Evolving timescale

The names of geologic periods on the geologic timescale we use today represent an international melting pot of terms derived from earlier efforts to divide up the history of the Earth. The period names Triassic and Quaternary are holdovers from earlier version of the timescale that divided Earth history into three (tri-) and four (quater-) slices. The names for the Periods of the most geologically recent Cenozoic Era continue to evolve. The Tertiary Period has been divided into two periods, the Paleogene and Neogene (meaning, old recent and neo recent, respectively). This fine-tuning permits more precise correlation of rock units around the world allowing for greater detail in our reconstructing the history of the Earth. The chart shown here is the work of an international committee.

Putting a name on time

-->When geologists talk about events that occurred 200 million years ago, they are referring to dates on the geological time scale.  Adding dates to the time scale was made possible with the discovery in the late 19^th century of the use of radioactive decay as a tool for determining the age of geologic materials.

The geological time scale is an evolving document, and radiometric dates are subject to refinement as technology advances.

But the geological time scale has always been an evolving document, even before dates were added. The names of the geologic eras, periods, and epochs on the geologic time scale we use today were not written in stone (so to speak!) but represent the latest version of a document that has its origins in principles enunciated almost 400 years ago.

Figuring out the order in which rock layers were deposited or emplaced was the first step to developing a time scale.

Steno and stratigraphy

Nicholas Steno, born in 1638, was a renaissance era scholar, whose experience in comparative anatomy provided some of the earliest evidence recognizing fossils as the preserved remains of animals, which he famously demonstrated by comparing modern shark teeth to fossils shark teeth.

Steno is also credited with enunciating basic principles of geology that help determine the relative order of geological events that led to the development of a geological time scale.

Steno reasoned that sedimentary rock layers were formed when particles in a fluid such as water fell to the bottom, resulting in horizontal layers. Thus Steno's principle of original horizontality states that these rock layers form in the horizontal position.

Steno realized that tilted or folded rock layers, like the ones shown in the illustration here, meant that the layers were disturbed after they were deposited. It may seem obvious to us today, but this insight represented a huge step toward understanding 4.6 billion years of earth history.

Image and more information available here.

Telling time with sedimentary rock layers

Nicholas Steno is also credited with being the first to understand (and communicate to others) the most basic principle of ordering sedimentary strata, the principle of superposition, which states that in a sequence of undeformed rock strata, the oldest beds are on the bottom and the youngest are on the top. Simple, yes?

It is important to remember that this principle refers specifically to sedimentary rocks, deposited layer by layer as strata. Some metamorphic rocks may show banding that resembles layering (compare the layered gneiss show above to the sandstone), but these “layers” do not follow the principle of superposition—they reflect reorganization of grains due to stresses in the Earth’s crust associated with plate tectonics. It is sometimes difficult for aspiring geologists to tell the difference, especially in an isolated hand-sample. Familiarity with the three main types of rocks is the key to making this distinction.

The diagram, above (from here), shows how the process of superposition could happen in a lake setting.

Finding order among the rocks

When geologists refer to events that occurred 200 million years ago, they are referring to dates on the geological time scale that have been added since the discovery in the late 19^th century of the use of radioactive decay as a tool for determining the age of geologic materials.

The geological time scale is an evolving document, and radiometric dates are subject to refinement as technology advances.

But the geological time scale has always been an evolving document, even before dates were added. The names of the geologic eras, periods, and epochs on the geologic time scale we use today were not written in stone (so to speak!) but represent the latest version of a document that has its origins in principles enunciated almost 400 years ago.

Figuring out the order in which rock layers were deposited or emplaced was the first step to developing a time scale.

What is time?

How can geologists use rocks to create a chronology of Earth history?

To address this issue we need to understand the basic measure of time.

The orbit of the Earth around the Sun measures a year, the rotation of the Earth on its axis defines a day, and the vibration of the Cesium atom in an observatory near London marks seconds, but what do these different phenomena have in common?

A year is measured by the change in the position of the Earth in its orbit relative to the sun; a day is measured by change due to Earth’s rotation, and a second is measured by the change in position of a vibrating atom. Change is the fundamental measure of time.

Changes are recorded in rocks, for example, the layers of sedimentary rock record periods of deposition alternating with periods of non-deposition; metamorphic rocks record change from a previous non-metamorphosed state, igneous rocks record the change of cooling from a molten state to a crystalline state. So it’s valid to use rocks as geochronometers in sorting out the history of the Earth.

The "spiral of time" graphic (a classic!) can be found here, along with more information.

Stories pebbles tell

From their first discovery, fossils had been interpreted as stones that fell from the sky, tricks of the devil, or objects that grew in the rocks in which they are found.

Nicholas Steno’s work with modern shark teeth convinced him that the objects we recognize as fossil shark teeth looked like shark teeth because they were shark teeth, and that they must have been buried in mud or that was now dry land.

Steno also reasoned that if the fossil were a structure that had grown within solid rock, its shape would have been distorted by the enclosing rock. The pristine condition of the fossil shark teeth indicated instead that the tooth must have been buried in soft sediments which hardened later.

Steno’s realization that objects entombed within rock, like fossils or pebbles in a conglomerate were formed before the rock itself was deposited. This was another way to distinguish the relative order of events and is called the principle of included fragments.

Simply put, this principle states that the included fragment (be it a fossil in limestone or a rounded pebble in a conglomerate, shown above) is older than the rock that encloses it.

Image and more information available here.

At the beginning

To put the Earth’s history in context, our planet and our solar system formed about 4 and a half billion years ago, in a Universe that had its beginning at least 13 billion years ago.

In this beginning there was a tremendous expansion of all matter and energy in the Universe, in an event that astronomers call the Big Bang. From this event the first elementary particles such as quarks formed; after several hundred million years atoms, and elements such as hydrogen and helium formed, eventually, after a 100 million years, these elements condensed and formed the first stars.

Perhaps a billion years after the Big Bang galaxies began to form, including our own Milky Way Galaxy.

To be continued...

Links for more information on the Big Bang from the University of Michigan and NASA.

Big Bang timeline (illustration, above) from here.

Big numbers. Really. Big.

This week GeoLog will begin a trip through time, exploring the physical and biological changes on Earth through the more than four-and-a-half billion years of its history, as told through the rocks and fossils that are the physical evidence of these changes. But first, a few words about our frame of reference:

Geologists tend to use numbers like 4.6 billion without a second thought. In fact, the scope of what we call geologic time is ENORMOUS and extremely hard to comprehend, especially for those of us who spend our days counting seconds, minutes and hours.

After all, our own life spans are on the order of 10s to one hundred years; known civilization goes back only thousands of years, and there were no modern humans around millions of years ago.

Our ability to comprehend the vast eras of geologic time is severely challenged by the remoteness and scope of these events.

Links for some help in visualizing these large numbers:

This site measures millions and billions in pennies, and the image of one billion pennies (above) is from here.

Teachers can find an exercise for class use on big numbers here, and the geologic time scale here and here.