Explainer: Understanding geologic time
Imagine the nearly unimaginable: 4.6 billion years. That’s how old the Earth is — a mind-boggling length of time. And to measure it, scientists use special terms, most of which focus on the planet’s changing geology. That’s why, in fact, it’s known as geologic time.
To grasp just how old Earth is, imagine fitting its entire history into one calendar year. If Earth formed on January 1, the earliest primitive life (think algae) wouldn’t appear until March. Fish first swam onto the scene in late November. Dinosaurs stomped around from December 16 until December 26. The first modern humans — Homo sapiens — were real late-comers. They didn’t show up until just 12 minutes before midnight on New Year’s Eve.
Almost as mind-boggling is how geologists figured this all out. Like chapters in a very, very thick book, layers of rock chronicle Earth’s history. Put together, the rock records the long saga of life on Earth. It shows how and when species evolved. It also marks when they thrived — and when, over millions of years, most of them went extinct.
Limestone or shale, for example, may be the remains of long-gone oceans. These rocks contain traces of life that existed in those oceans over time. Sandstone might once have been an ancient desert, where early land animals scurried. As species evolve or go extinct, the fossils trapped in the rock layers reflect these shifts.
How to track such a long, complex history? Using dazzling detective skills, geologists created a calendar of geologic time. They call it the Geologic Time Scale. It divides Earth’s entire 4.6 billion years into four major time periods. The oldest — and by far the longest — is called the Precambrian. It is divided into Eons known as the Hadean (HAY-dee-un), Archean (Ar-KEY-un) and Proterozoic (Pro-tur-oh-ZOE-ik). After the Precambrian come the Paleozoic Era and Mesozoic Era. Last but not least is the Cenozoic (Sen-oh-ZOE-ik) Era, the one in which we live. The Cenozoic started about 65 million years ago. Each of these Eras, in turn, are divided into increasingly smaller divisions known as Periods, Epochs and Ages.
Unlike months in a year, geologic time periods aren’t equally long. That’s because Earth’s timeline of natural change is episodic. That means changes happen in spurts, rather than at some slow and steady pace.
Take the Precambrian Era. It lasted more than 4 billion years — or for more than 90 percent of Earth’s history. It ran from Earth’s formation until life burst out some 542 million years ago. That burst marked the beginning of the Paleozoic Era. Sea creatures like trilobites and fish emerged and came to dominate. Then, 251 million years ago, the Mesozoic Era burst into being. It marked the biggest mass extinction of all. It also kicked off the spread of life on land. This era then ended abruptly — and famously — 65.5 million years ago. That’s the moment when dinosaurs (and 80 percent of everything else) vanished.
Relative versus absolute ages
So here’s the 4.6-billion-year question: How do we know the actual ages on the Geologic Time line? The scientists who developed it in the 1800s didn’t. But they did understand relative ages, based on a simple, yet powerful principle. That principle is called the Law of Superposition. It states that in an undisturbed stack of rock layers, the oldest layers will always be on the bottom, and the youngest on top.
The Law of Superposition allows geologists to compare the age of one rock or fossil to another. It makes the sequence of geologic events more clear. It also gives clues into how species evolved, and what creatures co-existed — or didn’t. A trilobite, for example, literally wouldn’t be caught dead in the same rock as a pterosaur. After all, they lived millions of years apart.
Still, how can we make sense of a calendar with no dates on it? To assign such absolute ages to the Geologic Time Scale, scientists had to wait until the 1900s. That’s when dating methods developed that relied on radiometric methods. Certain isotopes — forms of elements — are unstable. Physicists refer to them as being radioactive. Over time, these elements shed energy. The process is called decay and will involve shedding one or more subatomic particles. Eventually, this process will leave the element nonradioactive, or stable. And a radioactive isotope always decays at the same rate.
Radiometric age dating is based on how much of one radioactive “parent” isotope has decayed into its stable daughter.
Scientists measure how much of the parent element still exists in a rock or mineral. Then they compare that to how its “daughter” element now exists there. This comparison tells them how much time has passed since the rock formed.
What element they choose to measure depends on many factors. Those can include the rock’s composition, its approximate age and its condition. It also depends on whether the rock had been heated or chemically altered in the past. The decay of potassium to argon, uranium to lead, and one isotope of lead to another are some common yardsticks used to date very old rocks.
These dating methods allow scientists to put real ages on rocks with astonishing accuracy. By about the 1950s, most of the Geologic Time Scale had real dates (described as “years before the present time”).
The exact timing and even the names of some geologic divisions are still not set in stone. Every year, geochronologists (GEE-oh-kron-OL-oh-gizts) — scientists who specialize in dating geologic ages — improve the methods to zoom in more accurately. They can now distinguish events that occurred just a few thousand years apart, back tens of millions of years ago.
“This is an exciting time,” says Sid Hemming. She’s a geochronologist at Columbia University in New York City. “We are refining our analyses of geologic dates. And this is allowing ever more control on the time scale,” she says.
Right now, new layers of limestone and shale are forming at the bottoms of Earth’s oceans and lakes. Rivers move gravel and clay that will someday become rock. Volcanoes spew out new lava. Meanwhile, landslides, volcanoes and shifting tectonic plates constantly re-shape Earth’s surface. These deposits slowly add layers that will end up marking the current geologic period. It’s known as the Holocene.
And now that people have been around for the equivalent of 12 seconds, some geologists propose adding a new period to the Geologic Time Scale. It will mark the time since humans began altering Earth. Starting about 10,000 years ago, it is tentatively being called the Anthropocene.
Its geologic layers will be quite a mix. They will hold plastics, petrified food wastes, graveyards, discarded cell phones, old tires, construction debris and millions of miles of pavement.
“Far-future geologists will have a huge set of puzzles on their hands,” says Jan Zalasiewicz. He works at the University of Leicester in England. As a paleobiologist, he studies organisms that lived in the distant past (such as at the time of the dinosaurs). Zalasiewicz recently proposed a name for this growing layer of human-made debris. He calls it the Technosphere.
In Earth’s never-ending story, we are creating our very own addition to the Geologic Time Scale.
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
Anthropocene Term coined by scientists to describe the age in which humans have been the strongest force of change on the planet. It is generally believed to date from at least the dawn of the Nuclear Age (in the middle 1940s), and possibly even earlier — from the beginning of the Industrial Revolution in the early 1800s.
argon An element first discovered on August 13, 1894, by Scottish chemist Sir William Ramsay and English physicist John William Strutt, better known as Lord Rayleigh. Argon was the first “noble” gas, meaning one that does not react chemically with other elements. For its discovery, Ramsey would receive the 1904 Nobel Prize in chemistry and Rayleigh the 1904 Nobel Prize in Physics.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
clay Fine-grained particles of soil that stick together and can be molded when wet. When fired under intense heat, clay can become hard and brittle. That’s why it’s used to fashion pottery and bricks.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
decay (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs.
element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
epoch (in geology) A span of time in the geologic past that was shorter than a period (which is itself, part of some era ) and marked when some dramatic changes occurred.
extinct An adjective that describes a species for which there are no living members.
extinction The permanent loss of a species, family or larger group of organisms.
factor Something that plays a role in a particular condition or event; a contributor.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
geochronology The study of the age of rocks. Geochronologists often use known rates of radioactive decay of elements in rocks to determine the age of the rock.
geologic An adjective that refers to things that are related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.
geology The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.
Holocene The current period in geologic time. Meaning “entirely recent,” the Holocene began at the end of the last ice age, 11,700 years ago, and continues to the present time.
Homo A genus of species that includes modern humans (Homo sapiens). All had large brains and used tools. This genus is believed to have first evolved in Africa and over time evolved and radiated throughout the rest of the world.
insight The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.
isotopes Different forms of an element that vary somewhat in mass (and potentially in lifetime). All have the same number of protons in their nucleus, but different numbers of neutrons.
lava Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano.
Law of Superposition (in geology) A rule for the relative ages of things present in undisturbed rock. It holds that the oldest rock will always be at the bottom and the youngest on top.
lead A toxic heavy metal (abbreviated as Pb).
limestone A natural rock formed by the accumulation of calcium carbonate over time, then compressed under great pressure. Most of the starting calcium carbonate came from the shells of sea animals after they died. However, that chemical also can settle out of water, especially after carbon dioxide is removed (by plants, for instance).
Mesozoic An era in geologic history that contained three related periods which became renowned for their large reptiles: the Triassic (which spanned from 251 to 199.6 million years ago), the Jurassic (which spanned from 199.6 to 145.5 million years ago), and the Cretaceous (which spanned from 145.5 to 65.5 million years ago).
mineral Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).
paleobiology The study of organisms that lived in ancient times — especially geologically ancient periods, such as the dinosaur era. Scientists who work in this field are known as paleobiologists.
physicist A scientist who studies the nature and properties of matter and energy.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
potassium A chemical element that occurs as a soft, silver-colored metal. Highly reactive, it burns on contact with air or water with a violet flame. It is found not only in ocean water (including as part of sea salt) but also in many minerals.
Precambrian Era The period of geologic time that ran from around the time of Earth’s formation, roughly 4.6 billion years ago, to 540 million years ago. During this period, complex life forms — organisms containing many cells — emerged. The later Precambrian, from about one billion years to 540 million years ago, is also called the Neoproterozoic.
pterosaur Any of various extinct flying reptiles of the order Pterosauria. These animals lived 245 million years ago to 65 million years ago. Although not true dinosaurs, they lived during the reign of dinosaurs. Among members of this order were the pterodactyls of the Jurassic and Cretaceous periods, which were characterized by wings consisting of a flap of skin supported by the very long fourth digit on each forelimb.
radioactive An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.
radiometric dating A means to measure geologic time. It dates very old rocks by measuring the share of one or more radioactive elements in rocks that have decayed into their “daughter” isotopes.
sandstone A type of sedimentary rock. It formed as sand-size grains of mineral grit became compacted or glued together over time.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sequence The precise order of related things within some series. (in genetics) n. The precise order of the nucleotides within a gene. (v.) To figure out the precise order of the nucleotides making up a gene.
shale A fine-grained sedimentary rock made by compressing layers of clay-rich sediment for millions of years.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
subatomic Anything smaller than an atom, which is the smallest bit of matter that has all the properties of whatever chemical element it is (like hydrogen, iron or calcium).
technosphere The accumulation of human-created and altered materials, debris and other visible alterations to Earth's surface.
tectonic plates The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.
trilobite An extinct group of arthropods that were related to modern-day insects.
uranium The heaviest naturally occurring element known. It’s called element 92, which refers to the number of protons in its nucleus. Uranium atoms are radioactive, which means they decay into different atomic nuclei.
waste Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.