What are the time divisions that are used for the geologic time scale and what determines how they are divided?

Deep Time, Decoded: Cracking the Earth’s Billion-Year Calendar

Ever wonder how we wrap our heads around the Earth’s crazy-long history? I mean, we’re talking 4.54 billion years, give or take a few million. That’s a number that’s almost impossible to fathom! To make sense of it all, geologists came up with the geologic time scale (GTS). Think of it as Earth’s own calendar, but instead of tracking days and weeks, it charts eons, eras, periods – the whole shebang.

Eons, Eras, and Everything In Between: The Time Scale’s Building Blocks

The geologic time scale is basically a set of nested boxes, each one fitting inside a bigger one. It’s a hierarchy, with the biggest chunks of time at the top and the smaller, more detailed slices at the bottom. Let’s break it down:

• Eons: These are the granddaddies of time divisions, stretching across hundreds of millions, even billions, of years. We’re talking serious time here. There are four main eons: Hadean, Archean, Proterozoic, and Phanerozoic. Those first three? Sometimes they get lumped together as the “Precambrian,” which is kind of like saying “everything before things got really interesting.” Speaking of interesting, we’re currently chilling in the Phanerozoic Eon, which, fittingly, means “visible life.”
• Eras: Eons get chopped up into eras, which mark major milestones in the story of life and the Earth’s geology. Take the Phanerozoic Eon, for instance. It’s neatly divided into the Paleozoic (“old life”), Mesozoic (“middle life,” think dinosaurs!), and Cenozoic (“new life”) eras.
• Periods: Now we’re getting down to more manageable chunks of time. Eras are split into periods. Remember “Jurassic Park”? Yep, that’s a period – the Jurassic Period, to be exact, from the Mesozoic Era. The Paleozoic Era is divided into six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. Fun fact: in the US, the Carboniferous is often split into the Mississippian and Pennsylvanian periods.
• Epochs: Periods get even more granular, breaking down into epochs. The Paleogene Period, for example, has the Paleocene, Eocene, and Oligocene epochs within it.
• Ages: Finally, we have ages, which are the smallest divisions of time in the GTS. Epochs can be further divided into these ages.

What’s the Secret Sauce? How Do We Draw These Lines?

So, how do geologists decide where to draw the lines between these different time divisions? It’s not like they just picked dates out of a hat! The divisions are based on major changes that happened in Earth’s history, changes that left their mark in the rocks. Think of it like reading a really, really old diary.

• Big Earth-Shaking Events: Major geological shifts, like mountains popping up or continents drifting around, can signal the start or end of a time unit.
• Life’s Big Moments: The arrival or disappearance of important life forms is a huge clue. The “first of first” species, like the first critter to breathe oxygen or the first plant to make seeds, are key indicators. When a new species suddenly becomes widespread in the fossil record, that’s a good sign that a geologic interval is starting or ending.
• Bye-Bye, Species! Mass extinctions, where tons of species vanish in a geological blink of an eye, are major boundary markers. The Cretaceous-Paleogene extinction – the one that took out the dinosaurs – is a classic example. It marks the end of the Mesozoic Era and the beginning of the Cenozoic.
• Climate Chaos: Big swings in global climate, like ice ages or super-hot periods, can also influence where we draw the lines.

Digging into the Details: Stratigraphy and Radiometric Dating

How do we actually figure out when these events happened? Two main tools come into play: stratigraphy and radiometric dating.

• Stratigraphy: This is all about studying layers of rock, or strata. The basic idea is that, unless things have been flipped upside down, the oldest layers are on the bottom, and the youngest are on top. By comparing rock layers from different places – looking at the type of rock, their magnetic properties, and the fossils they contain – geologists can build a relative timeline.
• Radiometric Dating: This is where we get actual numbers. Certain elements decay at a known rate, like a super-reliable clock. By measuring the amounts of the original element and what it decays into, scientists can figure out how old a rock is. Radiocarbon dating, potassium-argon dating, and uranium-lead dating are some of the common methods. Radiometric dating helps us put dates on the relative timeline we build with stratigraphy.

Who’s in Charge? The International Commission on Stratigraphy

The official keeper of the geologic time scale is the International Commission on Stratigraphy (ICS). They’re part of the International Union of Geological Sciences (IUGS). The ICS maintains the International Chronostratigraphic Chart, which is the go-to standard for the GTS. Of course, you might still see some local terms and variations floating around.

Always Evolving

The geologic time scale isn’t set in stone (pun intended!). As we discover new things and improve our dating methods, the GTS gets tweaked and updated. It’s a never-ending process of refining our understanding of Earth’s incredible history. And that’s what makes it so fascinating!