05 Geologic Time and Dating Free Online FlashCards

How do relative dating and numerical dating differ?

Relative dating orders rocks or events without assigning ages in years; numerical dating estimates an age in years, with uncertainty.

What does superposition tell you about undisturbed layers?

In an undisturbed sequence of deposited layers, the bottom layer is oldest, and each layer is older than the one above it.

What can original horizontality reveal about tilted layers?

Sediments are generally deposited in horizontal or nearly horizontal layers. Tilting or folding therefore occurred after deposition.

What is the age relationship in cross-cutting relationships?

A feature that cuts or disrupts another is younger than the feature it cuts; for example, a fault is younger than the layers it offsets.

What does the principle of inclusions indicate about enclosed fragments?

The fragments are older than the rock surrounding them because they existed before that rock formed.

How do index fossils help geologists compare rock layers?

Fossil succession is the recognizable order in which species appear and disappear. Widespread, short-lived index fossils help compare the relative ages of layers in different places.

What does an unconformity represent?

An unconformity is a surface representing erosion or a time when sediment was not deposited; it marks missing time in the rock record.

What does stratigraphic correlation identify?

Stratigraphy studies layered rocks and their relationships. Correlation identifies strata in separate locations that formed at roughly the same time or under related conditions.

What is an isotope’s half-life?

A parent isotope decays into a daughter isotope. A half-life is the time required for half of the parent atoms in a sample to decay.

How does radiometric dating estimate a mineral’s age?

Scientists measure parent and daughter isotopes and apply the known decay rate to estimate when the mineral’s isotopic clock began, such as when it crystallized.

How can geologists bracket the age of a sedimentary layer?

Sedimentary grains may be older than the sedimentary rock. Dating volcanic ash layers above and below can bracket the sedimentary layer’s age.

Why must geologists assess whether a sample remained a closed system?

The sample must preserve its isotope record. Heating, fluids, or weathering can alter it, so geologists assess whether it remained a sufficiently closed system.