A Trip Through Geologic Time

Finding the Age of Rocks Through Rock Dating

D A T I N G R O C K

Ages of Rocks The relative age of a rock is its age compared to other rocks. Use words like: “older or younger” The absolute age of a rock is the number of years since the rock was formed. Ex: 358-360 mya

The Position of Rock Layers It can be difficult to determine a rocks absolute age. So… scientists use the law of superposition. According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Rock Joke!! How do rocks wash their clothes? The Rock Cycle!!

Other Clues to Relative Age Clues From Igneous Rock Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older. Magma that cools beneath the surface is called an intrusion. An intrusion is always younger than the rock layers around an beneath it.

Other Clues to Relative Age Faults (a break in the rock) are always younger than the rock it cuts through! Unconformities: An unconformity is a gap in the geological record that can occur when erosion wears away rock layers and other rock layers form on top of the eroded surface.

Rock Joke!! What does a rock want to be when it grows up? A Rock Star!!

What are Fossils? What are different types of fossils? The Fossil Record What are Fossils? What are different types of fossils?

Fossil Record The fossil record provides evidence about the history of life on Earth.

The fossil record also shows how different groups of organisms have changed over time. It also provides some evidence to support the theory of evolution.

Remember! A scientific theory is a well-tested concept that explains a wide range of observations. The fossil record shows that millions of types of organisms have evolved. However, many others became extinct.

Fossils

Fossils are preserved remains or traces of living things. Scientists who study fossils are paleontologists. “Paleontology” Most fossils form when living things die and are buried by sediments. The sediments slowly harden into rock and preserve the shape of the organisms.

Fossils Fossils are usually found in sedimentary rocks. When an organism dies, its soft parts often decay quickly leaving only the hard parts to fossilize. Ex. Bones, Shells, Teeth, or Seeds

Kinds of Fossils Petrified Fossils: fossils in which minerals replace all or part of the organism. Ex: petrified wood (Permineralization) When the object is buried by sediment, water rich in minerals seeps into the cells. After the water evaporates, hardened minerals are left behind.

Kinds of Fossils 2. Molds and Casts A mold is a hollow area in sediment in the shape of an organism or part of an organism. A cast is a copy of the shape of an organism.

Kinds of Fossils 3. Carbon Films: an extremely thin coating of carbon on rock that forms when materials that make up an organism become gases and escape leaving only carbon behind.

4. Actual Body. Fossils in Amber Organisms stuck in hardened tree sap

Preserved Remains are formed when an organism is preserved with little or no change. For example when organisms become preserved in tar, amber (tree sap), and freezing.

Mummification

4. Trapped in Ice-Frozen Fossils

5. Trapped in Tar Fossils in La Brea Tar Pits.

6. Buried in Rock

Go to: http://www.tarpits.org/page-museum/exhibits

7.Trace Fossils provide evidence of the activities of ancient organisms. Ex: footprints, animal trails, or animal burrows.

Permian mammal-like reptile tracks, Arizona

Coprolite (Fossilized dung). Probably tortoise Miocene Epoch (Approx Coprolite (Fossilized dung). Probably tortoise Miocene Epoch (Approx. 22 million years) Washington state

Dinosaur nest from Lower Jurassic of South Africa, showing embryos in some of the eggs Credit: D. Scott

Why Study Fossils? Scientists study fossils to learn what past life forms were like. Paleontologists classify organisms in the order in which they lived. All the information scientists have gathered is called the fossil record.

Fossils Tell us what?? About environmental changes over time How life forms have changed over time

Using Fossils to Date Rocks! Scientists use index fossils to match rock layers. An index fossil must be widely distributed and represent a type of organism that existed only briefly. They are useful because they tell the relative ages of the rock layers they are found in.

The Trilobite An index fossil. A group of hard-shelled animals whose bodies had three distinct parts. Evolved in shallow seas more than 500 million years ago.