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The Deep Present


One of geology's great intellectual contributions to the public is deep time: the knowledge that Earth is, in effect, infinitely old. The Earth's past goes back an unimaginable 4.56 billion years; the future is less certain but also beyond comprehension. Then there's the present. Has geology given us a deep present too? I think so.

The Human Present

Today is seductive. As we picture the places we live in, we tend to favor the good days over the bad. The river is well-behaved so much of the time that the possibility of a hundred-year or thousand-year flood seems distant. The "big one" earthquake lasts just a minute or two, while the centuries in between are placid and prosperous. Volcanic eruptions can wipe out everything around, but they happen so seldom. On the individual scale, we enjoy the good times and place our bets on the odds that we'll be OK.

Cities, states and civilizations cannot place such bets, not if they plan to endure. They help ensure, through laws and institutions, that they can be relied upon for generations. Today there are demands for civilization to do even better, to become more nearly sustainable. Geology is essential for reaching that goal.

The Present in the Geologic Past

The present is an instant, and we see very few instants in the geologic record. Dinosaur tracks and other biological traces might come to mind—part of their fascination is their snapshot nature. Sedimentary structures, like the tool marks left by sticks and pebbles bouncing along a streambed, are also signs that are created instantaneously. More leisurely are storm-related features like hummocky cross-bedding, or products of volcanic eruptions like lahars and lava flows. They were laid down in hours or days. But almost everything else in the rock record represents many years of very slow activity.

The geologic record, even at its most detailed, is a series of whole ages of time. A meter of rock might represent the average conditions of a thousand-year span. Deep time is irretrievably blurred and will never be revealed in the clarity and detail with which we see today. But being able to see the present day as an age, rather than a moment, gives us new and valuable insight.

The Deep Present

Running a sustainable civilization requires us to be ready for long-term challenges: rare earthquakes, unusual flood events, climate changes. We need to know the range of the present on the thousand-year scale. That's too long for historical records, but well within the resolution of the geologic record.

The most relevant geologic records of the last thousand years are not found in rocks or fossils, but in sediments. We have many ways of determining the age of young sediments, then determining environmental conditions at the time they were laid down in the bottom of lakes, wetlands or the sea. Here are the principal dating methods.

  • Radiocarbon is a well-calibrated dating method for carbon-bearing materials younger than about 50,000 years.
  • Conventional dating methods like potassium-argon and uranium-series have advanced to the point that they can be used on even young materials like lava flows and volcanic ash beds.
  • Cosmogenic nuclides, short-lived isotopes that are created by cosmic radiation, are good for estimating the ages of rock surfaces.

Once we have ages, we have a timeline to make sense of other information that sediments may yield. Here are some of the major techniques.

  • The mineral mix of the sediment itself can help document erosion patterns in the nearby countryside.
  • Unusual bedding features can record large storms and earthquake disturbances.
  • Remains of plankton indicate temperature and salinity conditions of the past.
  • Gypsum and other evaporite minerals may indicate intense droughts.
  • Pollen grains are direct evidence of the surrounding vegetation.

In addition, tree ring records in some regions have climatic information for many thousands of years.

This kind of data can give regions a good sense of their typical range of conditions on the thousand-year scale: their deep present. With that knowledge, planners can design realistically based on deep history, not hopefully based on today's nice weather. And the more citizens and their leaders who share the geologist's sense of the deep present, the better.

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