Chapter 10 THE ROCK RECORD AND THE GEOLOGIC TIME SCALE

Earth formed 4.5-4.6 billion years ago

Principle of Uniformitarianism - the present is the key to the past

Relative Ages of Rocks

• Principal of original horizontality
• Principle of superposition
• Fossils
  • Principle of faunal succession, evolution
  • Index fossils - widespread geographically, short time span
• Unconformities
• Cross-cutting relations
• Lateral correlation of strata
• Sequence stratigraphy - seismic discontinuities

Geologic Time Scale - based on fossil record & stratigraphic correlation

• Know - Cenozoic Era, Mesozoic Era , Paleozoic Era, Precambrian Era
• When did dinosaurs live? When did mammals appear? When did creatures with hard shells appear? What killed the dinosaurs?

Evidence for old age of the earth

• Tree rings (8000 yrs)
• Ice cores from Greenland and the Antarctic (100,000 yrs)
• Annual lake sediments (varves - up to 6 million yrs)
• Erosion rates on continents - a few mm per century. Would take 10's to 100's of millions of years to erode high (300 m) mountains.
• Rates of tectonics processes. For example, the San Andreas Fault at its current rate of about 5 cm/yr would take over 10 million yrs to offset rock formations by as much as 500 km (as is observed). Another example, at present spreading rates it would take100 million yrs for Atlantic Ocean to reach its present width.
• Salt in the oceans comes from chemical weathering of rocks on continents (would take 100 - 300 million yrs at current rates)
• Cooling of the Earth - would take 30 million years or more for Earth to cool to present temperature (this calculation does not take effects of radioactive heating into account)

Absolute Ages of Rocks - Radiometric age determination

Radioactivity and radiometric dating - certain isotopes of certain elements undergo spontaneous nuclear decay (the nucleus breaks apart) to form different elements. This process occurs in rocks and can be used to determine the age of rocks.

• Decay of each atom is governed by probability (50 % chance of decaying in one half life) and is independent of all the other atoms around it
• Isotope, half-life, parent, daughter product - how much parent and daughter exist after several half lives? To determine ages, the amount of parent and daughter isotopes as well as the half life must be known.
• C14 used for ages up to about 75,000 years (half life = 5730 years)
• Uranium decay (238 U to 206 lead, half life 4.5 billion years), potassium decay to argon, rubidium to strontium. These systems are used for older rocks (millions to billions of years)
• Radiometric age determination is most useful for igneous rocks. In many situations, the ages of igneous rock layers above and below a sedimentary layer of interest are used to bracket the age of the sedimetary rocks.

Results of radiometric age determinations

• Oldest rocks on earth - ~3.9 billion years
• Oldest minerals (zircon [a zirconium silicate] clasts in sedimentary rocks in Australia) - ~4.1 billion years
• Oldest Moon rocks are4.4 to 4.5 billion years
• Oldest meteorites are 4.5 - 4.6 billion years
• So, age of Earth is estimated to be 4.5 to 4.6 billion years

Origins of Life

• Oldest fossils - bacteria, algae, about 3.5 billion years old - ancient stromatolites (small knobs on beaches made of algae plus sand plus calcium carbonate) similar to those found today in a few places in Australia
• DNA tree of life suggests that origin of life may have been in hot springs environment

Implications of 'Deep Time' (the fact that geological times scales are so very very long)

• 1) Very slow geological processes will have big effects (plate tectonics occurs at a speed of a few cm/year, but the effects are large)
• 2) Events of low probability on short time scales, have high probabilities over the time frame of the age of the earth (example - we see evidence of over 200 large meteorite impacts on the earth over geologic time)