Geologic time scale

Earth is very old.

Scientists use rocks to tell its story. They look at layers of rock. Old rocks are at the bottom. New rocks sit on top.

This helps them see what happened long ago. They also look for fossils. Fossils are remains of things that lived. These help name different times. It is like a giant book of Earth's life.

Earth is very old. It is about 4.54 billion years old.

Scientists use a geologic time scale to track Earth's history. This scale is a way to organize time. They do this by studying rock layers. They also look at fossils. Fossils are remains of living things.

To find the age of rocks, scientists use geochronology. This is the study of how old rocks are. They use several rules to understand rock layers. One rule is the law of superposition. It says that old rocks are at the bottom. Newer rocks sit on top.

Another rule is faunal succession. This means that rock layers have special fossils. These fossils follow a steady order. This helps scientists match layers from different places.

Time is split into many parts. The largest parts are called eons. There are four eons: Hadean, Archean, Proterozoic, and Phanerozoic. Eons are split into eras. Eras are split into periods. The current period is the Quaternary.

The geologic time scale is a way to organize Earth's history. Earth is about 4.54 billion years old.

This scale helps scientists understand the timing of major events. Geologists and paleontologists use it to describe the past. They look at how different events relate to one another. It is a vital tool for studying our planet.

Scientists figure out the age of rocks using several rules. One rule is the law of superposition. This says that the oldest rock layers are at the bottom. Newer layers stack on top of them. Another rule is the principle of original horizontality. This means sediment layers usually settle in flat, horizontal rows. They also use the principle of faunal succession. This rule says that fossils follow a steady, reliable order in rock layers.

Experts work together to keep this scale organized. The International Commission on Stratigraphy, or ICS, is a key group. They define the global units used in the International Chronostratigraphic Chart. This helps scientists all over the world use the same names. Before this, different regions used their own local scales. Now, they use standard names for the same time periods. This makes it easier to share discoveries about Earth's past.

Time is divided into many different sized chunks. The largest units are called eons. There are four main eons: the Hadean, Archean, Proterozoic, and Phanerozoic.

Eons are broken down into eras. There are ten defined eras, such as the Mesozoic. Eras are split into periods, and there are 22 of them. The current period is the Quaternary. Eras and periods are further divided into epochs and ages. The current age is called the Meghalayan.

This scale links the rocks we see to the history of life. For example, the Phanerozoic eon means "visible life." The names of eras often reflect changes in living things. The Mesozoic era means "middle life." The Cenozoic era means "new life." Scientists use these names to tell a story of change. They can see how life grew and changed through the layers. By studying these layers, we learn how our world became what it is today.

The geologic time scale is a system used to organize Earth's history. Earth has existed for about 4.54 billion years.

Scientists use this scale to describe the timing of major events. It helps them understand how the physical world and life have changed over time. This system relies on two main scientific processes. Chronostratigraphy is the study of relating rock layers, or strata, to time. Geochronology is the branch of geology that aims to determine the specific age of rocks. Together, these tools allow Earth scientists to map out the planet's long past.

To build this scale, geologists use several key principles to read the rock record. One important rule is the law of superposition. This states that in sequences that have not been moved, the oldest layers lie at the bottom. Newer material then stacks on top of the older surface. Another rule is the principle of original horizontality. This suggests that sediment layers are originally deposited in flat, horizontal rows due to gravity. Scientists also use the principle of lateral continuity. This means sediment layers extend sideways in all directions until they thin out or hit a different rock layer.

Other rules help scientists solve more complex puzzles in the rock layers. The principle of cross-cutting relationships states that a rock cutting through another must be the younger of the two. There is also the law of included fragments. This says that small pieces of one rock inside a second type of rock must have formed first. Scientists also look for unconformities. These are gaps in the geologic record caused by erosion or periods where no sediment was deposited. Finally, the principle of faunal succession is used. This principle states that rock layers contain distinctive sets of fossils that appear in a specific, reliable order. These fossils allow scientists to match rock layers even if they are not touching.

The geologic time scale is organized into a hierarchy of different units. The largest unit is an eon, which is equivalent to a chronostratigraphic eonothem. There are four formally defined eons: the Hadean, Archean, Proterozoic, and Phanerozoic. The next largest unit is an era, or erathem. There are ten defined eras, such as the Mesozoic or the Cenozoic. Eras are divided into periods, or systems. There are 22 defined periods, and the current one is the Quaternary. These units are further broken down into smaller parts. These include epochs (series), subepochs (subseries), and ages (stages). The current age is known as the Meghalayan.

Standardizing these names is a global effort led by the International Commission on Stratigraphy (ICS). The ICS is part of the International Union of Geological Sciences (IUGS). Their goal is to define precise global units for the International Chronostratigraphic Chart. In the past, different regions used their own local time scales. This made it hard for scientists to compare findings. The ICS works to reconcile these differences by using identifiable stratigraphic horizons. These horizons serve as global boundaries. This allows scientists everywhere to use the same names for the same moments in history.

Scientists use specific measurements to describe these vast spans of time. They often use the unit Ma, which stands for megaannum. One Ma represents one million years. For example, the boundary of the Jurassic Period is defined as 201,400,000 years old. Scientists also use Ga for billion years and ka for thousand years. When defining boundaries, they use Global Standard Stratigraphic Ages (GSSAs) for very old time periods. For more recent periods, they use Global Boundary Stratotype Sections and Points (GSSPs). A GSSP is an internationally agreed-upon reference point in a rock section. This point defines the lower boundary of a specific stage.

The names of these time units often tell a story about Earth. Many names are chosen to reflect major changes in life. For example, the Phanerozoic eon means "visible life" in Greek. The names of the three eras in this eon are very descriptive. The Paleozoic means "old life." The Mesozoic means "middle life." The Cenozoic means "new life." Other names come from different sources. Some names describe the type of rock, like the Cretaceous. Others come from specific geographic locations. By using this organized system, scientists can track how the Earth and its inhabitants have evolved through the ages.