive decay, meaning its elemental composition changes as it gives off radiation. Hafnium-182 only existed during Earth’s first 50 million years. The time it takes for half of any quantity of hafnium-182 to decay into tungsten-182 is 9 million years.

The team determined that the basalts from Baffin Island and the Ontong Java Plateau contain slightly more tungsten-182 than other young volcanic rocks. Because all the hafnium-182 decayed to tungsten-182 early in our planet's history, these findings indicate that the mantle material that melted to form the flood basalt rocks that the team studied originally had more hafnium than the rest of the mantle.

"This demonstrates that some remnants of the early Earth's interior, the composition of which was determined by the planet's formation processes, still exist today," explained lead author Rizo.

Now try and answer these questions:

  1. How long ago was the Earth formed?
  2. What are isotopes?
  3. What events took place during the Earth's formation?
  4. Where did the research team conduct its research? Why do you think they chose those spots?
  5. What did the researchers discover?
  6. What is the significance of this discovery for our understanding of how the Earth was formed?

You can listen to High Pressure to learn more about how scientists study difficult environments like Earth's interior.

You can read more about Hanika Rizo's research at Evidence of Earth's Early Years May Still Lurk in the Mantle.

Going Further

For Educators

This Science Update could be a good way to get your students excited about studying Earth's formation and history. You also could use the feature to demonstrate how new discoveries can add to scientific knowledge and understanding.

In addition to this Science Update, you could have your students listen to Quake Side Effects to learn why days are now three millionths of a second shorter because of the earthquake that caused the tsunamis in southeast Asia.

Related Resources

How Sedimentary Rocks are Formed
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Earth Movers
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Fossils and Geologic Time
9-12 |
Pumpkin Planet
6-12 | Audio

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