Determining Ages of Rocks

• Explain how rock strata and fossils help create the geologic time scale.

• Differentiate between relative age and absolute age using key geological principles.

• Describe how major geologic events help define different eras in Earth's history.

• Use evidence from rock layers to determine the sequence of geologic events.

Key Vocabulary

Relative Age

The age of a rock or geologic feature when it is compared to other rocks.

Absolute Age

The exact numerical age of a rock, often given in millions of years since it formed.

Rock Strata

These are the distinct layers of rock, typically of sedimentary rock, that have been deposited.

Law of Superposition

This scientific law states that in undisturbed rock layers, the oldest strata are at the bottom.

Cross-Cutting Relations

This principle states that any geologic feature that cuts across another is the younger feature.

Fault

A fault is a large crack in the Earth's crust where blocks of rock move.

Key Vocabulary

Index Fossil

An index fossil is the preserved remains of a widespread organism that existed for a short period.

Unconformity

An unconformity represents a significant gap in the geologic record, often caused by long periods of erosion.

Geologic Time Scale

The geologic time scale is a chronological timeline that represents Earth's entire 4.6-billion-year history.

Relative Age

• Compares the age of a rock to the ages of other rocks.

• Places rock layers and events in a sequence without an exact date.

• It determines if a rock is older or younger than another.

Absolute Age

• Provides the specific number of years since the rock was formed.

• It gives a precise numerical date for the rock or event.

• Used with relative dating to reconstruct the history of Earth.

Law of Superposition

• In undisturbed rock layers, the oldest layer is always found at the very bottom.

• The youngest rock layer is located at the top of the sequence of rock layers.

• This principle helps determine the sequence of events and the relative age of rocks.

Cross-Cutting Relationships

• Any geologic feature that cuts through rock layers is younger than those layers.

• These features can be faults, which are breaks in the rock, or igneous intrusions.

• The original rock layers had to be present first for the feature to cut them.

Using Fossils to Date Rocks

• Fossils are the preserved remains or traces of once-living things.

• The fossil record shows when organisms appeared and went extinct.

• Index fossils are from widespread organisms that lived for a short time.

• Matching index fossils means different rock layers are the same age.

Gaps in the Record

• An unconformity is a surface where new rock layers meet a much older rock surface beneath them.

• This represents a gap in time where older rock layers were lost before new sediment was deposited.

• The loss of these older rock layers is typically caused by the natural process of erosion.

Folding

• Powerful forces inside the Earth can bend and squeeze rock layers, causing them to fold.

• An overturned fold occurs when these rock layers are flipped upside down due to extreme pressure.

• This can result in the oldest rock layer being on top, which is a reversal of superposition.

The Geologic Time Scale

• The geologic time scale organizes Earth's 4.6-billion-year history using rock and fossil evidence.

• Its timeline is divided by major events like extinctions or asteroid impacts.

• It shows Earth's history is immense compared to the short timescale of human history.

• The geologic time scale organizes Earth's history by analyzing rock strata and fossils.

• The law of superposition and cross-cutting relationships determine the relative age of rocks.

• Index fossils correlate rock layers, but unconformities and folding can disrupt these layers.

• Major events, like mass extinctions, define the main divisions in the geologic time scale.