o Figure to be shown: GET FROM BOUTTE ← ← Important example of dating: ← ← U-Pb method. ← ← a.) Age of the Earth- 4500 m.y.-based on dating of meteorites formed at the same time as the Earth ← b.) Explosion of life forms at base of Paleozoic occurred~550 m.y. ago ← c.) Faunal extinction event at Paleozoic -Mesozoic boundary occurred ~250 m.y. ago ← ← 3/7/07 ← ← K-At method ← d.) Faunal extinction event at Mesozoic- Cenozoic boundary occurred ~65 m.y. ago ← e.) Man-like creatures first appeared in East Africa~ 5 m.y. ago ← f.) Correct age (1925 y) obtained on ash material from Vesuvius eruption of 79 A.D. (death knoll for creationists) ← ← Radiocarbon method 39
← g.) Native Americans enter “New World”, ~ 30 k.y. ago. ← h.)Ice man frozen to death in the Alps ~5500 years ago ← i.) Cloth on which Shroud of Turin is seen, was made in about 1300 A.D. ← ← Other important geological event “dated”: ← ← 1.) Oldest rocks dated on Earth ~4200 m.y. Older record wiped out by (big) accretion event. ← ← 2.) Oldest rocks showing signs of life (algae?) on Earth ~4000 m.y. old ← ← 3.) Appalachian Mountains formed in complex series of event ~400-300 m.y. ago, when Africa and Europe “crashed into” North America ← ← 4.) Current cycle of plate tectonics began ~200 m.y. ago. Pangaea breaks up. North American from Africa and Europe at about `180 m.y. South America from Africa at about 130 m.y. ← ← 5.) India comes into hard collision with Eurasia ~50 m.y. ago. Himalayas rise rapidly in the last ~15 m.y. Big effect on world climate. ← ← Radiometric methods “work” ← ← i) Ages obtained by these methods agree with those obtained by tree-ring counting. ← ← ii) Correct radiometric age obtained for 70 AD Mt. Vesuvius eruption. ← ← iii) Radiometric ages always in right order, i.e. agree with relative dating. ← ← iv) sear-floor spreading (plate) velocities obtained from radiometric result, in excellent agreement with those obtained by modern “physics” methods e.g. measuring “distances” using laser. 40
Unit 3 ← ← 3/12/07 ← ← Chapter 7 Rock Deformation. ← Look at rocks I the field, we see that they can be broken ( faulted ) or bent ( folded ) by (deforming) forces. (Fig. 7-1) ← Faulting shows brittle behavior of rocks, folding shows ductile behavior . ← The latter occurs only for rocks at considerable depth inside the Earth. • To understand “structures” formed by rocks in the field, we need to measure the orientation of rocks in outcrops. Measure dip and strike of beds (sedimentary rocks). • Strike is a compass direction – e.g. N-S, NE-SW, whereas • Dip has a compass direction as well as an angle associated with it (Fig. 7-2/3) • Strike is the direction of a rock layer as it intersects the horizontal surface • Dip is measured at right angles to this, and measures the amount of tilting.