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.