Fossils and Geologic Time

You can't travel back in time. But rocks can tell you what happened millions of years ago, if you know how to read them. Geologists are basically detectives working a case where the evidence is buried in layers and the witnesses have been dead for 100 million years.

A fossil is any preserved remains or trace of an ancient living thing. Most fossils form when a plant or animal is buried quickly in sediment, like mud at the bottom of a lake. Soft parts rot away, but hard parts like bones, shells, and teeth can be preserved. Fossils are almost always found in sedimentary rock.

Types of fossils:

• Petrified (permineralized) fossils: minerals fill in the tiny spaces of bone or wood, turning it to stone. Petrified forests are made this way.
• Molds and casts: an organism leaves an empty print in the rock (a mold). If minerals later fill it, they make a copy (a cast). Like making a cookie cutter, then a cookie.
• Carbon films: a thin layer of carbon left behind, showing outlines of leaves or insects.
• Trace fossils: evidence of activity, not the body itself. Footprints, burrows, nests, and even fossilized poop (called coprolites). Yes, really.
• Preserved remains: whole organisms trapped in amber (tree sap), ice, or tar. Woolly mammoths have been found frozen in Siberia.

• The Hagerman Horse: near Hagerman, Idaho, scientists dug up five nearly complete skeletons, more than 100 skulls, and dozens of jaws of a zebra-like horse that lived about 3.5 million years ago. It's Idaho's official state fossil. The site is now Hagerman Fossil Beds National Monument.
• Clarkia fossil beds: in northern Idaho, leaves about 15 million years old are so well preserved that some are still green when first split open, and scientists have even pulled ancient DNA from them.
• Helicoprion: a bizarre ancient shark with a spiral "buzz saw" of teeth. Some of the best specimens were found in Idaho's phosphate mines.

Relative dating puts events in order without giving exact ages. It answers "older or younger?" using a few simple rules:

• Law of superposition: in undisturbed layers, the oldest is on the bottom and the youngest is on top. Like a pile of dirty laundry: the stuff on the bottom has been there longest.
• Principle of original horizontality: sediment is laid down in flat layers. If layers are tilted or folded, something happened to them later.
• Law of crosscutting relationships: anything that cuts across a rock, like a fault or a squirt of magma, is younger than the rock it cuts. You can't cut a cake before it's baked.
• Inclusions: rock pieces inside another rock are older than the rock around them.
• Unconformities: gaps in the rock record where layers were eroded away or never deposited. Missing pages from the history book.

Index fossils are fossils of organisms that lived for only a short time but were found all over the world. If you find the same index fossil in rock layers in Idaho and in England, those layers are about the same age. Trilobites and ammonites are famous index fossils. Matching layers in different places this way is called correlation.

Absolute dating gives an actual age in years. The main method is radiometric dating.

Some elements are radioactive: their atoms are unstable and slowly break down, or decay, into other elements at a steady rate. The original element is the parent isotope, and what it turns into is the daughter isotope.

The time it takes for half of the parent atoms to decay is called the half-life. It never changes, so it works like a natural clock.

Example: imagine a rock starts with 100 parent atoms. After one half-life, 50 are left. After two half-lives, 25. After three, 12.5. By measuring the ratio of parent to daughter atoms, scientists can calculate how many half-lives have passed.

• Carbon-14: half-life about 5,730 years. Used to date things that were once alive (bones, wood, cloth) up to about 50,000 years old.
• Uranium-238: half-life about 4.5 billion years. Used to date very old rocks.
• Potassium-40: half-life about 1.3 billion years. Great for dating volcanic rocks.

The oldest Earth minerals found, tiny zircon crystals from Australia, are about 4.4 billion years old. Meteorites and Moon rocks show Earth formed about 4.6 billion years ago.

Scientists divided Earth's history into a calendar called the geologic time scale. It's split into eons, then eras, periods, and epochs. Big boundaries are often marked by major changes in life, like mass extinctions.

• Precambrian Time: from Earth's formation 4.6 billion years ago to 541 million years ago. About 88% of Earth's history! Mostly single-celled life.
• Paleozoic Era ("ancient life," 541–252 million years ago): an explosion of life in the oceans, then fish, plants, insects, and amphibians on land. Ended with the biggest mass extinction ever.
• Mesozoic Era ("middle life," 252–66 million years ago): the Age of Reptiles. Dinosaurs ruled. Ended with an asteroid impact.
• Cenozoic Era ("recent life," 66 million years ago to now): the Age of Mammals. Humans showed up only about 300,000 years ago.

If Earth's whole history were squeezed into one 24-hour day, humans would show up in the last 4 seconds before midnight.