Chapter 9 – Geologic Time

Numerical date

actual number of years since an event happened

Relative date

places events in order (older vs. younger) without assigning numbers

Principle of Superposition

In undisturbed sedimentary layers, oldest on bottom, youngest on top

Sedimentary layers

stacked beds of deposited sediment

Principle of Original Horizontality

Sediments are deposited in horizontal layers

If sedimentary layers are tilted or folded

deformation occurred after deposition

Principle of Lateral Continuity

Sedimentary beds originally extend in all directions until they thin out or change type

Principle of Lateral Continuity Used to

match layers across canyons or distances

Principle of Cross-Cutting Relationships

A feature that cuts across rock (fault, dike, intrusion) is younger than the rock it cuts

Principle of Inclusions

Rock fragments inside another rock → inclusions are older than the surrounding rock

Unconformities

Gaps in the rock record

Unconformities Examples

• Angular unconformity

• Disconformity

• Nonconformity

Angular unconformity

tilted/folded rocks overlain by flat layers

Disconformity

parallel layers with an erosional gap

Nonconformity

sedimentary rocks on top of older igneous/metamorphic rocks

Sedimentary rocks

Rocks formed from deposited sediments that get compacted and cemented over time. (Examples: sandstone, shale, limestone)

Igneous rocks

Rocks formed from cooled and solidified magma or lava. (Examples: basalt, granite)

Metamorphic rocks

Rocks changed by heat, pressure, or chemical processes without melting. (Examples: marble, schist)

Fossil

Remains or traces of prehistoric life

Fossils found mainly

in sedimentary rocks

Ways Fossils Are Preserved

• Permineralization

• Molds

• Casts

• Carbonization

• Impressions

• Amber

• Trace fossils

Permineralization

minerals fill pores (ex: petrified wood)

Molds

empty cavity left when shell dissolves

Casts

cavity later filled with minerals

Carbonization

thin film of carbon remains (plants, delicate animals)

Impressions

imprint remains after carbon film is lost

Amber

insects trapped in tree resin

Trace fossils

tracks, burrows, coprolites (dung), gastroliths

Trace fossils - Tracks

Footprints or trails left by organisms

Trace fossils - Burrows

Holes or tunnels dug by animals into sediment

Trace fossils - Coprolites (Dung)

Fossilized animal droppings

Conditions Favoring Fossilization

• Rapid burial.

• Possession of hard parts (bones, shells).

• Soft-bodied organisms rarely preserved.

Correlation

Matching rocks of the same age at different places

Principle of fossil succession (Fossils as Correlation Tools)

fossils occur in a definite, recognizable order

Global Fossil Order

The same fossil order occurs worldwide

Index Fossils

Widespread, short time range → excellent for dating layers.

Fossil Assemblages

Group of fossils used together to pinpoint a specific time

Environmental Indicators

Fossils help identify past environments

Environmental Indicators - Shoreline Energy from Fossils

Thick shells indicate high-energy shorelines

Environmental Indicators - Water Temperature from Fossils

Corals indicate warm, shallow water

Atomic number

# of protons

Mass number

protons + neutrons

Isotopes

same element, different neutrons

Types of Radioactive Decay

• Alpha decay

• Beta decay

• Electron capture

Alpha decay

loses 2 protons + 2 neutrons (mass −4, atomic # −2)

Beta decay

neutron becomes proton + electron (atomic # +1)

Electron capture

proton + electron = neutron (atomic # −1)

Half-Life

Time for half of radioactive parent atoms to decay

Parent decreases

→ daughter increases predictably

Radiometric Dating

Uses parent/daughter ratios + half-life to determine rock ages

Radiometric Dating Requires

a closed system (no loss of isotopes)

Common Isotopes for Dating

• U-238 → Pb-206 (4.5 billion yrs)

• U-235 → Pb-207

• Th-232 → Pb-208

• K-40 → Ar-40 (1.3 billion yrs)

• Rb-87 → Sr-87

Carbon-14 Dating

• Dates once-living materials.

• Half-life = 5,730 years.

• Works up to ~70,000 years.

Why Sedimentary Rocks Are Hard to Date

Sediment grains come from older rocks → not same age as the layer

Dating Sedimentary Layers Solutions

• Use igneous intrusions, lava flows, or volcanic ash beds to bracket ages.

• Apply relative dating principles + radiometric dates from igneous rocks.

Eon

Largest division of geologic time.

Phanerozoic

Eon meaning “visible life.”

Precambrian

Eon containing Archean + Proterozoic.

Era

Major life changes (Paleozoic, Mesozoic, Cenozoic).

Period

More specific life changes (e.g., Jurassic).

Epoch

Finer time divisions (e.g., Holocene, Pleistocene).

Precambrian – Characteristics

88% of Earth history.

Precambrian – Fossils

Few fossils; mostly soft-bodied life.

Precambrian – Rocks

Rocks heavily metamorphosed; harder to interpret.

Dynamic Time Scale

Updated as new data emerges (e.g., Quaternary revised to 2.6 Ma).

Geologic time scale from largest to smallest

Eon → Era → Period → Epoch