Chapter 21 Fossils and Rock Record. Vocabulary Uniformitarianism- processes happening today have been happening since Earth formed. (ex. Earthquakes and.

Chapter 21

Fossils and Rock Record

Vocabulary• Uniformitarianism- processes happening today

have been happening since Earth formed. (ex. Earthquakes and volcanic eruptions)

• Relative-age dating- order in which geological event occur.

• Original horizontality- sedimentary rocks are naturally formed in flat or horizontal layers.

• Superposition- oldest rock layers are on the bottom and the youngest rock layers are on top.

Vocabulary • Cross-Cutting Relationships- intrusions or faults

are younger than the layers that they cut across.• Principle of Inclusion- inclusions must be older

than the rock layer that contain it.• Unconformity- a gap in the rock record caused

by erosion or weathering. • Correlation- matching outcrops of one

geographic region to another.• Key Bed- a rock or sediment layer used as a

marker.

Principles of Determining Relative Age

1.Original Horizontality2.Superposition3.Cross-cutting Relationships

Draw a Diagram

B

A

Intrusion

Intrusions form when magma is pushed up in between cracks in Earth.

Create a Diagram

Intrusion EIntrusion F

A

B

DC

Questions

Rock layers oldest to youngest A, B, C and D- superposition.

Intrusion E is younger than layer D, but older than intrusion F- Cross-cutting relationships.

Rock layers A,B,C, and D are flat because of original horizontality.

Compare Unconformities

Unconformities

Type: disconformityHow formed? Sedimentary rock layer deposited on top of an eroded sedimentary rock layer

Type: angular unconformityHow formed? Horizontal sedimentary rocks are uplifted and tilted then weather and erode; sedimentary rocks are deposited on top of them, but they are at an angle to the eroded surface.

Type: nonconformityHow formed? Sedimentary rock layer is deposited on top of an eroded metamorphic or igneous rock layer.

OtherJames Hutton and Charles Lyell (Founders of Modern Geology) thought that the “present was the key to the past” – Earth works in an orderly fashion in which natural phenomenon will recur given the same set of conditions.

VocabularyDrought-extended period of low rainfall.Radioactive Decay- emission of atomic particlesRadiometric Dating- process used to determine the absolute age of a rock or fossil by determining the ratio of parent nuclei to daughter nuclei within a given sample.Half-life- period of time it takes a radioactive isotope to decay to ½ of its original amount?Dendochronology- science of comparing annual growth rings in trees to date events and environmental changes.Varve- alternating light-colored and dark-colored sedimentary layers of sand, clay, and silt deposited in a lake that can be used to date cyclic events and changes in the environment.Key Bed- sediment layer that serves as a time marker in the rock record.Similar- nearly the same or alike

Compare absolute and relative dating

Relative-Age Dating

Absolute-Age Dating

Measures

Ages of rocks and events that formed them in order.

The actual age of a rock, fossil, or other object.

Half-Life Drawing

http://glencoe.com/sec/science/earthscience/2007/concept_motion/animated_art/HalfLife21_15.mpg

Watch the video clip on half life and draw a simple picture showing what half life is.

Radioactive Decay of Uranium 238

PercentParent Element

PercentDaughterElement

ElapsedYears

Number of Half-Lives

Time 1 100 0 0 0

Time 2 50 50 4.5 billion

1

Time 3 25 75 9 billion 2

Time 4 12.5 87.5 13.5 billion

3

Time 5 6.25 93.75 18 billion 4

Radioactive Decay of Uranium-238 to Lead-206

21.4 Vocabulary

Trace Fossil- imprints left by organisms that give insight as to how the organisms lived, moved, or obtained food.

Index Fossil- the remains of a plant or animal geologists use to correlate or date rock layers.

Useful Fossils

1.Provide information about past environmental conditions

2.Be used to correlate rock layers from one area to another

3.Provide evidence that population have evolved in response to changes in their environments.

PetrificationMineral solutions such as groundwater replace the original organic materials that were covered by layers of sediment with new materials.

CoproliteFossilized dung or feces from ancient animals.

Gastrulates

Dinosaurs had stones in their digestive systems to help digest their food.

Mold and Casts

When a shell is buried, its remains decay, and sand or mud fills it in.

A cast is a replica of the original organism.

ImprintsCarbonized imprints of leaves, stems, flowers, and fish made in soft mud or clay have been found preserved in sedimentary rock.

Freezing

The low temperatures of frozen soil and ice can protect and preserve organisms.

AmberHardened tree sap is called amber. Insects become trapped in the sticky sap and are preserved when the sap hardens.

MummificationMummified remains are often found in very dry places, because most bacteria which cause decay cannot survive in these places.

What is needed for a index fossil?It must be:

1. It must be distinct 2. abundant 3. widespread 4. existed for only a short span of

geologic time.

+ Write down somewhere on pg. 250

Index Fossils and Relative Age