CHAPTER 5: INTERPRETING EVIDENCE IN

ROCKS

Topic 1: What You Can Learn From Rocks

Interpreting Clues From Rocks

◦Rocks are solid material that make up the outer layer of the Earth.

◦Geologists study the physical properties of rocks to tell how the rocks may have formed.

◦Geologists look for minerals (naturally occurring substances that are neither plants nor animals) in rocks. ◦Minerals are the building blocks of rocks. ◦Minerals have properties that help identify them. Ex: color

Telling Minerals Apart ◦Mineral detectives look beyond color. See Mineral Identification Table on page 150. 1. Luster – the way light bounces off the mineral2. Rubbing the mineral gently but firmly on a steak

plate to see what color it streaks. 3. Hardness of the mineral.4. Whether it splits easily on a flat surface. 5. Whether they break evenly or unevenly.6. Special properties. Ex: magnetite is attracted by

a magnet.

Classifying Rocks

◦Geologists study properties of rocks and classify them into three main groups based on how they are formed.

1. Igneous rocks2. Sedimentary rocks3. Metamorphic rocks

Types of Rocks: Igneous Rocks◦“Fire made” rock formed from melted rock material that has cooled and hardened.

◦Examples:

Granite

Types of Rocks: Sedimentary Rocks

◦Formed from bits or layers of rocks cemented together.

◦Older sediments are on the bottom layers and newer towards the top.

◦Examples:

◦Geologists use layers in sedimentary rocks to tell the rocks relative age – describes the age of something compared to the age of another thing.

Types of Rocks: Metamorphic Rocks

◦Rocks whose form has been changed by heat and/or pressure.

◦Classified into two groups: • Pressure causes minerals to spread out in bands. Rocks can break along these bands.

◦Do not have bands of minerals and do not break in layers.

Rock Cycle◦Rocks are always changing and all rocks are part of the rock cycle.

◦Rock Cycle – a process by which rocks are changed from one type to another.

CLUES FROM FOSSILS

Topic 2

Learning from Fossils

◦Clues from fossils teach scientists about the past. ◦Past events◦Past environments◦Past organisms

Other Evidence Fossils Provide

◦Age of organisms when they died. ◦Annual growth rings in petrified wood tell the age of fossil trees.

◦Footprints tell if the organism was young or old.

◦Characteristics of organisms.◦Footprint size tells us the animal’s size.◦Distance between footprints tells if it was walking or running.

◦Footprints tell if it had two or four legs.

Other Evidence Fossils Provide

◦What an animal ate. ◦Meat eaters – strong jaws with many pointed teeth

◦Plant eaters – weaker jaws with flat teeth◦Fossilized stomach content show what an animal actually ate.

Other Evidence Fossils Provide

◦Past environments◦Aquatic organisms tell us where rivers, lakes, or oceans once were.

◦Can tell what parts of the world were once colder or warmer◦Evergreen fossils tell us the area was once cool◦Fern fossils tell us the area was moist or warm

How Do Fossils Form?

1. A plant or organism dies.

2. Soft parts decay first.

3. Hard parts (teeth, bones, shells) last longer and could become fossils and are buried by sediments.

4. Sediments become rocks and fossils form.

Types of Fossils◦Imprint – a fossil created by a print or impression◦A shallow print or impression.

◦Ex: animal tracks, body outline, leaf prints, and grooves made by fish bones.

Types of Fossils

◦Mold – a hollow fossil form clearly showing the outside features of the organism. ◦Forms when water seeps into the rock where a shell is buried dissolving the shell leaving a hollow space showing the outside features of the shell.

Types of Fossils

◦Cast – a fossil formed or shaped within a mold.◦Forms when a mineral slowly accumulates in a mold and takes the shape of the original shell making a copy.

Other Ways Fossils Form

◦Amber – hardened tree sap; often the source of insect fossils◦whole insects get trapped in sap from trees

Other Ways Fossils Form

◦Whole animals can be preserved by being frozen. ◦Ex: Mammoths

Other Ways Fossils Form

◦Tar Pits◦Animals like saber tooth tigers, camels, lions, and mammoths got stuck and their flesh decayed while bones sank.

Other Ways Fossils Form

◦Mummies◦Animal remains slowly dried out in hot regions like deserts.

Other Ways Fossils Form

◦Carbon films◦Plants and animals decay slowly and leave a thin film of the element carbon that show detailed outlines.

◦Ex: ferns, leaves, and fish.

Other Ways Fossils Form◦Petrified

◦“turned to stone”◦Especially happens to wood and bones.

◦Forms when minerals seep into pores in bones or hollow space in wood and fill them. When the bone or wood is completely dissolved and replaced by mineral it is petrified.

What happens when fossils are found?

1. Fossilized bones are wrapped in plaster jackets and protected, labeled and sent to a museum or university.

2. The last bits of rock are removed from the fossil.

3. Bones are assembled.

4. Fossils are put away for further study.

* Museums display casts not the actual fossil.