Thursday, October 14, 2010
Monday, October 11, 2010
If the evolution of life on Earth seems to go back a long way, it may be put into a geological context by comparing the age of the planet with the life time of a person now 47 years old. Fossils only tell us about life since the Pre Cambrian era began 600 million years ago, but by then our person would have already celebrated their 40th birthday. Soon after, multi cellular life in the sea diversified into thousands of species. Two years later on the human timescale, planets and insects emerged onto land followed by amphibious animals. Then things began to speed up. It is only a year since the age of the dinosaurs, a week since that last ice age and a mere four hours since our own species, Homo sapiens, first walked on the planet.
During the Earth’s life time, the Solar System has moved around the galactic centre about 25 times. The ocean crust has been recycled 50 times. The continents have accumulated, crashed into each other and broken apart. Landscapes have been eroded and weathered. And the atmosphere has been altered by life forms. Now the globe is being transformed by humans. Judging from the evolutionary path of the Sun, the Earth has five billion years to go.
The first living things were microscopic bacteria and protozoa. The first visible sign of life was probably a film of algae. Some algae or filamentous bacteria grew in large mats in shallow after near the tide line, binding sand among them to form layered mounds. Still found growing in warm seas today, fossil stromatolites from the oldest macroscopic fossils in 3,500 million years old deposits in Australia.
Wednesday, October 6, 2010
Sedimentary rocks contain a record of the changing forms of life on earth. It is not a complete record; most creatures get eaten, rot, or are otherwise destroyed. Many do not have any resistant parts and, of those that do, the remains may later be eroded or never found. Although material from the original shell or bone may survive, it may alternatively be replaced by other minerals or leave just a faint imprint in the rock.
Even so, fossils provide a remarkably detailed picture of life on Earth. It is a picture of rapid diversification and great inventiveness to suit every ecological niche, punctuated by rapid extinctions when times get hard. Thus the changing fossil record provides a powerful means of dating rocks. So called zone or index fossils have been picked as key markers for each time. Ideally they are common, free swimming species that can help correlated rocks of the same age wherever in the world they are found. Sometimes, finding assemblages of different fossils together at one location can narrow the time down further.
Fossils and stratigraphy reveal relative ages of rocks, but not an absolute age. For that, geologists have other techniques. They can simply count the growth rings in trees. Modern mass spectrometers can measure even the slightest trace of an isotope, so tiny crystals can each be dated. Thus the oldest minerals on Earth were found: grains of zircon over four billion years old, eroded and redeposit later. The key to accuracy is purity of the sample. If the sample comes from a lava, its melting will have released any previous gas and reset the clock.
Monday, October 4, 2010
Reading the stratigraphical notebook is not just a question of opening the pages. A geological map of the world today is in effect a patch work of different environmental conditions, and the same was true at each stage in the past. The limestone was forming in one place a hundred million years ago does not imply that limestone was forming everywhere at that time. While rocks are deposited in one place, another place may be uplifted into mountains and eroded.
There are however, gaps in the record, and things are not always what they seem. Although younger rocks are deposited on top of older ones, folding and faulting can be so intense that the younger rocks end up underneath the old. Layers can be folded up to steep angles, or the original layers may be sloping such as current bedding at a river delta or a continental slope. Thick shale beds may have take a few hundred thousand years to form and be tens of meters thick; in the same strata there may be another layer only a few centimeters thick that was five million years in the making. So thickness is no certain clue to age; neither is apparent depth of water. Sea level can change by tens of meters, land level by hundreds. Nor is present latitude much help, when continents have skated across the globe. Britain was once on the equator and there were glaciers in what is now the Sahara desert. But there are plenty of clues for geologists and with their knowledge of processes at work can compute the stratigraphical evidence from around the world to confirm their theories on geological maps.