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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?. Measuring BIG TIME (BYA). You can measure seconds, minutes, and hours with a clock
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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?
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
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?
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
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)
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
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
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
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
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
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