Geology Chapter 12 Geologic Time We will cover: A.Relative Age Principals B.Unconformities & Stratigraphic Formations C.Absolute Age D.The Geologic Column Divisions

12A Relative Age Principles Relative Age- the age of one geologic feature compared to another Absolute (Numerical) age- the age of a geologic feature in years (cover this later in this chapter) We determine relative age through several principles (8 listed here).

Relative Age Principles 1.Uniformitarianism- the processes we observe today also operated in the past at comparable rates “The present is the key to the past.” 2. Superposition- in the sequence of sedimentary rock layers, each layer is younger than the one below. The layer at the bottom is the oldest, the layer at the top is the youngest.

James Hutton

Relative Age Principles 3.Original horizontality- sediment layers are horizontal when they accumulate. If they are no longer horizontal some other process must have been folded or otherwise tilted after formation. 4.Original continuity- sediments generally accumulate in continuous sheets. If you find a sedimentary bed cut by a canyon, you know that the bed once crossed the canyon, but was later eroded away.

Relative Age Principles 5.Cross-cutting relations- if one geologic feature cuts across another, the feature that has been cut must be older. If a rock is faulted, you know the fault must be younger than the rock. 6.Inclusions- if an igneous intrusion contains xenoliths (fragments) of another rock, the fragments must be older than the intrusion. If a layer of sediment contains fragments of an igneous rocks, the igneous rock must be older than the sediment layer.

Relative Age Principles 7.Baked contacts- The rock that has been baked (contact metamorphism) by an igneous intrusion must be older than the igneous intrusion. 8.Fossil succession- Since a specific group of species only exists for a limited amount of time, if a species appears in a limited time interval of strata (sedimentary layers) it will not appear above or below that time. Extinction is forever.

12 B Unconformities & Stratigraphic Formations Unconformity- a surface within rock layers that represents a period of non- deposition or erosion Basically there is no rock from a certain time and we may have lost some rock during this time.

Types of Unconformities 1.Angular unconformity- rocks below this unconformity were tilted or folded before the unconformity

Types of Unconformities 2. Nonconformity- a type of unconformity where sedimentary rocks overlie intrusive igneous rocks and/or metamorphic rocks. The igneous rocks must have been cooled, uplifted and been exposed to erosion in order for the sedimentary layer to form on top of it.

Nonconformity

Types of Unconformities 3. Disconformity- If there is an erosional boundary between two sedimentary layers representing a time when sea level sank and erosion was prevalent it is called a disconformity. (see picture p. 374)

Stratigraphic Formations Strata- several sedimentary beds together Stratigraphic column- a cross section diagram of a sequence of strata summarizing information about a sequence Stratigraphic formation- recognizable layers of a specific rock type or group of rock types deposited during a specific time interval that can be traced over a broad region

Stratigraphic Formations Group- several formations in a succession are often lumped into these Correlation- the process of defining the relationship between the strata at one locality to another Often uses: key beds (an unusual rock bed) and fossil evidence (including index fossils- a fossil known to a certain time period)

Trilobite- Common index fossil

12 C Absolute Age Absolute (Numeric) age- age in years of a rock, fossil or event Geochronology -the science of dating geologic events in years Ways to determine absolute age: (listed in following slides, 8 total)

Absolute Age Determination 1. Radiometric dating- Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay. Radioactive parent elements decay to stable daughter elements (a different element that the parent!). The time it takes for half of the parent isotope to decay into a daughter product is called a half life. Radiometric dating is generally accurate for 5 half lives, so different elements are used depending on the age of the rock.

Absolute Age Determination (radioactive dating continued) Uranium and Uranium-238 (both decay to lead) are useful in older rocks because there half lives are over a billion years. Carbon-14 (decays to nitrogen) has a half life of 5,730 years and therefore is only useful for about 28,000 years- not very useful in dating rocks.

Absolute Age Determination 2.Growth rate of trees- trees grow seasonally, so growth rings can be counted. 3.Organic productivity of lakes and seas- during winter, there are cool temperatures, which causes numbers of organisms to decrease.

Absolute Age Determination 4.Sediment supply carried by rivers- in regions that have a wet and dry season, the rainy season will be marked with more sediment, while the dry season will have little sediment accumulation. 5.Growth of chemically precipitated sedimentary rocks- chemical sedimentary rocks (such as travertine in caves) grow over time

Absolute Age Determination 6.Growth rate of shell-secreting organisms 7.Proportion of snow, relative to dust, that falls in cold- during the wet season more snow falls in proportion to dust than in the dry season. 8.Magnetostratigraphy- since geologists have determined when polarity reversals have taken place, they can be used to date rocks.

12D The Geologic Column Divisions The geologic history of Earth (all 4.6 billion years of it) is divided into intervals based on events during these times. Intervals: 1.Eons- the largest intervals. We have 4- from oldest to youngest: Hadean, Archean, Proterozoic, and Phanerozoic.

Hadean

Archean

Proterozoic

Time Intervals 2.Eras- Eons are divided into Eras defined on the basis of the presence or absence of fossils found in the rocks. The Archean and Proterozoic Eons are not usually divided into Eras due to absence of fossils, but the Phanerozoic is divided into the Palaeozoic (old life), the Mesozoic (middle life) and the Cenozoic (recent life).

Paleozoic

Mesozoic

Cenozoic

3.Periods- Eras of the Phanerozoic Eon are divided into Periods. Periods are sometimes named after the geographical location where originally studied (e.g. the Cambrian - 'Cambria' is the Roman name for Wales), or on the characteristics of the strata found (e.g. the Cretaceous - is the Latin for chalk).

Time Intervals 4.Epochs (pronounced eeee-pock)- Certain Phanerozic Periods are further subdivided into Epochs by the fossils they contain. Epochs are terms mainly used by specialists concerned with very detailed studies of strata and corresponding fossils.

How large is the history of Earth? If you lined up 4.6 billion pennies in a row, the line would go around Earth’s equator twice! (Human history would only be about 100 city blocks of it)

If we condensed the 4.6 billion years into one calendar year: the oldest rocks on the planet would be from early February The first bacteria would appear in the oceans on February 21 st The first invertebrates would appear on October 25 The first amphibians would crawl on to land on November 20 th Pangaea would form on December 7 th

1 st mammals and birds would appear on December 15 th Dinosaurs would die out on December 25 th The first human like creatures would appear on December 31 st at 3PM Homo sapiens (our species) would appear an hour before new years The last ice age would end an hour before midnight ALL of recorded human history would take place in the last 30 seconds before midnight (Human history is % of Earth history)