Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Shaul Hurwitz, Research Hydrologist with the U.S. Geological Survey.

The timing of major events in Earth’s long history are continuously being refined as the accuracy and precision of dating methods improves and as new discoveries are being made. Ongoing research in Yellowstone is also providing new insights into the region’s volcanic, hydrothermal, and glacial history. But when did these major events in Yellowstone take place within the context of Earth’s history?

Before we address that question, we need to answer a more fundamental question: how old is Earth and how do we know it?  Based on evidence from age-dating of iron meteorites, specifically of fragments from the Canyon Diablo meteorite, the age of Earth is 4.54 ± 0.05 billion years. This determination was made considering the Earth and meteorites as part of the same evolving system in which the isotopic composition of lead changes over time owing to the decay of radioactive uranium. The determined age represents the last time that lead isotopes were homogeneous throughout the inner solar system, and the time that lead and uranium were incorporated into the solid bodies of the solar system. The determined age for the Earth is consistent with the ages of the oldest-known terrestrial and lunar samples.

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Geologists divide the approximately 4.6 billion years of Earth’s history into geologic units based on rock sequences that are calibrated against well-dated rocks. Over the years, development of new dating methods and the refinement of previous ones have resulted in revisions to the ages of different Eons, Eras, Periods and Epochs in Earth’s history.

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How do major volcanic events in Yellowstone relate to the major events in Earth’s history? To answer this question, let’s use our imagination and scale the approximately 4.6 billion years of Earth’s history to one calendar year. In that “geologic calendar year”, each of the 12 months represents 383 million years, each day represents 12.6 million years, each hour represents 525,114 years, each minute represents 8,752 years, and each second represents 146 years.

Now let’s place some of Earth’s major events on the “geologic calendar.” To find out when major events in Yellowstone took place, you’ll have to be patient—it’s not until late December!

• On January 1 – The Earth formed
• On February 13 – The oldest known rocks on Earth were formed
• In early March - Earth’s first oceans formed
• On March 27 - The first recorded forms of life; simple single-celled organisms
• On June 27 - Oxygen appears in Earth’s atmosphere

Let’s skip a few months….

• On November 19 - The Cambrian “explosion” which marked a profound change in life on Earth when most major groups of complex animals appeared in the fossils record
• On November 26 – The first known mass extinction at the end of Ordovician time
• On December 12 - The largest mass extinction at end of Permian time caused by large changes in Earth’s climate
• On December 15 - End-Triassic mass extinction triggered by widespread volcanic eruptions, and an increase in atmospheric CO₂ that caused acidification of the oceans and global warming that killed most of the marine and terrestrial species on Earth
• December 19 – The most famous mass extinction, when the dinosaurs vanished

It’s late December and we are finally ready for some activity in Yellowstone.

• On December 29 at 5 pm - Volcanism started along the Yellowstone hotspot track at the McDermitt Caldera, in northern Nevada and southeastern Oregon
• On December 31 at 8 pm - Volcanism in the Yellowstone Plateau started with the eruption of the Junction Butte Basalt, exposed in northeastern Yellowstone National Park, and the rhyolitic lava flow of Snake River Butte, in southern Island Park, Idaho, after which followed the eruption of the Huckleberry Ridge Tuff.
• On December 31, 9:30 pm – Eruption of the Mesa Falls Tuff and the formation of the Henry's Fork Caldera
• On December 31, 11:38 pm – Not in Yellowstone, but Homo sapiens appear in Africa
• On December 31 at 10:48 pm The Yellowstone caldera formed following the eruption of the Lava Creek Tuff
• On December 31 at 11:43 pm (or slightly before) – The penultimate Bull Lake glaciation covered Yellowstone in ice for about 2.5 minutes
• On December 31 at 11:52 pm - The most recent eruption of rhyolitic lava flows formed the Pitchstone Plateau in southwestern Yellowstone National Park
• On December 31 at 11:58 pm –The last glacial retreat from Yellowstone following approximately one minute of the Pinedale glaciation and the largest hydrothermal explosion that formed the Mary Bay crater (1.5-mile (2.6 km)-diameter)
• On December 31, 11:59 pm - Beginning of the Holocene Epoch (the past 10,000 years)

With just over one second remaining before the year draws to a close, Yellowstone was declared as the first National Park in the United States. With just a tenth of a second left in the “geologic calendar year”, the Yellowstone Volcano Observatory was formed, and with just a few milliseconds to spare, the first Yellowstone Caldera Chronicles article was published!

Following Scottish naturalist James Hutton (1726–1797), who is considered to be the founder of modern geology, many discoveries have been made by geologists that tell the story of our planet’s history. Geologists also spent decades putting together the pieces of Yellowstone’s complicated “jigsaw puzzle”. This fascinating puzzle is not yet complete, and our understanding of Yellowstone’s volcanic history will continuously be refined as new discoveries are made.

Related links about geologic time:

• Activities for Kids about Geologic Time
• Smithsonian Deep Time Exhibit
• National Park Service Geologic Time
• Is Time Travel Possible? (NASA)