How old is Earth? This question is continually debated and revised by scientists. The currently accepted scientific estimate of Earth’s age is about 4.5 billion years old. So, how did we arrive at this number?

In the 17th century, Danish scientist Nicolaus Steno spent time observing the different layers of rock that had formed in Tuscany, Italy. The ideas he developed from his observations started a new branch of geology called stratigraphy – the study of rock layers. Inspired by observations of rock layers, Steno established the basis for relative dating. The concept of relative dating is that layers of rocks are ordered chronologically, so their ages can be compared.

Relative Age of Rock Layers
Horizontal rock layers can be easily distinguished between oldest and youngest; however, layers can shift as Earth exerts its geologic forces over time.

Law of Original Horizontality

Steno developed two laws concerning relative dating. The first was the Law of Original Horizontality, which states that all sedimentary rock layers initially form in horizontal layers, and that any change from that horizontal position is due to the rock being disturbed later.

Law of Superposition

Steno then developed the Law of Superposition to describe the relative ages of the rock layers in an area. His idea was that, in sedimentary rock layers, the older layers of rock are deposited first. Then, newer, younger layers are deposited and formed on top of these older ones. So, if you observe an undisturbed layer of rock, you can assume that the oldest rocks are at the bottom and the youngest are on the top.

Changing Ideas

Steno’s laws revolutionized the way people viewed the age of Earth. In the 1800s, scientists measured the approximate thickness of all sedimentary rock layers at Earth’s surface. These measurements ranged from about 25–112 m (15­–70 mi). They then observed and measured the rate at which sedimentary layers form, and calculated it to be around 0.3 m (1 ft) every 1,000 years. Using these rates, scientists then calculated how long it would take for all of the sedimentary layers in the world to build up. Based on these calculations, these scientists determined that Earth had to be perhaps a hundred million years old! This was much older than Steno’s estimate of a few thousand years.

Changing Stratigraphy

Even though estimates of Earth’s age were revised to be near 100 million years old, scientists were still underestimating the age of Earth. The revised age did not take into account processes, such as weathering, erosion, and underground geologic activities, that cause change at the surface. These processes are gradual and sometimes are not noticeable for extremely long periods of time. It could take millions of years for weathering and erosion to unearth underground structures. The folding and twisting of rock layers resulting from pressure below may also take millions of years.

For example, geologic forces can exert enough pressure to cause horizontal layers of rock to fold. When the layers fold downwards and form a bowl-like shape, it is called a syncline. When the layers of rock fold upward like an arch, it is called an anticline. The forces that cause these features can often bring old layers of rock to the surface, where they are exposed to the agents of weathering and erosion.

Over time, parts of the older layers in an anticline can be weathered and eroded away. New, horizontal layers can then form on top of this eroded surface. These gaps in the geologic record, due to layers of rocks lost to weathering and erosion, are called unconformities.

Implications for Carbon Sequestration

The presence of synclines and anticlines do more than make the relative dating of rock layers difficult. To Dr. Guthrie, it also has an impact on a region’s suitability to be a carbon sequestration location.

How these syncline or anticline patterns in the rock layers affect a region’s suitability for carbon sequestration can vary. On the one hand, the stress caused by pushing layers of rock together can cause cracks in the rock layers. Carbon dioxide gas sequestered below anticline and syncline regions could leak through these cracks, which could make it impractical for the safe storage of carbon dioxide. However, anticlines could create favorable carbon storage sites, because their shapes might act as a container to hold the gas.

Relative Intrusions and Extrusions

Analyzing the positions of rock layers is not the only method geologists use to determine the relative age of rocks. Geologists can also find clues about relative age by examining the location of igneous rock formations. When molten rock cools and solidifies, it forms igneous rock. Igneous rock usually forms as slabs of giant rock. However, hot molten rock can sometimes pierce through layers of other rock. When this hot molten rock cools and solidifies within the pre-existing rock, it forms an intrusion. An intrusion is always younger than the layers of rock that it pierces.

An extrusion is also an igneous formation. How­ever, an extrusion forms when lava cools and solidifies on top of older rock formations. In undisturbed areas, extrusions are always younger than the rock layers below them.

Clues from Faults

A fault is a break in Earth’s crust usually caused by geologic forces within Earth. These forces move or shift opposite sides of a fault. Faults provide valuable insight into the relative age of rock layers. Since the layer of rock had to be present in order to break, the fault is always younger than the youngest layer of rock cut by the fault.