Geologic Time

• Faults and intrusions that cut across layers are younger than the layers that they cut through.
• Younger features cut across older features.
• "Cutting a cake"

• Placing rocks in their proper sequence of formation.
• How old a rock is compared to surrounding rocks.
• Non-specific - something is older than something else.

Actual number of years since the rock was formed.

In a sequence of undisturbed layered rocks, the oldest rocks are on the bottom.

Layered strata are deposited horizontally, or nearly parallel to the Earth's surface.

Groupings of fossil plants and animals follow or succeed each other in time in a predictable manner.

• An inclusion is a piece of rock that is enclosed within another rock.
• The inclusion is younger than the rock it is within.

An uncomformity is a break or gap in the rock record produced by erosion and/or nondeposition of rock units.

• Disconformity
• Angular Unconformity
• Noncomformity

• Strata on either side of the unconformity are parallel; usually sedimentary layers of greatly differing ages.
• In a succession of rock layers (sedimentary strata or lava flows) parallel to one another, the disconformity surface is a gap in the layering. The gap may be a non-depositional surface where some layers never formed for a while, or the gap may be an erosional surface where some layers were removed before younger layers covered up the surface.

• Tilted rocks are overlain by flat-lying rocks.
• An angular unconformity is an erosional surface between two bodies of layered sedimentary strata or lava flows that are not parallel. The gap is because the older body of layered rock was tilted and partly eroded (rock was removed) before a younger body of horizontal rock layers covered the eroded surface.

• Metamorphic or igneous rocks in contact with sedimentary strata.
• A nonconformity is an erosional surface between older igneous and/or metamorphic rocks and younger rock layers (sedimentary strata or lava flows). The gap is because some of the older igneous and/or metamorphic rocks were partly eroded (rock was removed) before the younger rock layers covered the eroded surface.

The percentage of radioactive atoms that decay during one half-life is always the same (50 percent) no matter how many there are to begin with.

The time it takes for 1/2 of the atoms in a rock sample to decay

The actual number of atoms that decay continually decreases. Comparing the ratio of parent to daughter yields the age of the sample.

Spontaneous changes (decay) in the structure of atomic nuclei.

An unstable radioactive isotope.

The isotopes resulting from the decay of a parent

• Radiocarbon dating
• Half-life of only 5730 years
• Used for very recent events
• Carbon-14 is produced in upper atmosphere
• Useful tool for anthropologists, archaeologists, and geologists who study very recent Earth history.

• Radiometric dating is a complex procedure that requires precise measurement
• Rocks from several localities have been dated at more than 3 billion years
• Confirms the idea that geologic time is immense

• Not all rocks can be dated by radiometric methods.
• Grains comprising detrital sedimentary rocks are not the same age as the rock in which they formed.
• The age of a particular mineral in a metamorphic rock may not necessarily represent the time when the rock formed.
• Datable materials (such a volcanic ash beds and igneous intrusions) are often used to bracket various episodes in EArth history and arrive at ages.

Fossils are the remains/traces of prehistoric life.

• Molds and casts
• Carbonization
• Tracks
• Burrows
• Coprolites (fossil dung)
• Gastroliths (polished stomach stones)

• Cenozoic (recent life - now)
• Mesozoic (middle life)
• Paleozoic (ancient life)
• Precambrian (hidden life)

• Eras are subdivided by into Periods.
• Periods are subdivided into Epochs.