How Do You Measure Deep Time?

1. How Do You Measure Deep Time? How old is the oldest rock on Earth? When did the dinosaurs become extinct? How long does it take for a mountain chain to form?

2. Measuring BIG TIME (BYA) • You can measure seconds, minutes, and hours with a clock • How do you measure “deep” time, as in billions of years? • Scientists use zircon crystals – little time capsules - in magma from erupting volcanoes to measure “big” time • zircons are composed of zirconium, silicon, and oxygen

3. Earth’s Most Ancient Crystals: Zircon • Time Capsules for Deep Time • 1/5 of a millimeter long • Grow by incorporating elements around them • Ratio of element isotopes date the crystal and indicate whether formed in an ocean or a volcano • Growth rings visible under microscope – formed in volcano, moved to an ocean environment, sucked deep into E’s crust. Some rings too small for existing technology – more secrets can be squeezed out of zircon! • Oldest formed 4.4 bya – was E an ocean already?

4. Stability and Instability = Perfect Time Problem Solver • Incredibly stable • One tough crystal • Dense, inert, non-magnetic • Resists weathering • Withstands temperature extremes • Rejects lead when it forms • Contains radioactive element uranium-238

5. Useful Instability • Zircon contains unstable, or radioactive, uranium • From moment it forms, it starts to decay into stable lead • Over time, amount of uranium decreases and amount of lead (daughter) increases • Measuring ratio of U-238 to Pb-206 is precise stop watch from a few kya to several bya (it takes 4.468 by for half of U to change to Pb)

6. Formation of Zircon Crystals • Volcano erupts • Gases originally dissolved in magma form bubbles quickly • Magma explosively shredded into microscopic glass shards and mineral crystals = volcanic ash • Common minerals in volcanic ash are feldspar, quartz, mica, and zircon • When formed in same way as rest of minerals, unique because extremely durable and contains radioactive uranium but no lead

7. Deposition • After volcanic ash falls down, many things can happen • May wash away • Fall into small depression • Land in lake or swamp • Overtime, ash is buried and becomes an identifiable layer in a stack of layered sedimentary rocks, which may also contain fossils

8. Erosion and Discovery • Erosion exposes buried layers of rock • Geologists looks for ash layers as they study stacks of sedimentary rocks and fossils • Each layer contains zircon stopwatches • Geologist can get very precise idea of timing of events within the stack

9. Easy To Collect Ash Samples • Big pickax • Garden trowel to scrape • Marker • Duct tape • Enough bags to collect several pounds of ash to ensure plenty of zircon in the sample

10. Concentration • In lab, geologist crush the ash sample • Wash away all fine dust and clay • Zircon’s unique properties make it possible to separate it from other ingredients in ash sample.Dense but not magnetic so geologists use large magnet to get rid of magnetic minerals • Separate zircon crystal from remaining minerals using high density liquids. Zircon will sink, while lower density crystals float

11. Reading the Stopwatch • After separation, scientists dissolve tiny individual zircon crystals and separate uranium and lead from other elements in zircon • To read stopwatch, uranium and lead put in mass spectrometer, which separates and counts individual atoms (high voltage electricity accelerates unsorted atoms into the machine) • Ratio of U to Pb atoms allows scientists to calculate how many years have passed since volcano erupted and the lead atoms started to accumulate in zircon crystals