Fundamental to all ordering of events of the Earth's history is the principle of the positional relationships of rock and mineral bodies. For example, in any stratified rock sequence, younger rocks overlie older ones. Similarly, stratified sedimentary rocks intruded by formerly molten igneous rocks are clearly older than the igneous rocks. Study of positional relationships allows the geologist to establish a relative sequence of events. Positional relationships underlie all attempts to decipher geological history, and they allowed the development in the 19th century of a relative time scale based on the sequences of fossil assemblages which also provided evidence for the theory of evolution.
At the end of the 18th century, James Hutton formulated another major contribution to our understanding of the rock record, the principle of uniformitarianism, which states that the same geological processes now operating also acted in the past, producing similar results. This principle underlies the use of modern geological processes, rates and products as guides in interpreting and explaining the rock record. On a broad scale, 3 geological phenomena exhibit systematic changes which are essential to the construction and continued refinement of the geological time scale: the evolution of life, the radioactive decay of unstable isotopes and the paleomagnetic signature of rock and mineral bodies.
Whereas earlier attempts to understand the Earth's history focused on specific kinds of rocks, with crystalline igneous rocks considered oldest and sedimentary rocks progressively younger, the recognition of distinctive fossils within the younger part of the rock record led to rapid progress. Most major divisions of the Phanerozoic [Gk "visible life"] part of the geological time scale, the last 570 million years, were established in the 19th century, based on their distinctive fossil content.
Cambrian, Permian, Triassic and most other names of the Phanerozoic systems were in common use before 1900 to refer to the rocks and time periods in which particular organisms were abundant. Recognition of the geological periods and eras permitted relative dating of rocks to move from local divisions, based on positional relationships, to regional and international dating; however, the time scale remained relative.
Calibration of the 19th-century geological time scale had to await 2 major advances of 20th-century earth science: discovery of natural radioactivity and development of tools to measure this radioactivity accurately (see Geological Dating). Radiometric dating techniques have permitted calibration of the geological time scale and are essential to subdivision of the vast Precambrian part of the rock record which lacks hard-shelled fossils.
About five-sixths of geological time is assigned to the Precambrian, which ended about 570 million years ago. Less is known about it than about the Phanerozoic (the most recent 570 million years) because more of the Phanerozoic is preserved and exposed, and because it contains most of the fossil record.
The concept of geological eras came from the Phanerozoic part of the rock record, and the names of its 3 eras - Paleozoic (ancient life), Mesozoic (middle life) and Cenozoic (modern life) - are based on how closely the fossils resemble living forms.
Each era had its own most characteristic organisms, and these and others are used to identify Phanerozoic rocks around the world. The hard-shelled arthropods and corals of the Paleozoic oceans gave way to the reptiles of the Mesozoic oceans and, in particular, to the land-dwelling dinosaurs. These, in turn, were replaced by the more adaptable, warm-blooded mammals of the Cenozoic.
Canada has world-famous exposures of Lower and Middle Paleozoic sedimentary rocks in the Rocky Mountains (see Burgess Shale); of classic, Upper Paleozoic rocks in the upper islands of the arctic; and of abundant, widespread Mesozoic and Cenozoic successions in the sedimentary basin of the prairies (see Badlands; Dinosaur Hunting in Western Canada), in the Arctic Archipelago and on the Continental Shelf off the Atlantic Provinces.
Proterozoic rocks, also abundant in the Shield, are about 570 million to 2.5 billion years old. These rocks begin to have a modern look: large sequences of shallow marine and continental sedimentary rocks can be distinguished, as can mountain belts similar to modern, continental-margin mountain belts.
Refinement of the Geological Time Scale
Cretaceous/Tertiary Boundary Event
The multidisciplinary approach in arriving at a precise and universally applicable record of geological time has been developed for the time spanning the Cretaceous/Tertiary (K/T) boundary event, popularized by its coincidence with the extinction of the dinosaurs. This time of biological crisis occurs within a major reversed interval - by comparison to a world standard containing the record of the K/T biological crisis, the 29th major reversal interval (29r) counting back from the present. There is a record of these events in rocks of the Western Canada Basin, where a very precise multidisciplinary-based chronology has been established.
This chronology uses the fossil record of the K/T biological crisis, as reflected in plant distinctions or changes in the overall composition of the flora; the absolute age (in millions of years) of volcanic-derived minerals; and a detailed magnetic polarity record.
The magnetic polarity chronology rigorously establishes a short time (of about 50 000 years' duration) of normal polarity (N) spanning the immediate interval containing the K/T boundary within 29r (of about 500 000 years' duration). Together these events provide 4 universal time surfaces or datums, marking the change from the lower part of 29r to the short N; the K/T boundary, a physical as well as paleontologically defined datum; the change from the N to the upper part of 29r; and the change from the upper part of 29r to N and to a well-established Paleocene fossil flora.
This detailed and precise division of geological time, established with certainty by combining these different types of information, provides a necessary tool in attempting to answer a fundamental question of geological history - what are the cause and effect relationships between changes in the Earth's biota associated with the physical K/T boundary event, considered by many to involve the collision of an extraterrestrial body with the Earth? The same principles can be equally well applied to many geological problems.
The 20th century has seen the development and continued refinement of radiometric dating techniques. These techniques also permit continued calibration of the geological time scale, including the Phanerozoic portion. In recent years the third geological phenomenon that exhibits systematic changes, the record of the earth's magnetic field, is being used increasingly to calibrate the geological time scale and resolve geological problems. It has been known for some years that the earth's magnetic field has reversed a large number of times in the past, and it is the discovery of the pattern of these reversals in the rock record that has provided a new tool in dating past geological events. Through painstaking studies over the past several decades, a detailed magnetic anomaly scale of normal and reversed magnetic polarities has been constructed for the interval from the present back to the mid-Mesozoic time, about 160 million years ago.
Elegant refinements of the geological time scale occur by combining the 3 approaches - biostratigraphy based on the fossil record, radiometric dating of suitable materials, including volcanic ash falls and magnetic reversals. Reversals in the earth's magnetic field throughout geological time have been recorded in the rocks by the paleopolarity of tiny crystals of magnetic minerals. There have been 58 major reversals since the time of the dinosaurs in the latest Cretaceous some 66.4 million years ago, or on average one about every million plus years.
As each shift from a reversed to a normal state or vice versa represents only a moment in geological time, and as each of these polarity events is worldwide in scope, they have the potential of providing geologists with a series of universal time datums-a tool of immense value in understanding the chronological history of the earth. However, each interval of geological time when the poles were as they are at present (called normal or N) or when they were the opposite of the present (called reversed or R), does not in itself carry a unique signature. Each individual imprint left by times of "normal" or of "reversed" polarity looks the same, being comparable to geological time recorded by a series of 0s and 1s. The dilemma in applying magnetic polarity changes to time problems is one of developing a means of recognizing individual 0s and 1s.
Two approaches are available to resolve this dilemma. It is known that the length of time between individual polarity events differs widely. There have been times in the earth's history when its polarity was very stable and times when its polarity changed relatively frequently. By comparing a stratigraphic profile of reversals of varying lengths as recorded in the rocks of a specific area with a polarity standard, it is possible to arrive at a highly plausible, unique solution to the age of the unknown profile.
The second approach is to overlay on the profile of magnetic reversals absolute age data as determined by radiometric dating, relative age data from the study of fossils, or both types of age data. Each of these dating methods provides an unidirectional, although somewhat imprecise, record of geological time, but nevertheless a means of uniquely differentiating between a series of 0s and 1s.
Since the 1960s the advent and acceptance of the concept of plate tectonics has resulted in great interest in detailed reconstruction of the Earth's crust and surface in past geological times. Canadian earth scientists are leaders in this work because of the long emphasis on regional geological studies in Canada and because of the diversity and complexity of the country - with its enormous Precambrian Shield core, its widespread interior-platform sediments, its continent-bounding mountain belts on east, west and north sides, and its well-developed, continental-margin sedimentary sequences.
This work depends fundamentally on continued refinement of subdivisions and methods of dating the rock record, through paleontological studies, radiometric dating, magnetic reversal chronology and other methods. The selection of suitable field localities as standards or references for geological systems (such as the Devonian) is a vital part of this work. Government earth scientists, especially those of the Geological Survey of Canada, play a key role in regional geological studies, particularly in remote areas of northern Canada (see also Geological Regions; Plate Tectonics).