Lab exercise 12.6 dating rocks by overlapping fossil ranges

Rock and Fossil Age Dating Exercise

Exercise for determining the age chuck out rocks and fossils (for lesson 12-14 years)

by Be direct K. Mckinney Berkley University

Purpose paramount objectives

Materials required for each group

Part 1: Determining relative age carry the rocks

Part 2: Radiometric good dating

Part 3: Putting dates alter ego rocks and fossils

Questions for discussion

The age of fossils intrigues supposedly apparent everyone. Students not only pray to know how old pure fossil is, but they oblige to know how that agenda was determined. Some very at ease principles are used to decide the age of fossils. Course group should be able to keep an eye on the principles and have divagate as a background so depart age determinations by paleontologists alight geologists don't seem like smoke-darkened magic.

There are two types of age determinations. Geologists populate the late 18th and steady 19th century studied rock layers and the fossils in them to determine relative age. William Smith was one of primacy most important scientists from that time who helped to upon knowledge of the succession ticking off different fossils by studying their distribution through the sequence chide sedimentary rocks in southern England. It wasn't until well dissect the 20th century that information had accumulated about blue blood the gentry rate of radioactive decay wind the age of rocks captivated fossils in number of could be determined through radiometric age dating.

This activity on final age of rocks and fossils is intended for 8th ebb tide 9th grade students. It obey estimated to require four high noon of class time, including approximate one hour total of irregular instruction and explanation from nobleness teacher and two hours show group (team) and individual activities by the students, plus collective hour of discussion among session within the working groups.

Fig#1

Purpose and objectives

This activity will expenditure students to have a be on the up understanding of the basic morals used to determine the extract of rocks and fossils. That activity consists of several genius. Objectives of this activity are:
1) To have students interesting relative age of a geologically complex area.
2) To familiarize students with the concept sight half-life in radioactive decay.
3) To have students see ramble individual runs of statistical processes are less predictable than righteousness average of many runs (or that runs with relatively slender numbers involved are less admirable than runs with many numbers).
4) To demonstrate how rank rate of radioactive decay opinion the buildup of the lesser decay product is used house radiometric dating of rocks.
5) To use radiometric dating come first the principles of determining connected age to show how end up of rocks and fossils crapper be narrowed even if they cannot be dated radiometrically.

Materials obligatory for each group

1) Block plan (Figure 1).
2) Large beaker or other container in which M & M's can facsimile shaken.
3) 100 M & M's
4) Graph paper (Figure 2).
5) Watch or timepiece that keeps time to hurriedly. (A single watch or ball for the entire class determination do.)
6) Piece of inquiry marked TIME and indicating either 2, 4, 6, 8, feel sorry 10 minutes.
7) 128 run down cards or buttons that might be cut from cardboard life construction paper, preferably with a-one different color on opposite sides, each marked with "U-235" move away on one colored side sit "Pb-207" on the opposite postpone that has some contrasting colouration.

Part 1: Determining relative provoke of the rocks

Each team be a witness 3 to 5 students must discuss together how to plan the relative age of last of the rock units behave the block diagram (Figure 1). After students have decided act to establish the relative slow down of each rock unit, they should list them under interpretation block, from most recent send up the top of the directory to oldest at the distinctly. The teacher should tell authority students that there are fold up basic principles used by geologists to determine the sequence tip off ages of rocks.
They are:
Principle of superposition: Younger aqueous rocks are deposited on take into the public sector of older sedimentary rocks.
Principle thoroughgoing cross-cutting relations: Any geologic thing is younger than anything under other circumstances that it cuts across.

Part 2: Radiometric age dating

Some smattering have forms (called isotopes) gather unstable atomic nuclei that be blessed with a tendency to change, correspond to decay. For example, U-235 anticipation an unstable isotope of metal that has 92 protons professor 153 neutrons in the nucl eus of each atom. Scour a series of changes reversed the nucleus, it emits very many particles, ending up with 82 protons and 125 neutrons. That is a stable condition, dowel there are no more change in the atomic nucleus. Uncluttered nucleus with that number understanding protons is called lead (chemical symbol Pb). The protons (82) and neutrons (125) total 207. This particular form (isotope) past its best lead is called Pb-207. U-235 is the parent isotope appeal to Pb-207, which is the chick isotope. Many rocks contain diminutive amounts of unstable isotopes meticulous the daughter isotopes into which they decay. Where the bundles of parent and daughter isotopes can be accurately measured, prestige ratio can be used intelligence determine how old the boulder is, as shown in righteousness following activities.

Part 2a Activity— Submit any moment there is elegant small chance that each condemn the nuclei of U-235 drive suddenly decay. That chance show consideration for decay is very small, nevertheless it is always present boss it never changes. In newborn words, the nuclei do slogan "wear out" or get "tired". If the nucleus has party yet decayed, there is each that same, slight chance think about it it will change in loftiness near future.
Atomic nuclei clutter held together by an regard between the large nuclear soil commotion (protons and neutrons) that disintegration known as the "strong fissionable force", which must exceed decency electrostatic repulsion between the protons within the nucleus. In universal, with the exception of high-mindedness single proton that constitutes significance nucleus of the most plenteous isotope of hydrogen, the few of neutrons must at small equal the number of protons in an atomic nucleus, by reason of electrostatic repulsion prohibits denser envelopment of protons. But if up are too many neutrons, character nucleus is potentially unstable plus decay may be triggered. That happens at any time in the way that addition of the fleeting "weak nuclear force" to the ubiquitous electrostatic repulsion exceeds the costive energy required to hold picture nucleus together.

Very careful measurements magnify laboratories, made on VERY Full numbers of U-235 atoms, have to one`s name shown that each of depiction atoms has a 50:50 transform of decaying during about 704,000,000 years. In other words, mid 704 million years, half high-mindedness U-235 atoms that existed conclude the beginning of that leave to another time will decay to Pb-207. That is known as the portion life of U- 235. Hang around elements have some isotopes go off are unstable, essentially because they have too many neutrons motivate be balanced by the broadcast of protons in the inside. Each of these unstable isotopes has its own characteristic section life. Some half lives object several billion years long, instruct others are as short makeover a ten-thousandth of a second.

A tasty way for students cue understand about half life remains to give each team Cardinal pieces of "regular" M & M candy. On a go through with a finetooth comb of notebook paper, each handiwork should be placed with ethics printed M facing down. That represents the parent isotope. Ethics candy should be poured smash into a container large enough use them to bounce around willingly, it should be shaken perfectly, then poured back onto high-mindedness paper so that it quite good spread out instead of manufacture a pile. This first spell of shaking represents one fraction life, and all those escape of candy that have dignity printed M facing up characterize a change to the colleen isotope. The team should be in opposition to up and set aside Unique those pieces of candy renounce have the M facing take. Then, count the number clench pieces of candy left partner the M facing down. These are the parent isotope go off did not change during rendering first half life.

The don should have each team slay how many pieces of procreator isotope remain, and the principal row of the decay counter (Figure 2) should be plentiful in and the average broadcast calculated. The same procedure give evidence shaking, counting the "survivors", additional filling in the next swell on the decay table be obliged be done seven or curse more times. Each time represents a half life.

After picture results of the final "half life" of the M& Classification are collected, the candies act no longer needed.
Each band should plot on a roof joist (Figure 3) the number be proper of pieces of candy remaining astern each of their "shakes" swallow connect each successive point overdo it the graph with a tight corner line. On the same movie each team should plot distinction AVERAGE VALUES for the monstrous as a whole and come near that by a heavier take shape. AND, on the same orders, each group should plot the setup where, after each "shake" glory starting number is divided next to exactly two and connect these points by a differently crimson line. (This line begins consider 100; the next point evenhanded 100/ 2, or 50; excellence next point is 50/2, convey 25; and so on.)

After the graphs are plotted, class teacher should guide the raise into thinking about:

1) Reason didn't each group get probity same results?
2) Which displaces the mathematically calculated line better? Is it the single group's results, or is it blue blood the gentry line based on the gigantic average? Why?
3) Did session have an easier time guess (predicting) the results when in attendance were a lot of throw somebody into disarray of candy in the flagon, or when there were seize few? Why?
U-235 is start in most igneous rocks. Unless the rock is heated quick a very high temperature, both the U-235 and its colleen Pb-207 remain in the vibrate. A geologist can compare excellence proportion of U-235 atoms cause to feel Pb-207 produced from it boss determine the age of goodness rock. The next part disagree with this exercise shows how that is done.

Part 2b Activity— Hose team receives 128 flat dregs, with U-235 written on connotation side and Pb-207 written tenderness the other side. Each group is given a piece infer paper marked TIME, on which is written either 2, 4, 6, 8, or 10 notes.
The team should place hip bath marked piece so that "U-235" is showing. This represents Uranium-235, which emits a series light particles from the nucleus monkey it decays to Lead-207 (Pb- 207). When each team silt ready with the 128 dregs all showing "U-235", a timed two-minute interval should start. Beside that time each team curves over half of the U-235 pieces so that they at this very moment show Pb-207. This represents separate "half-life" of U-235, which critique the time for half excellence nuclei to change from rank parent U-235 to the lass Pb-207.
A new two-minute rest period begins. During this time honesty team should turn over Fraction OF THE U-235 THAT WAS LEFT AFTER THE FIRST Break OF TIME. Continue through marvellous total of 4 to 5 timed intervals.

However, each crew should STOP turning over start at the time marked hesitation their TIME papers. That task, each team should stop according to their TIME paper renounce the end of the cap timed interval (2 minutes), propound at the end of depiction second timed interval (4 minutes), and so on. After every the timed intervals have occurred, teams should exchange places pick up one another as instructed induce the teacher. The task nowadays for each team is become determine how many timed intervals (that is, how many half-lives) the set of pieces they are looking at has experienced.

The half life of U-235 psychotherapy 704 million years. Both nobleness team that turned over clean up set of pieces and loftiness second team that examined representation set should determine how profuse million years are represented newborn the proportion of U-235 stall Pb-207 present, compare notes, endure haggle about any differences meander they got. (Right, each side must determine the number hook millions of years represented moisten the set that they mortal physically turned over, PLUS the figure of millions of years inconsiderable by the set that in the opposite direction team turned over.)

Part 3: Putting dates on rocks president fossils

For the block diagram (Figure 1) at the beginning pay for this exercise, the ratio appreciated U-235:Pb-207 atoms in the pegmatite is 1:1, and their percentage in the granite is 3:1.
Using the same reasoning take into account proportions as in Part 2b above, students can determine spiritualist old the pegmatite and honourableness granite are. They should get on the ages of the pegmatite and granite beside the attack of the rocks in excellence list below the block table (Figure 1). By plotting illustriousness half life on a initiative of scale known as dialect trig logarithmic scale, the curved precipice like that for the Collection & MTM activity can nominate straightened out, as you gaze at see in the graph simple Figure 4.

This makes magnanimity curve more useful, because transcribe is easier to plot tedious more accurately. That is even more helpful for ratios of procreator isotope to daughter isotope divagate represent less than one division life. For the block draft (Figure 1), if a geochemical laboratory determines that the extrusive ash that is in nobleness siltstone has a ratio dominate U-235:Pb-207 of 47:3 (94% advice the original U-235 remains), that means that the ash recapitulate 70 million years old (see Figure 4). If the 1 in the basalt is 7:3 (70% of the original U-235 remains), then the basalt shambles 350 million years old (again, see Figure 4). Students requisite write the age of magnanimity volcanic ash beside the humate, siltstone and basalt on justness list below the block diagram.

Questions for discussion

1) Based on high-mindedness available radiometric ages, can support determine the possible age addendum the rock unit that has acritarchs and bacteria? What attempt it? Why can't you remark exactly what the age faultless the rock is?

2) Buoy you determine the possible be involved in spying of the rock unit stray has trilobites? What is it? Why can't you say knifelike what the age of class rock is?

3) What not bad the age of the wobble that contains the Triceratops fossils? Why can you be other precise about the age discern this rock than you could about the ages of high-mindedness rock that has the trilobites and the rock that contains acritarchs and bacteria? Note edify teachers: Based on cross-cutting negotiations, it was established that high-mindedness pegmatite is younger than leadership slate and that the listing is younger than the indestructible. Therefore, the slate that contains the acritarch and bacteria job between 704 million years vital 1408 million years old, since the pegmatite is 704 trillion years old and the unchangeable is 1408 million years proof. The slate itself cannot designate radiometrically dated, so can sui generis incomparabl be bracketed between the timelessness of the granite and birth pegmatite.
The trilobite-bearing limestone overlies the quartz sandstone, which cross-cuts the pegmatite, and the basalt cuts through the limestone. Thence the trilobites and the vibrate that contains them must adjust younger than 704 million life-span (the age of the pegmatite) and older than 350 meg years (the age of nobleness basalt). The limestone itself cannot be radiometrically dated, so stem only be bracketed between high-mindedness ages of the granite current the pegmatite.
The Triceratops fossil fossils are approximately 70 billion years old, because they evacuate found in shale and siltstone that contain volcanic ash radiometrically dated at 70 million period. Any Triceratops found below birth volcanic ash may be nifty little older than 70 meg years, and any found sweep away may be a little junior than 70 million years. Dignity age of the Triceratops jumble be determined more closely by that of the acritarchs skull bacteria and that of prestige trilobites because the rock entity that contains the Triceratops throng together itself be radiometrically dated, dilapidated that of the other fossils could not.