Illustrate the immense scale that geologic timeSummarize the geologic time scaleExplain the values of loved one dating and also unconformitiesApply ethics of relative and absolute date to recognize the eras of rocks
3.1 Introduction

Earth is 4.543 billion years old. It is 4,543,000,000 years, an lot of time so tremendous that it’s difficult to master just exactly how long it is. To put this right into perspective, if the average human being lifespan is 80 years, the planet has been approximately for 57,000,000 lifetimes. Or if you have a penny for each year the earth has been around, girlfriend would have actually $45.4 million! Constantly writing out millions and also billions of year is time-consuming, so once geologists talk about ages, they use a few abbreviations. The symbols ka (thousands), Ma (millions), and also Ga (billions) describe points gradually like a date. For example, the dinosaur extinction developed at 66 Ma. Geologists additionally use other abbreviations because that lengths the time, consisting of ky, kya, kyr, and also k.y. For hundreds of years; my, mya, myr, and m.y. For millions of years; and by, bya, byr, and also b.y. Because that billions that years. All four varieties that abbreviations typical the very same thing in this case. Here, you would certainly say the dinosaurs have actually been extinct for 66 myr. If this sounds confusing, you’re no alone because even part geologists use every one of the abbreviations interchangeably. There is a debate amongst geologists, and also other sciences, end the notation used for geologic time.

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Fun fact: The Tyrannosaurus rex was one of the critical dinosaurs come evolve around 70.6 Ma (a particular date). The very first dinosaurs evolved around 230 Ma (a certain date), 159 myr (a length of time) before T-rex evolved. We are closer with time to the T-rex than the T-rex is come its earliest dinosaur ancestor! see the distinction in abbreviation yet?

3.2 Geologic Time

Since 4.54 byr is a big chunk of time, geologists have split it into more manageable chunks by developing a time scale. The frequently accepted time scale comes from the international Commission ~ above Stratigraphy (Figure 3.1). It is continually revised as brand-new research fine-tunes numbers between time scale divisions. The one in figure 3.1 is the many up-to-date at the moment of this writing and will it is in referenced transparent this manual. The departments on the time scale are regularly based on significant events that have taken ar tectonically, biologically, or climatically, and the numerical ages are obtained from radiometric dating of rocks, minerals, and also fossils.

Geologic time is an initial divided right into eons; these room the Hadean, Archean, Proterozoic, and also Phanerozoic. The first three eons are regularly referred to together the Precambrian, i beg your pardon we’ll speak to a “super” eon. The eons room subdivided right into eras, and also eras room subdivided right into periods, and periods into epochs, and also epochs into ages. For the objectives of this rap manual, us will refer to this nomenclature. You’ll notification there is a second way to refer to these departments on the time scale. The difference between the two is discussed here, yet it is not important for ours purposes. Transparent this laboratory manual, friend will mainly see united state referencing durations of geologic time.

Figure 3.1 – The geologic time range subdivided by eon, era, system, series, epoch, and stage. What perform you observe around each duration of time? that is simple; they room color-coded. Us will use these typical colors throughout this rap manual. The golden spikes at age limits indicate a certain place used to define the period boundary. Image credit: global Commission top top Stratigraphy.

Many depictions that the geologic time scale don’t present the divisions of geologic time on the very same scale. Look at the time range in figure 3.1, for example. The far-right obelisk goes indigenous 4.6 Ga come 541 Ma; that’s around 4 billion year of history in one little column! The various other three columns consist of the staying 500 myrs. The factor for this is the geologists know much an ext about the last 500 myrs the Earth’s background than the an initial 4 byrs. So, let’s make a geologic time range where all geologic time is presented at the same scale.

Using a 2.5 m lengthy roll of paper, produce your very own geologic time range using the complying with scale: 1 centimeter = 20 million years. Because that the purpose of this exercise, round Earth’s period to 4.6 Ga and use a tick note spacing of every 100 myrs.Label the Precambrian and its associated eons.Label the Phanerozoic eon and its linked eras and also periods.For the Cenozoic era, brand the epochs.Using a mobile or laptop, watch up as soon as these occasions actually occurred and also label lock on the other side of her time scale. Brand physical occasions in one color and biological events in a different color. Few of the occasions will have a selection of ages.How close were your guesses?Table 3.1 – A perform of major events in earth history
Formation of earth MoonGreat oxidation event
Dinosaur extinctionFormation the the Himalayan Mountains
First fossil evidence of lifeFirst homo sapiens
First fishFirst mammals
First reptilesFirst amphibians
First major glaciation, called Snowball EarthEnd of the last ice cream age
Breakup the PangeaFormation that Rocky Mountains
Earliest trace of life (bacteria)Oldest oceanic crust
Come up v a mnemonic an equipment to help you mental the durations of geologic time in the Phanerozoic Eon. For example, did you recognize that the word “scuba,” as in scuba divers, is actually a mnemonic machine that represents Self-Contained Underwater breathing Apparatus?

Table 3.2 – abbreviation for the Geologic Time ScaleGeologic Time PeriodAbbreviation

Geologists usage abbreviations to refer to the different portions that geologic time (Table 3.2), specifically on geologic maps. A few of this abbreviations might seem puzzling because the abbreviation isn’t always the an initial letter the the name. Carboniferous obtained the “C”, therefore Cambrian acquired a C with a slash v it “Ꞓ“. This left Cretaceous v a “K.” Tertiary had actually originally received the “T”, i m sorry left Triassic to it is in symbolized v a T that had an R subscript. In 2003, “Tertiary” was stricken native the geologic timescale and also replaced by the Paleogene and Neogene. You will certainly still check out “T” on larger maps for Tertiary, and some geologists still usage the term today. So, it is in careful! there were currently some M’s and also P’s; so, Proterozoic eras were assigned together X, Y, and also Z.

Geologists use two methods for dating occasions in earth’s history. The an initial is called relative dating, an interpretation how events relate to each other in time, or an ext plainly, they figure out the succession of events (what come first, second, third, etc.). Family member dating has no regard because that numerical ages. The second an approach is pure dating, wherein geologists use radioactive isotopes to number out the numerical period of a absent or mineral.

3.3 relative Dating and also Correlation

There space several values geologists use for family member dating. The very first four ethics were arisen in the 17th century by an early geologist named Nicolas Steno, 3 of i beg your pardon pertain to sedimentary rocks. The first is the regulation of superposition, which says that in layers of horizontal sedimentary rocks, the oldest rock layer is in ~ the bottom, and the youngest is at the top (Figure 3.2). The 2nd rule is the rule of original horizontality, which claims that class of sediment are initially deposited horizontally (Figure 3.2). So, any tilting or urgent of the rock occurred after it was deposited (Figure 3.3). The third principle claims that layers of sedimentary rock are continuous, and also anything the interrupts the class (like a flow or canyon) taken place after the absent formed. This is referred to as the rule of lateral continuous (Figures 3.4 and also 3.5).

Figure 3.2 – Horizontal layers of red sedimentary rocks indigenous the Moenkopi and also Chinle Formations indigenous the Triassic period in the Virgin flow Valley close to Zion national Park. Great of sedimentary rock are initially deposited horizontally, v the earliest layer in ~ the bottom. This reflects both the values of superposition and also original horizontality. Photo credit: James St. John, CC-BY.
Figure 3.3 – These room tilted layers of the Moenkopi formation from the reduced Triassic, just south of separation Mountain, Dinosaur national Monument in Utah. They were initially deposited horizontally and also were titled at some allude in geologic time. Image credit: James St. John, CC BY.
Figure 3.4 – A relenten of the rule of lateral continuity. The great of absent on one next of this sink were once linked to the same rocks top top the other side. The sink was sculpted after these rocks to be deposited. The dashed red lines display how the devices were connected. Picture credit: Wikimedia user Wouldloper, publicly Domain.
Figure 3.5 – A panoramic watch of meanders in the mountain Juan river cutting with layers that sedimentary rock in Goosenecks State Park, southwestern Utah. If you look carefully, you deserve to trace great of sedimentary absent that have actually been reduced by the river across the whole image; an example of lateral continuity. Picture Credit: Gernot Keller, CC BY.

Steno’s 4th principle is cross-cutting relationships. This principle is used when various other geologic events reduced through sedimentary rocks, prefer an igneous dike or a fault. This principle basically claims that once a geologic event cuts across another, the event doing the cut is younger than the one being reduced (Figure 3.6). Because that example, if sedimentary rocks are reduced by one igneous dike, the igneous dike is younger 보다 the sedimentary rocks it’s cut through. The same have the right to be claimed of a fault that cuts through any type of rock; the fault needs to be younger because the rocks had actually to exist first to it is in faulted.

Figure 3.6 – A mafic dike (dark rock) cutting through granitic pegmatite (light rock) in Ruggles Mine, new Hampshire. To determine which is younger, look at to check out which unit cross-cuts the other. Due to the fact that the mafic dike is cutting through the pegmatite, the dike is younger through the principle of cross-cutting relationships. Photo credit: James St. John, CC BY.

Some 200 year later, the 5th principle of loved one dating was occurred by Charles Lyell called the rule of inclusions. This principle described that a clast, or a different-looking rock that is contained inside of one more rock, is older 보다 the absent that has it (Figure 3.7). How deserve to this happen? Originally, a mafic magma to be cooling quickly, developing the finer-grained mafic rock in the middle of figure 3.7. Then, something taken place to readjust the chemistry that the magma to felsic and also slowed the cooling rate to produce the surrounding, coarse-grained granite. The mafic rock created first, and then the felsic absent formed roughly it.

Figure 3.7 – A mafic inclusion in granitic rock. Inclusions are older 보다 the surrounding rocks. A penny mirrors the scale of the image. Picture credit: Marli Miller, CC BY.


Simply speaking, an unconformity is a pattern that you look for in a team of rocks the tells you erosion has taken place. Rocks exposed ~ above the Earth’s surface ar are affected by physical and chemical weathering procedures that work to break them right into smaller pieces or dissolve them in water. This material is then transported far by wind, water, or ice, a procedure known as erosion. Many civilization use weathering and also erosion interchangeably, however they carry out mean different things: weathering is the break down of rocks, when erosion gets rid of the damaged down material.

There room four species of unconformities, and each develops in a contempt different method (Figure 3.8). They every involve sedimentary rocks, alters in sea level, and/or uplift from an orogeny. Each unconformity speak a distinct story of the geologic background of the area they’ve to be found.

Figure 3.8 – varieties of unconformities. (a) Disconformity; (b) Nonconformity; (c) Angular unconformity; (d) Paraconformity. Image credit: Wikimedia user דקי, CC BY-SA.

A disconformity (Figure 3.8a) is an erosional surface where the rocks listed below the unconformity are lot older than the rocks above. This form of unconformity typically forms when horizontal layers of sedimentary rock room deposited in a shallow marine environment; climate sea level lowers to disclose these rocks and enables erosion come occur; climate sea level rises again, and new horizontal class of sedimentary rock space deposited. Erosion gotten rid of some that the initial rock, producing a large age gap between the rocks above and below the erosional surface. This age gap is the disconformity and is situated at the contact allude between the enlarge rock and also younger rock. Oftentimes the erosion procedure leaves behind proof of river channels or soil development, which administer clues to geologists to find the unconformity in what looks like a continuous succession of sedimentary layers.

A nonconformity (Figure 3.8b) forms when igneous or metamorphic radical is eroded, and also then horizontal great of sedimentary rock room deposited directly on height of it. The unconformity is where the radical meets the sedimentary rock. Because that example, as soon as a mountain belt is eroded listed below sea level, and also afterward sediments are deposited on peak of the igneous or metamorphic rock, the call is a nonconformity.

An angular unconformity (Figure 3.8c) is developed when horizontal layers of sedimentary absent lie on peak of tilted great of sedimentary rock. The most well known angular unconformity is native Siccar allude in Scotland. Figure 3.9 mirrors the procedure of creating an angular unconformity. For this come occur, sedimentary rocks deposit in the marine atmosphere are lifted over sea level by one orogeny or comparable event. The orogeny causes the sedimentary rocks to become tilted or folded. Since these rocks space exposed above sea level, erosion takes place. The rocks deserve to either be eroded listed below sea level, or sea level have the right to rise, i beg your pardon would enable new, horizontal class of sedimentary rock to be deposit on optimal of the titled ones. This create an angle between the younger, horizontal layers on top and also the older, tilted layers below.

Figure 3.9 – A relenten of exactly how an angular unconformity forms. 1) Deposition the sedimentary rocks; 2) uplift and also folding of the sedimentary layers; 3) erosion; 4) resumed deposition on top of the urgent layers. Image credit: Utah geology Survey, windy Domain.

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A paraconformity (Figure 3.8d) is very similar to a disconformity, except the proof for erosion is not present. Either no proof of the erosion to be left behind, or erosion no happen, and instead, over there was just a stop in sediment deposition. We understand these occur since sediment over and below the paraconformity have been radiometrically dated and also reveal a large gap in time.