9.1 The beginnings of geology In 1666, Nicholas Steno, a Danish anatomist, studied a shark’s head and noticed that the shark’s teeth resembled mysterious stones called “tonguestones”.

9.1 Evidence from Rock Steno theorized that tonguestones looked like shark’s teeth because they actually were shark’s teeth that had been buried and became fossils.

9.1 What is relative dating? Steno’s principles are used by geologists to determine the age of fossils and rocks in a process called relative dating. Relative dating is a method of sequencing events in the order they happened.

9.1 What is relative dating? James Hutton (1726–1797) showed how processes today might explain what happened a long time ago. For example, grooves left behind by flowing rainwater helped explain the formation of the Grand Canyon from the Colorado River.

9.1 Superposition Steno’s ideas for relative dating include superposition, original horizontality, and lateral continuity. Superposition states that the bottom layer of sedimentary rock is older than the layer on top because the bottom layer formed first.

Rock layers may bend or shift and are found standing vertically, or tilted, or rolled into curves.

9.1 Lateral continuity Horizontal layers of rock are continuous. By comparing rock layers in the Grand Canyon, geologists have found that the layers on one side of the canyon match up with the layers on the other side.

9.1 More relative dating The idea of cross-cutting relationships states that a vein of rock that cuts across a rock’s layers is younger than the layers. The middle and top layers formed after the bottom layer but before the vein.

9.1 Faunal succession Faunal succession means that fossils can be used to identify the relative age of the layers of a rock formation. The organisms found in the top layers appeared after the organisms found in the layers below them.

9.1 Fossils and Earth’s changing surface Most of the land on Earth was part of a large landmass called Pangaea about 250 millions of years ago.

9.1 Fossils and Earth’s changing surface Fossils provide evidence for how Earth’s surface has changed over time. Scientists map fossil locations. Understanding Earth’s past helps explain how similar plants and animals ended up in different locations.

9.2 Geologic Time Scientists have developed a model of the history of life on Earth called the geologic time scale. Paleontologists divide the geologic time scale into blocks of time called eras and periods.

9.2 Precambrian era The Precambrian era lasted from Earth’s formation 4750 until 542 million years ago (mya). The first cells appeared in the Precambrian era.

9.2 Paleozoic era The Paleozoic era lasted from 542 to 251 mya. Paleozoic is a Greek word meaning “ancient life.”

9.2 Paleozoic era Rocks from the Paleozoic Era contain fossils of snails, clams, corals, and trilobites.

9.2 Paleozoic era Therapsids are a group of animals that dominated the land in the Permian Period of the Paleozoic era. Scientists have determined that mammals evolved from therapsids.

9.2 Mesozoic era The Mesozoic era lasted from 251 to 65 mya. This era is often called the Age of Reptiles.

9.2 Cenozoic era The Cenozoic era began 65 mya and is still going on. Fossils from the Cenozoic era are closest to Earth’s surface, making them easier to find.

9.2 Cenozoic era The Cenozoic Era is often called the Age of Mammals because mammals diversified into a variety of species including land mammals, sea mammals, and flying mammals.

9.2 Absolute Dating Absolute dating is a method of measuring the age of an object such as a rock or fossil in years. Scientists use both absolute and relative dating to develop the geologic time scale.

9.2 Absolute Dating Radioactive decay refers to how unstable atoms lose energy and matter over time. As a result of radioactive decay, an element turns into another element over a period of time. Carbon turns in to nitrogen over time.

9.2 The half life of uranium Scientists know that it takes 4.5 billion years for one half of the uranium atoms in a specimen to turn into lead. We say that 4.5 billion years is the half-life for the radioactive decay of uranium.

9.2 Trees and absolute dating A tree grows one tree ring for every year that it is alive. Andrew Douglass (1867–1962) was an astronomer who discovered the significance of tree rings. In the early 1900s, Douglass hypothesized that trees might record what Earth’s climate was like in the past.

9.2 Trees and absolute dating Trees are like history books. Each tree ring is a record of what the environment was like that year. Wide tree rings indicated a very wet year and narrow rings indicated a dry year. Douglass named this new field of science dendrochronology.

9.2 Trees and absolute dating The oldest tree on record is a bristlecone pine called “Methuselah.” It is 4,765 years old. These trees grow in the mountains of California. Bristlecone pine trees grow very slowly.

Biology Connection It’s All in the Rings Dendrochronologists are scientists who study tree rings to date past events. By studying the rings, they can tell how old the wood is to the exact year.