The History of the Earth

The Data are in the Strata

Go outside and pick up a rock. Take a close look at it. You can determine the rock’s volume, mass, color, and texture easily. Using laboratory methods, you could even determine the rock’s chemical composition. But how could you determine the age of the rock? Would this rock be as old as the Earth or would it be younger? Could this rock’s age give you a clue to the history of our planet?

For a very long time, geologists examined rock and sediment layers on the sides of hills, in deep canyons, or on the sides of mountains all over the world. Upon inspection, it is easy to assign a relative order to the geologic events that laid down these layers. The sandstone on the bottom came first, then later the layer of limestone, and much later the clay layer, and so on. The basic concepts of a geologic time scale began late in the 17th century when scientists correctly deduced that each rock stratum represented a different geologic time period.

During the 18th century, scientists realized that they could not read rock strata as simple layers because geological processes deep in the earth complicate the layer order. Strata once on the ocean floor millions of years ago could now be on the top of a mountain. It can be distorted, eroded, or tilted long after it was deposited. Rock strata deposited at the same time in different parts of the world can vary greatly in composition. Clearly, marking distant geologic times by types of rock strata in layers only gives part of the picture.

By the 19th century, geologists began to classify strata according to the types of fossils found within it. This made it possible to identify similar strata on different continents if they contained the same kinds of fossils. These observations lead to the development of the relative geologic time scale. However, the actual age of these rock strata and fossils they contained remained a topic for heated debate for many decades. It wasn’t until after the discovery of radioactivity in the late 1890s, that a method would be developed to determine the absolute age of rocks, fossils, and other material found in any rock strata. This tool would give geologist what was needed to date rock strata and to develop a chronology of the Earth’s history.

The Geologic Time Scale

Geologists have known for some time that the entire history of the Earth is preserved in the strata. There you can easily see the clues to the geological and biological processes of ancient times. The geologic time scale displayed below was developed over a long period of time and was most recently updated in 2004 by the International Commission on Stratigraphy.

*Millions of years ago

The accepted age of the Earth (and the rest of our solar system) is about 4.6 billion years. This age is computed through a number of different methods. Unfortunately, the age of the Earth can not be determined from terrestrial rocks since they have not been able to withstand the effects of a volatile planet such as Earth.

The Earth is a vibrant and thriving planet whose surface is completely reworked through the process of plate tectonics every 500 million years or so. New rock structures are constantly being forged and brought to the surface through our planet’s dynamic geological machine. This means that it would be almost impossible to find rocks on the Earth’s surface that date back to 4.5 billion years. The processes of weathering and erosion compound this problem.

The most common method for determining the age of rocks is radioactive dating. Radioactive isotopes change into stable isotopes through the process of radioactive decay. This process changes the total number of protons in the nucleus and thus changes the isotope through time. This is commonly referred to as an object’s isotopic signature. The time it takes for half of the radioactive materials in a sample to decay into another isotope is called the half-life. Carbon-14 is a well known isotope that has a half-life of 5730 years.

Carbon-14 dating is very useful for archaeologists wanting to date an ancient artifact or to paleontologists wanting to determine the age of a fossil. Over time, carbon-14 decays into nitrogen-14. The amount of carbon-14 that is left in a sample is an indication of the age of the sample. Although carbon-14 can be used to determine the age of materials up to 40,000 years old, this is too short of a time span to be useful in determining the age of the Earth.

This table lists some of the many radioactive isotopes with half-lives longer than carbon-14.

Several radioactive dating methods based on isotopes with long half-lives are used to determine the age of rocks. These methods reveal that the oldest rocks on Earth date from between 3.8 and 3.9 billion years. Most of the rocks dated were metamorphic. The radiological information stored in these rocks is reset or lost due to the heat and pressures of formation inside the Earth.

Some sedimentary rocks that have been dated included minerals in their structure as old as 4.1 – 4.2 billion years. To find older rocks, we must look towards extraterrestrial sources. The Earth formed at the same time and from the same materials as the rest of the solar system. If we can collect extraterrestrial rocks and date them, we could find rocks older than the Earth that could give us a clue to Earth’s age.

Every year the earth is bombarded by tons of meteorites. Meteorites come from comets and asteroids, which are made of the leftover material from which the planets formed. The surfaces of comets and asteroids are not subject to the same geological and erosion processes which affect rocks on the Earth’s surface. This means that the rocks on comets and asteroids remain relatively unchanged from the time of the solar system’s formation.

This table shows the age of several meteorites tested using radioactive dating methods.

These samples (as well as other meteorite samples) show that the average age of meteorites hovers around 4.5 billion years. But what about rocks collected from another extraterrestrial site? From 1969 to 1972, the Apollo moon missions collected several hundred pounds of moon rocks. Many of these lunar rock samples (especially those collected during the Apollo 16 and Apollo 17 missions) also dated to around 4.5 billion years. Since we know that the Earth formed out of the same material and at about the same time as all of the other objects in the solar system, then the age of the Earth must be about 4.5 billion years.