Geologic Time

Lecture notes Geologic Time GLG 101 – Physical Geology Remember…..The principle of Uniformitarianism‐ The present is the key to the past. In past lectures we applied that principle to study WHAT the Earth is made of (ROCKS) and the processes and environments by which the rocks are formed. Now we will look at WHEN and HOW LONG. Remember…..Historical Geology is the study of the Evolution of the Earth through Time How old is the Earth…. Approx. 4.6 billion (Ga) yrs Trying to wrap our heads around that number is extremely difficult because our concept of time is relative to what we know best, and we usually think about time in terms of days/years/centuries. • Geologic time – more difficult to understand • Processes can be catastrophic or occur more slowly Although most changes occur slowly, there can also be quicker, more catastrophic changes. • Examples: Volcanic eruptions, earthquakes, tsunamis (tidal waves), floods • We have witnessed these, either in the news or in‐person • Seem unusual….but they are NOT unusual or rare if witnessed over a long period of time There are also uniform/subtle changes that occur. For example: • Sea level rise (1.2 mm/yr) • Uplift of land (1.5 meters/century) • Cutting the Grand Canyon (less than 0.7 mm/yr) These occur slowly/uniformly over time. They don’t seem important, but create impressive geologic features. Let’s “compress” Geologic Time into One Year to help us understand geo time January 1 is 4.6 Ga (billion years ago)‐Earth formed by…. March: oldest known rocks are formed Late March: first forms of life (bacteria and algae) are preserved as fossils September 3: first multi‐celled creatures appear December 5: Reptiles appear December 14: first mammals appear December 26: Dinosaurs go extinct 5 PM on December 31: first hominids (human‐like ancestors) appear 11:48 PM on December 31: first modern humans (Homo sapiens) appear 3 seconds before midnight: Columbus lands in the Americas You were born 1/10th of a second before midnight Let’s review relative dating again ‐ Placing events in proper sequence without knowing their age in years. There are some principles we need to remember and apply to help us with geologic time. Principle of Superposition • In an undeformed sequence of sedimentary rocks (or layered igneous rocks), the oldest rocks are on the bottom • Developed by Nicolaus Steno in 1669 Principle of Original Horizontality • Layers of sediment are generally deposited in a horizontal position • Rock layers that are flat have not been disturbed Principle of Lateral Continuity • layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous Principle of Cross‐cutting relationships • Younger features cut across older features Cross‐cutting relationships‐ dikes Principles of Inclusions and Baked Contacts • Inclusions are older than the rock that include them (in other words ‐ The rock containing the inclusion is younger) • Rock that has been “baked” is older than the intrusion Unconformities • Unconformity = a break in the rock record produced by erosion and/or nondeposition of rock units Types of Unconformities • Angular unconformity—tilted rocks are overlain by flat‐lying rocks • Disconformity—strata on either side of the unconformity are parallel • Nonconformity—metamorphic or igneous rocks in contact with sedimentary strata You can practice recognizing these. Refer to the diagram from end of Chapter in the text. Also, I recommend that you go through the tutorial on the GCC Geology Dept. home page. On the Geology Home Page – left side Department News & Info - Faculty Home Pages & Contacts - Campus Location - GeoAssist (for help in geology) ‐ Geologic Time, Structures & Maps Tutorial ‐‐ THIS ONE Faunal Succession‐ Life has changed. Dinosaurs are a good example of how things have changed. They have been extinct for a long time. Age of the Dinosaurs • Became extinct 65 million years ago • They roamed the earth from 250 to 65 million years ago in the Mesozoic Era Fossils – Evidence of Past Life How do we know life has varied through time? • By studying the traces or remains of prehistoric life that are now preserved in rock (sedimentary‐mostly) – Aid in interpretation of geologic past – Serve as important time indicators – Allow for correlation of rocks with similar ages from different places Paleontology is the scientific study of fossils Special circumstances have to occur for fossils to be created. Rapid burial & possession of hard parts (skeleton, shell, etc.) favor preservation Fossil types • The remains of relatively recent organisms—teeth, bones, etc. • Entire animals, flesh included • Given enough time, remains may be petrified (literally “turned into stone”). • Molds and casts • Carbonization • Others – tracks, burrows, coprolites, gastroliths Rock Units and Correlations. Here are some names that we give to rock units and groups. Note: Names given here are from formations etc. in the Grand Canyon. • Formation = fundamental rock unit. This is a single mappable rock type or lithology(e.g. Coconino Sandstone)or multiple lithologies with common characteristics (e.g. Kayenta Formation) • Members = divisons of a formation (e.g. Shinarump Member of the Chinle Formation) • Groups and Supergroups = groups of formations (e.g. the Unkar Group and the Grand Canyon Supergroup) Correlations ‐ Rock units can be traced laterally great distances. Formations may be traced laterally for hundreds of miles. This is called lithologic correlation. Although the same lithology (rock types/formations) may correlate across large areas, they may not have been deposited at the same geologic time. This is because of transgression and regression (covered under “Sedimentary Rocks”). Continents and the locations of the oceans have shifted over time. At the same geologic time, seas may cover one area, and another area may be characterized by continental sedimentary environments. Principle of fossil (faunal and floral) succession. Another relative dating principle. • Fossil organisms succeed one another in a definite and determinable order • Therefore, any time period can be recognized by its fossil content Note: Index fossil = a geographically widespread fossil that is limited to a short span of geologic time The succession of fossils or fossil assemblages from oldest to youngest is the same everywhere. This allows us to make a relative time scale based on the life forms found in the rocks. Relative ages of fossil assemblages can be determined using the other principles of relative time. Here are some things to remember about correlations (correlating rock units) of rocks units in different regions. • Often relies on fossils. Biostratigraphic correlations‐ rocks with the same fossil assemblages are the same age. Guide or Index Fossils are easily recognized, abundant & geographically widespread, and have a narrow range of existence. These are useful when a full assemblage is not present. • Key beds (e.g. volcanic ash) can also be used as they indicate a single time event • Correlations based on lithology alone may not work due to changes in depositional environments The Relative (Geologic) Time Scale (refer to the time scale at the end of these notes) • Can construct a relative time scale based on fossils • Time Rock Units – units with distinct fossil assemblages (e.g. Cambrian) • Time Units – Eon‐Era‐Period, etc. Geologic Periods & Eras & Eons – Time Units Period = time unit based on faunal assemblage/place. This is the basic unit of Geologic Time. These were named based on the places where a rock units contain a distinct faunal assemblage. For example, the Cambrian is named based on the old Roman name for Wales = Cambria. And, Silurian was named for the Silures, an ancient Welsh tribe that occupied that region‐ and so forth. The periods of the relative geologic time scale was developed by Sedgewick and Murchison‐ 1835. Periods are grouped into Eras which are named for the degree to which life is similar to life today, in the present. The major Eras are: Paleozoic‐ "ancient life"; life very different from today (Cambrian – Permian) Mesozoic‐ "middle life"; life between ancient and recent (Triassic‐ Cretaceous) Cenozoic‐ "recent life; life resembles today's fauna and flora. (Tertiary‐Quaternary) Eras are grouped into Eons which are named for visibility of life. The major Eons are: Phanerozoic‐ "visible life"; forms are visible to naked eye. Proterozoic‐ "early or proto life"; microscopic forms‐ mostly primitive algae Archean‐ this is named for a distinct assemblage of rocks assumed to be older than life although recent evidence suggests life also existed in Archean time. From bottom to top (oldest to youngest), let’s look briefly at the time scale. Precambrian Era (4.6 Ga‐billion years ago to 570 Ma – million years ago) Why is there so little detailed info about the Precambrian? • Abundant fossil evidence does not appear until the Cambrian (early organisms appear in the Precambrian. However, there are fewer hard parts to preserve) • Most Precambrian rocks are very old and distorted (mostly metamorphic) Paleozoic Era. The Paleozoic begins at the end of the Precambrian at approx. 570 Ma (million years ago) and ends at the beginning of the Mesozoic, approx. 245 Ma. Here are some examples of the different life forms that dominated the Paleozoic (Oldest to Youngest) Cambrian Period(Earliest shelled organisms appear, including Trilobites) Ordovician (The first fishes appear) Silurian/Devonian (Age of the fishes). The first land plants also appear in the Silurian. Mississippian/Pennsylvanian & Permian(Age of Amphibians. The first amphibians and reptiles appear in the Mississippian. The Mississippian and Pennsylvanian are also called the Carboniferous as they were char. by abundant coal swamps) Mesozoic Era – This begins at approx. 245 Ma (million years ago) when the first dinosaurs appear and ends when they become extinct at approx. 66 Ma, and includes, from oldest to youngest, the Triassic, Jurassic (of “Jurassic Park” fame) and Cretaceous Periods. Cenozoic Era – From oldest to youngest, this includes the Tertiary and Quaternary Periods. This Era began at 66 Ma to the present time. The Quaternary begins at about the same time that the first humans appear, at approx. 2 Ma. So……..How can I remember the sequence of Periods above the Precambrian? TRY THIS memory aid. Remember this phrase that represents the first letter of each Period. Can Old Senators Demand More Political Power Than Junior Congressmen? Tough Question Oldest to Youngest: Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian, Triassic, Jurassic, Cretaceous, Tertiary, Quaternary Absolute Time Now we’ll look at how we can determine more precisely how old the rocks are. We will use a little of the chemistry that we covered earlier, when we looked at minerals. Tree‐ Ring Dating ‐ Tree rings can be used to date regular events ‐ annual drought/rainfall etc. However, we need to use other methods to determine the ages of much older things. Dating with Radioactivity ‐Radiometric or isotopic dating Remember the Isotope? An isotope is a variant of the same parent atom that differs in the number of neutrons. Radioactive isotopes: An unstable isotope of an element. Emits radiation to decay to a stable daughter isotope of another element. Radioactivity ‐ Spontaneous changes (decay) in the structure of atomic nuclei There are three (3) types of radioactive decay. • • • Alpha emission – Emission of two protons and two neutrons (an alpha particle) – Mass number is reduced by 4, and the atomic number is lowered by 2. Beta emission – An electron (beta particle) is ejected from the nucleus. – Mass number remains unchanged and the atomic number increases by 1. Electron capture – An electron is captured by the nucleus and combines with a proton to form a neutron. – Mass number remains unchanged and the atomic number decreases by 1. When we talk about decay, we refer to the original isotope at the “parent” and the isotopes that are created after the parent decays are the “daughter products”. • Parent—an unstable radioactive isotope • Daughter product—the isotopes resulting from the decay of a parent An important concept for radiometric dating is “half‐life”. Half‐life = the time required for one‐half of the radioactive nuclei in a sample to decay • The percentage of radioactive atoms that decay during one half‐life is always the same (50%). • However, the actual number of atoms that decay continually decreases. • Comparing the ratio of parent to daughter yields the age of the sample. How are the ages calculated? • Determine amount of parent/daughter in a mineral or glass to determine age. • For most circumstances, these quantities are then fed into the radioactive decay equation for the particular isotope system: N = Noe-λt, where N is the amount of parent remaining, No is the original amount of the parent (= remaining parent + daughter), λ is the radioactive decay constant for that parentdaughter pair, and t is time. The equation is then solved for time. • In much simpler terms, for even multiples of ½, the age can be calculated by multiplying the half-life by the number of multiples of ½ that it takes to equal the amount of radioactive parent left in the sample. For example, ½ = 1 half life; ¼ = 2 half lives; 1/8 = 3 half lives; and so on. Common Long‐lived Isotopes (1/2 lives = millions and billions of years) Uranium (238 U) ‐> decays to Lead (206 Pb) – ½ life = 48 billion years Rubidium (87 Rb) ‐> decays to Strontium (87 Sr)) – ½ life = 48 billion years Potassium (40 K) ‐> decays to Argon (40 Ar) – ½ life = 1.3 billion years. Potassium is abundant in many common minerals. Practice: In this example Potassium 40 decays to Argon 40 (40K  40Ar) The half life =1.3 Billion (Ga) years A feldspar crystal from a basalt is found to have….. 25% (1/4) K (parent isotope) present, and 75% (3/4) Ar (daughter/decay product) present in the feldspar What is the absolute age in years? Here is the answer….. It takes one ½ life (1.3 Ga) to reach 50%K / 50%Ar Now you only have 50% (1/2) of the K remaining ….which continues to decay After another 1.3 Ga elapses, 50% or ½ of the remaining K will decay Note: (50% or ½ of the remaining K is equal to 25% or ¼ of what was there initially) Therefore…. 50% + another 25% = 75% or ¾ of the original K decays to Argon (Ar) over 1.3Ga + 1.3Ga years….Which equals a total of = 2.6 Ga In the simplest approach to remembering how all of this works, as mentioned above, we can look at multiples of ½ for any situation where there is radioactive decay. In that case…. If ½ of parent is left = 1 half life If ¼ of parent is left = 2 half lives If 1/8 of parent is left = 3 half lives Short lived isotopes (1/2 lives = thousands of years) Carbon (14 C) ‐> decays to Nitrogen (14N) – ½ life = 5730 years Can be used for plants, etc. but not for much older rocks What is being dated with radiometric methods? • Minerals and glasses in igneous and metamorphic rocks • Cannot typically use this method for sedimentary rocks How are the numbers attached to the time scale? By combining radiometric (absolute) dating with relative dating principles (e.g. superposition and cross‐ cutting relationships) of igneous and metamorphic rocks relative to sedimentary rocks, such as when there are sedimentary rocks that are intruded by dikes and lava flows. If we know the absolute age of some of the layers, we can use those to help determine the ages of the other layers above and below (bracketing). When this is done in many places on Earth, a time scale emerges which is always being examined and revised as necessary. NOTE: The geologic time scale is on the next page………….