Sabancı Üniversitesi Proficiency Sınavı Hazırlık 5
Sabancı Üniversitesi Proficiency Sınavı Hazırlık
1 – Sabancı Üniversitesi Proficiency Sınavı Hazırlık (Sabancı Üniversitesi Proficiency Sınavı Hazırlık için Bireysel – Özel Ders )
2 – Sabancı Üniversitesi Proficiency Sınavı Hazırlık ( Sabancı Üniversitesi Proficiency Sınavı Hazırlık için 4 kişilik Gruplarla Ders )
The Geological Column
11. Look at a section of sedimentary rocks in, for example, a cliff face and you will see that it is made up of layers. Sometimes annual layers corresponding to floods and droughts are visible. More often, the layers represent occasional catastrophic events or slow but steady sedimentation across hundreds of thousands or even millions of years, followed by a change of environment leading to a layer of slightly different rock.
In the case of a really deep section of ancient rock, such as that seen in the Grand Canyon in Arizona, hundreds of millions of years of deposits are represented. It is a natural human instinct to divide up and classify things, and sedimentary rock with its many layers is an obvious candidate. But, when viewing a spatially narrow cliff face of flat layers, it is easy to forget that the layers are not continuous around the world. The entire globe was never covered by a single shallow ocean depositing similar sediments! Just as today, there are rivers, lakes, and seas, deserts, forests, and grasslands, so in ancient times there was apanoply of sedimentary environments.
12. In the early l9th century, William Smith, an English civil engineer, began to make sense of the sections of rock layers and produced diagrams to show the associations between them. He was surveying for Britain’s new canal network and started to realize that rocks in different parts of the country sometimes contained similar fossils. In some cases the rock types too were the same, sometimes only the fossils were similar. This enabled him to correlate the rocks in different places and work out an overall sequence. As a
result, he published the first geological map. Once the dates were added in the 20th century, and the rocks correlated between different continents, it was possible to publish a single sequence of layers representing periods of geological time for the whole world. The geological column we know today is the product of many techniques, refined over the years and agreed by international collaboration.
Extinctions, Unconformities and Catastrophes
13. It became clear that some of the changes in the geological column were bigger than others, and these provided convenient places to divide the geological past into separate eras, periods, and epochs. Sometimes there was a sudden and significant change in the nature of the rocks across such a boundary, indicating a major environmental change. Sometimes there was what is known as an unconformity, a break in deposition, caused, for example, by a change in sea level so that either deposition stopped or the
layers were eroded away before the column continued. They are often also marked by major changes in fauna, represented by fossils, with many species becoming extinct and new ones beginning to arise.
14. A few intervals in the geological record stand out for the severity of the extinctions across them. The end of the Cambrian period and the end of the Permian period were both marked by the extinctions of around 50% of families and up to 95% of individual species of marine invertebrates. The extinctions that marked the late Triassic and late Devonian saw the loss of about 30% of families and, slightly smaller at 26%, but the most recent and the most famous, is the mass extinction at the end of the Cretaceous period 65 million
years ago. That so-called K/T boundary is famous not only because it saw the extinction of the last of the dinosaurs but also because there is good evidence for the cause.
15. The first suggestion, by Walter and Louis Alvarez, that the extinction might be due to an astronomical impact at first received little scientific support. However, they soon discovered that sediments in a narrow band at that point in the geological column were enriched in iridium, an element abundant in some types of meteorite. But there was no sign of an impact crater of that age. Then evidence began to emerge, not from the land but from the sea just off the Yucatan Peninsula of Mexico, of a buried crater 200 kilometres
across. There is evidence of debris from a much wider area. If, as is calculated, it marks the point where an asteroid or comet, maybe 16 kilometres across, hit the Earth, the results would indeed have been devastating. Apart from the effects of the impact itself and the tsunami that resulted, so much rock would have been vaporized that it would have spread round the Earth in the atmosphere. At first it would have been so hot that its radiant heat would have triggered forest fires on the ground. The dust would have stayed in the atmosphere for several years, blocking out sunlight, creating a global winter, and causing
food plants and plankton to die. The sea bed at the impact site included rocks rich in sulphate minerals and these would have vaporized, leading to a deadly acid rain when it washed out of the atmosphere again. It is almost surprising that any living creatures survived.