Actividades formativas y método dialógico

Historical or stratigraphical geology is mainly concerned with the description and classification of rocks with a view to arranging them in chronological order in which they were laid down on the surface of the earth. Of the three groups of rocks – sedimentary, igneous and metamorphic – only the sedimentary rocks are amenable to such an arrangement since they have been deposited layer by layer and contain the remains of organisms which flourished while they were formed.

The time scales used by the hydraulic engineer and the geologist are quite different. The hydraulic engineer uses seconds or days as the unit of time when dealing with transport rates of water or sediment.

When he is dealing with the morphological changes such as aggradation or degradation of the riverbed he is concerned with bed level changes occurring in a few years or a few decades at the most. As against this in stratigraphy the unit time used is million years. Thus Lord Kelvin (Rice 1977) assumed that earth started as a molten body, and applying the theory of cooling to this mass he estimated that to attain the present day temperature the earth must have taken 20 to 40 million years.

For arranging the various geologic formations in the order of increasing antiquity, the geologist uses various means at his disposal. The first is the fundamental principle of superposition in which the upper beds in an un-inverted succession are dated as younger than the lower ones. The second means is the palaentological dating depending on the fossil content of the formation. Each formation encloses a fossil assemblage, which is characteristic and different from that of the underlying or over lying formations. The animal and vegetable organisms of each geological age bear special characters not found in those of other ages. It needs to be emphasised that the fossils present in a series of formations are not only a function of the period when the formation was laid down but are also a function of (i) the geological period when rocks were formed; (ii) the zoological or botanical provinces in which the locality was situated; and (iii) the physical conditions prevalent at the time, e.g. depth, salinity, muddiness of water, temperature, character of sea bottom and currents. The geological formations are named in such a manner that they indicate the stage of development of the organisms. Thus the Azoic era is completely devoid of organisms, while Proterozoic era shows traces of the most primitive life. The Palaeozoic era contains the remains of ancient plants and animals, and so on to the recent time. The third means used for determining the age of formation is the lithology. Each lithology unit may comprise a number of individual beds having more or less the same characters, when it is spoken of as a formation, and given a local or specific name to distinguish it from a similar formation of different age or belonging to a different area. Lithology is many times useful in the determination of chronology.

With the discovery of radioactive elements uranium and thorium at the end of 19th and beginning of 20th century a more powerful means was available for determining the chronology of rocks and other formations. It was found that uranium and thorium emit alpha and beta radiations; alpha radiation consists of positively charged helium nuclei with two positive charges while beta radiation consists of negatively charged electrons. Depending on the nature of radioactive elements half the atoms of the element will disintegrate in this manner in a period known as half-life of the element concerned. As a result of such emission of alpha and beta radiation a new element or daughter element is formed. If the quantity of the parent element to the daughter element is known at any time, the period during which the

radioactive decay has taken place can be calculated. Uranium and thorium, as a result of radioactive decay are finally converted to lead Pb²⁰⁶, Pb²⁰⁷, or Pb²⁰⁸ isotopes. The isotopic analysis of the minerals is carried out using the technique of mass spectroscopy. Using radioactive dating techniques, it is estimated that the age of crustal material of the earth is about 4,500 million years while the age of lunar rock ranges from 3,000 to 4,500 million years.

Table 4.1 gives the era, group, system or formation or rocks, and the chief fossils found in these formations. Even though geologists are interested in all the eras from Quaternary to Azoic or Archaen, geo-morphologists consider Quaternary and Tertiary periods as of primary significance to them; this is so because it is believed that the majority of landforms are about a million years old, and the remaining not more than 20 to 30 million years old. Hence, from the geo-morphologist’s point of view it becomes crucial that the dating between Quaternary and Tertiary periods is done carefully. This aspect has been discussed in detail by Rice (1977) and the following discussion is based on his comments. The dating of Cenozoic era has become complicated because of various reasons. Earlier stratigraphic column was constructed on the basis of marine sediments and faunas raised above the modern sea level. Such continuous marine successions are rare in the late Cenozoic age, and the contemporary terrestrial beds tend to be fragmentary, of short duration and local. The second difficulty arises because of the relatively brief duration of Cenozoic era because of which the biological evolution during this period was not adequate to delimit the era. Then there were at least eight or probably even more environmental changes during this period, which have complicated building up of the chronological sequence. Lastly, as regards methods of estimating the age of the formation, K/Ar dating is most suitable for Cenozoic era;

however because of the very small amount of Ar present in the formations the accuracy of the method is doubtful.

Sufficient light has been thrown on the chronology of Cenozoic era by obtaining cores of materials deposited on the beds of deep seas. These deposits contain fossils of marine organisms such as foraminiferal, globorotalia menardii and diatoms in large numbers. Since these organisms have different environmental requirements, their fossils give information on the changing temperature of seawater.

The change in the ratio O¹⁸/O¹⁶ of the isotopes of oxygen in the shells also indicates the temperature changes. Shells formed in the cold climate are relatively richer in O¹⁸. Such evidences have helped in fixing the chronology of later Cenozoic era.

Some investigators argue that the most distinctive characteristic of Pleistocene epoch is the development of large continental ice sheets in Europe and North America. Hence the beginning of Pleistocene should be equated with dramatic fall in temperature. However, glaciation in different parts of the world leads to a very large variation in the onset of Pleistocene. Therefore many argue that it is unwise to relate glaciation to Pleistocene.

Similar uncertainty prevails in fixing the Pleistocene– Holocene boundary. The term Holocene was originally intended to designate the post glaciation period. However, the melting of ice sheets being transgressive of time, the post glaciation period in one region could be glaciation period in another region. Therefore, this criterion was difficult to use. Hence the boundary between Pleistocene and Holocene is fixed arbitrarily. The most accepted boundary is that first proposed by Scandinavian workers using pollen analysis. Using this technique the period of rapid warming indicating the onset of Holocene has been fixed at about 10,000 years ago.

It is necessary to emphasize that during the Pleistocene era some areas were covered by ice sheets while some were not. In glaciated areas the attention was focussed on till sheets laid down one over the

Table 4.1 Geological time scale [Adapted from Wadia (1961), and Krishnan (1982)]

Era Period Epoch Duration Years before Chief fossils

M years present M years (Ma)

Cenozoic Quaternary Holocene 0.01 0.01 Living animals

(present)

Pleistocene 1 1 Man appears; many animals dies

(Glacial) off during glaciation

Mesozoic Secondary Jurassic 60 200

Triassic 40 240

Proterozoic Precambrian Precambrian 2500 Soft bodied animals and plants

Azoic Archaen Archaen 3600 Lifeless

Mamals, mollusca, and flowering plants dominate. Divisian largely based on proportion of living to extinct species of mollusca and the presence of mammal species Giant reptiles and ammonites disappear at the end. Flowering plants become numerous

Ammonites abundant. First birds, flowering plants and sea urchins Ammonites, reptiles, amphibia abundant. Arid climate

Palaezoic Primary

Trilobites disappear at the end Many non-flowering plants, first reptiles appear

Abundance of corals, branchiopoda, first amphibious and lung-fishes Graptolites disappear at the end;

first fishes; probably first land plants Abundance of trilobites and graptolites

Abundance of trilobites

other as a result of multiple glacial advances. Equal attention was given to the pollen analysis of biogenic materials entrapped within the tills. Similarly a thorough study was made of non-marine molluscs and beetles. However, it has been found that the record of terrestrial sediments in glaciated regions is small and is confined to the later part of Pleistocene era. In the unglaciated regions, at a limited number of places pollen analysis has been used. Some deep core samples have also been obtained from the desert areas where earlier lakes existed. Another technique used to determine the chronology of continental land surfaces in Pleistocene era is the carbon C¹⁴ dating which is very useful for dating of Pleistocene and Holocene eras because of short half-life of 5730 years of C¹⁴.

This method depends on the fact that the atmosphere and the hydrosphere represent reservoirs of radioactive carbon C¹⁴, which are tapped by animals and plants to build up their structures and tissues.

The source of radioactive carbon lies in the cosmic ray bombardment of nitrogen in the atmosphere, which converts it into C¹⁴. Carbon has three isotopes C¹², C¹³ and C¹⁴ and it is present in the atmosphere

in the form of CO₂. Out of the three isotopes C¹⁴ is the only unstable isotope with half-life of 5730 years.

In historical times a balance was reached between new C¹⁴ received from cosmic radiation and that disintegrated due to radioactive decay. Since living organisms absorb CO₂, each organism absorbs a fixed proportion of C¹⁴ of the total carbon absorbed during the lifetime. After the death of the living organism, the replenishment of C¹⁴ ceases and C¹⁴ content declines due to radioactive decay. The ratio of radioactive to the total carbon present at any time is, therefore, a measure of the age of the organic materials such as bones, tusks, grains, wood, hide, peat etc. The method is suitable for dating up to 50,000 years.

4.7 GLACIATION

Glacier is a slow moving mass of ice formed by accumulation of snow in mountain valleys and other places. Area of the continents that is covered by ice at present is close to 15 M km², the largest part of which is concentrated in Antarctica (12.5 M km²) and in Greenland (1.7 M km²). Glaciers today, except at high altitudes and in high latitudes, are of minor importance in shaping landforms; but those that existed during the Pleistocene epoch have left their imprints on many millions of square kilometres of the earth’s surface. About 10 M km² of the North America, 5 M km² of Europe, 4 M km² of Siberia and large parts of the Himalayas were glaciated. Pleistocene epoch consisted of four major glacial ages separated by interglacial ages of probably far greater duration than the glacial ones. The latest glaciation has left the most obvious imprints on the topography. Glaciers are classified into ice caps, valley glaciers, ice-streams and glacier ice.

Glaciers that are continuous sheets of snow from which ice may move in all directions are known as ice caps. Glaciers that are confined to courses, which direct their movement, are called valley-glaciers and ice-streams. Glacier, which spreads in cake-like sheets over level ground at the base of glaciated areas, is known as glacier ice.