Chapter 23 The Rock Record------Relative Dating

Unconformity

A break or gap in the geologic record, cause by erosion of preexisting rock or by an interruption in the sequence of deposition.

Radiometric Dating

A method for calculating the age of geologic materials based on the nuclear decay of naturally occuring radioactive isotopes

Time Gap

Although deposition is ongoing, so are the processes of weathering and erosion, folding and faulting, and crustal uplift. These processes remove rock layers or interrupt deposition.

Cross-cutting relationships

An igneous intrusion or faulc that cuts through preexisting rock is younger than the rock through which it cuts.

Angular unconformity

An uncongormity in which older tilted or folded rock sedimentary are covered by younger relatively horizontal rocks layers

Faunal succession

Fossil organisms follow one another in a definite, irreversible time sequence. Fossil communities change through time as some species become extinct and new ones appear. Such changes are reflected in the rock record. In this way, fossils provide a key tool for recognizing the relative age of rocks.

What can be used to chronologically order the vertical locations

Fossils

Superposition

In an undeformed (horizontal) sequence of sedimentary rocks, each layer is older than the one above and younger than the one below.

Original horizontality

Layers of sediment are deposited horizontally. Layers that are tilted or folded must have been moved into that position by disturbances, such as earthquakes and mountain building, after deposition.

Ma

Mega-annum, or "one million years (ago)."

Lateral continuity

Sedimentary layers are deposited in all directions over a large area unless some sort of obstruction or barrier limits their deposition. Faulting, folding, and erosion can separate originally continuous layers into isolated outcrops.

These four principles are fairly straightforward and can be used to determine the ages of rock formations relative to one another-which formation was formed first, second and so on

Superposition Relative dating Cross-cutting relationships Inclusions.

This immense span of time is divided into three eons

The Hadean The Archean The Proterozoic

The Phanerozoic eon is subdivided into three eras

The Paleozoic era (time of ancienc life), the Mesozoic era (time of middle life), and the Cenozoic era (time of recenc life).

What illustated the principle of inclusion?

The rocks locked in the sedimentary layer existed before the sedimentary layer formed.

Precambrian time

The time of hidden life, which began about 4.5 billion years age when Earth formed and lasted until 543 million years ago( beginning of Palezonic), and makes up almost 90% of Earth history. Sometimes referred to as a "super-eon," it does not have a formal rank-it is simply

Geologic Time Scale

The timeline for the history of Earth

Nonconformity

When overlying sedimentary rocks are found on an eroded surface of metamorphic or igneous plutonic rocks

What does the term unconformity mean?

a missing rock layer in a sequence that represents a period of erosion or nondeposition.

Stromatolites

are algae like colonies of cyanobacteria inter-layered with carbonate sediments

Inclusions

are piece of one rock contained within another. Any inclusion is older than the rock containing it, just as small pieces of rock incorporated in a slab of concrete were formed before the concrete was formed.

Periods

are the fundamental time interval because each period represents a major change in life forms.

Disconformity

is a time gap between parallel layers of sedimentary rock.

Relative dating

the approximately determination of the age of a rock or event in relation to the age of other rocks or events. The ordering of rocks in sequence by their comparative ages.

Eons

the largest unit of geologic time

Phanerozoic

visible life