Fossils and the Rock Fossils and the Rock RecordRecord

““The Geologic Time Scale”The Geologic Time Scale”

The Geologic Time ScaleThe Geologic Time Scale

• By studying the characteristics of rocks and the fossils within them, geologists can:– interpret the environments the rocks

were deposited in– Reconstruct Earth’s history– Possibly predict events or conditions in the

future

The Rock RecordThe Rock Record

• Geologists divided the history of Earth into time units based on the fossils contained within the rocks.

• Geologic time scale is a record of Earth’s history from its origin 4.6 billion years ago to the present.

• Names of the periods have not changed, but the years marking the beginning and the end of each unit of time are being changed.

Geologic TimeGeologic Time

• The oldest division of time is at the bottom of the geologic time scale.

• As you go up the younger the division.

• Time scale is divided into units:– Eons– Eras– Periods– epochs

• An eon is the longest time unit and is measured in billion of years.– Archean– Proterozoic– Phanerozoic

• An era is the next-longest time unit and is measured in hundreds of millions to billions of years:– Paleozoic (“old life”)– Mesozoic (“middle life”)– Cenozoic (“recent life”)

• Precambrian Time, which makes up approximately 90 percent of geologic time is divided into:– Archean eons– Proterozoic eons

Plants and Animals EvolvePlants and Animals Evolve

• Paleozoic Era - the oceans became full of a wide diversity of plants and animals.

• Cambrian Period – dominated by trilobites in the oceans.

• End of the Paleozoic Era is marked by the largest marine extinction event– 90 % of all marine invertebrate

• The Mesozoic Era– Dinosaurs emerged– Reef building corals– Large predatory reptiles developed in oceans– Amphibians began living on land

• Cretaceous Period– Mammals evolved– Flowering plants and trees

• End of the Mesozoic- beginning of Cenozoic:– large extinction– Remaining dinosaurs– Mammals increased (numbers and diversity)– Human ancestors developed– Grasses and flowering plants increased

Periods of Geologic TimePeriods of Geologic Time

• Periods are defined by the life-forms that were abundant or became extinct during the time in which specific rocks were deposited.– Usually 10 of millions – 100 of millions– Some named for geographic region in which

the rocks of that age were first observed• Mississippian• jurassic

Epochs of Geologic TimeEpochs of Geologic Time

• Epochs are even smaller divisions of geologic time and are usually measured in millions of years to tens of millions of years.– Rocks and fossils from this era are easily

accessed and studied– Cenozoic divided into epochs – Paleocene

and Oligocene.

• Regardless of how a geologic period was defined, each unit contains specific characteristics that set it apart from the rest of geologic history.

Relative-Age Dating of Relative-Age Dating of RocksRocks

Even into the 19th century, people believed that the world was only 6,000 years old.

James Hutton attempted to explain the forces that continually change the surface features of Earth.

The principle of uniformitarianism states that the processes occurring today have been occurring since Earth formed.

Only the rate, intensity, and scale have changed.

Ex. Ocean shore

Principles for Determining Relative Principles for Determining Relative AgeAge

Concept of relative-age dating places the age of rocks and the events that formed them in order, but w/o exact dates.

Done by comparing rocks to each other.

Geologic PrinciplesGeologic Principles

The principle of original horizontality states that sedimentary rocks are deposited in horizontal or nearly horizontal layers.

We may not know actual dates but we do know that the youngest layer is at the top and as you go down the layers get older.

This is an application of the principle of superposition.

The principle of cross-cutting relationships states that an intrusion or a fault is younger than the rock it cuts across.

InclusionsInclusions

Relative age also can be determined where an overlying rock layer contains particles of rock material from the layer beneath it.

The bottom layer was eroded, and the loose material on the surface became incorporated in the newly deposited top layer.

These particles, called inclusions, indicate that the rocks in the lower layer are older than those on top.

Other Means of Determining Other Means of Determining Relative AgeRelative Age

The fact that Earth is constantly changing as a result of processes such as weathering, erosion, earthquakes, and volcanism makes it difficult to find an undisturbed sequence of rock layers.

An erosional surface might become buried by the deposition of younger rocks.

This buried erosional surface results in a gap in the rock record and is called an unconformity.

Horizontal sedimentary rocks overlie horizontal sedimentary rocks, it is called disconformity.

A different type of unconformity exists when sedimentary rocks overlie nonsedimentary rocks such as granite or marble.

The contact point between the nonsedimentary and sedimentary rock is called a nonconformity.

Figure 21-7 pg. 561

Correlation of Rock StrataCorrelation of Rock Strata

Correlation is the matching of outcrops of one geographic region to another.

Geologists examine rocks for distinctive fossils and unique rock or mineral features to help correlate the rock layers.

Absolute-Age Dating of Absolute-Age Dating of RocksRocks

VocabularyVocabulary

• Radioactive decay

• Radiometric dating

• Half-life

• Dendrochronolgy

• Varve

• Key bed

• Absolute-age dating enables scientists to determine the actual age of a rock, fossil, or other object.

• Scientists use the decay rate of radioactive isotopes.

• Radioactive substances emit nuclear particles

• The number of protons and neutrons lower with each emission.

• Thus the element is changed to a different element.

• The old radioactive element is called the “parent”

• The new is called the “daughter”

• The emission of radioactive particles and the resulting change into other elements over time is called radioactive decay.

Use of Radioactive IsotopesUse of Radioactive Isotopes

• In a process called radiometric dating, scientists attempt to determine the ratio of parent nuclei to daughter nuclei within a given sample of a rock or fossil.

• Because it often takes a long time for the entire amount of an isotope to decay, geologists use the length of time it takes for one-half of the original amount to decay.

• This period of time is called the half-life.

Carbon - 14Carbon - 14

• Commonly used radioactive isotope in determined the absolute age of an object.

• Especially used for materials of organic origin.

• C-14 decays into non-radioactive N-14 .

• It has a half-life of 5730 years.

• Accurate up to 75,000 years.

• Different radioactive elements are used based on the relative age of the object.

Other Ways to Determine AgeOther Ways to Determine Age

• Naturally occurring materials, such as trees, lake-bottom sediment, and volcanic ash can also be used to determine age of objects.

Tree RingsTree Rings

• The age of a tree can be determined by counting the number of annual tree rings in a cross section of the tree.

• The widths of tree rings are directly related to the climatic conditions during growth periods.

• Dendochronology is the science of comparing annual growth rings in trees to determine events and changes in past environments.

Seasonal Climatic ChangesSeasonal Climatic Changes

• Summer deposits are generally light-colored and relatively thick compared to the thinner, organically enriched and dark-colored sediments of winter.

• These bands of alternating light and dark-colored sediments of sand, clay and silt are called varves.

Distinctive Sediment LayersDistinctive Sediment Layers

• 66 million years ago an asteroid 10 km in diameter hit the Yucatan peninsula in Mexico.

• It thru dust and crushed rock into the atmosphere

• This dust and rock then settled onto the Earth’s surface and became a sediment layer found in many places around the world.

• This layer became a key bed.• A key bed is a distinct layer that can be

used to correlate rock layers across large areas.

• These layers are easy to recognize, like a coal bed.

• Mt. St. Helen – the ash will eventually become a thin, clay layer that will mark the date of the eruption.