Crusty Old Australia

SCIENCE

Australia holds the oldest continental crust on Earth, researchers have confirmed, rocks some 4.4 billion years old. (National Geographic News)

Use our resources to learn more about the oldest stuff on Earth, and our planet’s other extremes.

These pretty zircons are NOT the oldest rocks on Earth. They're the younger, more polished, and much larger cousins of the sand-sized Jack Hills zircons. Photograph by John E. Fletcher and Anthony B. Stewart, National Geographic

These pretty zircons are NOT the oldest rocks on Earth. They’re the younger, more polished, and much larger cousins of the sand-sized Jack Hills zircons that formed 4.4 billion years ago.
Photograph by John E. Fletcher and Anthony B. Stewart, National Geographic

Discussion Ideas

  • The new study suggests the Jack Hills zircons were part of Earth’s primitive crust, formed about 100 million years after the Earth itself. So, what on Earth was going on for those 100 million years? Read our very short encyclopedic entry on Earth’s crust for some help.
    • According to the Nat Geo News article and our own entry, “the Earth started out as a hot, gooey ball of rock. The heaviest material, mostly iron and nickel, sank to the center of the Earth and became the core. The surface of the Earth slowly cooled off and hardened. These surface rocks became the crust.”
  • Skim our media spotlight on “Dating Fossils in the Rocks,” about how geologists and paleontologists date ancient fossils in Kenya, a “cradle of mankind.” These scientists use a form of radiometric dating involving the elements potassium and argon. Why didn’t the scientists in the Nat Geo News article use this technique to date the Jack Hills zircons? What technique did they use instead?
    • Well, it’s kind of a trick question. The scientists studying the Jack Hills zircons did use a type of radiometric dating. These geologists were looking for much, much older rocks than the 5-million-year-old samples in Kenya, however. They did not measure potassium and argon. Instead, they studied individual atoms (!!) of lead. By measuring the lead, scientists are actually measuring uranium. Stay with me: Two forms of uranium (U-235 and U-238) are radioactive, and decay at beautifully steady rates. In fact, these particles decay until they’re not uranium anymore—they turn to lead. Half of the original U-238 atoms in a zircon, for instance, will decay to lead in about 4.47 billion years. (This is U-238′s half-life.)
    • Measuring the lead content in zircon is an outstanding way to measure the age of the mineral. Andrew Alden, the utterly readable geology writer over at about.com, gives you four good reasons:
      • First, zircon’s chemical structure likes uranium and hates lead. This means the clock is truly set at zero when zircon forms—all the lead is probably a result of uranium decay, not natural occurence.
      • Second, zircon has a high trapping temperature of 900°C. This means the clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.
      • Third, zircon is widespread in igneous rocks as a primary mineral. This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.
      • Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.
  • Read through our breezy GeoStory on “Earth’s Extremes,” which gives you information on our planet’s hottest and coldest, highest and lowest, most isolated and most unpredictable geologic extremes. We’ve had to update the GeoStory three times in six months! The new dating of Jack Hills zircons was the latest update. (The previous “oldest material” was . . . other Jack Hills zircons.) Read through all the categories in the GeoStory and see if you can guess the other two updates.
    • new island! The Pacific Ocean gave birth to a new island, Niijima, about about 1,000 kilometers (600 miles) south of Tokyo, Japan, in November 2013. Read about it here! The previous “newest islands” were part of the Zubair Group, Yemen, in the Red Sea.
    • coldest! Researchers announced a new low temperature, -93.2 degrees C (-135.8 degrees F) in 2013, although the cold date was actually back in 2010. Read about it here! Both the new and previous coldest temperatures were recorded in Antarctica.

Related Material! Forget eHarmony—dating rocks can be much more interesting than dating people. A jewelry store is an archive of the Earth. Every gem fixed to every ring or necklace was forged deep inside our planet, according to its own recipe of elements, temperature and pressure. Read about it here!

Related Material! Ancient rocks can also hold ancient water—and help scientists search for life on other planets. Read about it here!

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