SGES 1302 INTRODUCTION TO EARTH SYSTEM LECTURE 9: Principles of Relative Dating

Lecture 9: GEOLOGIC TIME SCALE: Principles of relative dating

Geologic Time Time is an important element in all geological processes. Geologists in 19th century recognised that Earth had experience many episodes of mountain building and erosion, which required great spans of geologic time. Earth is very old. But how old? Before the advancement of radiometric dating, geologic events are based on relative dating principles. The age of the Earth and its inhabitants are now determined through two complementary lines of evidence: relative dating (stratigraphic age) and absolute dating (radiometric age).

Relative vs Absolute dating Relative dating places fossils in a temporal sequence by noting their positions in layers of rocks, known as strata. As shown in the diagram, fossils found in lower strata were typically deposited first and are deemed to be older. By studying and comparing strata from all over the world we can learn which came first and which came next, but we need further evidence to ascertain the specific, or numerical, ages of fossils. Absolute dating relies on the decay of radioactive elements that gives the actual number of years that have passed since an event occurred. By dating volcanic ash layers both above and below a fossil-bearing layer, as shown in the diagram, you can determine “older than X, but younger than Y” dates for the fossils. Geologists have assembled a geological time scale on the basis of numerical dating of rocks from around the world.

Geologic Time Scale Interpreting the history of the Earth is a prime goal of earth science. The complex history of the earth can be revealed by studying the clues preserved in the rocks, especially sedimentary rocks. Geological events by themselves (e.g. Age of Dinasour) have little meaning until they are put into a time perspective. The development of the GEOLOGIC TIME SCALE has change the way people think about time. Earth is much older than anyone had previously thought.

Geological Principles 17th & 18th century: Doctrine of CATASTROPHISM: Earth’s landscape had been formed primarily by great catastrophes. Mountains and canyons were explained as being produced by sudden and often worldwide disasters triggered by unknown causes that no longer operate. Late 1700s: Principle of UNIFORMITARIANISM by James Hutton. It is the pillar of modern geology. “the physical, chemical, and biological laws that operate today have also operated in the geologic past” This idea is commonly expressed as “The present is the key to the past”. Hutton demonstrated that geological processes occur over extremely long period of time. Weak, slow-acting processes could over a long period of time produce effects just as great as those resulting from major catatrophic events. Although the principle of uniformitarianism is still relevant today, some geologic processes may not be equally important or operate in the same rate. Earth is ever-changing through the geologic time. “…that we find no vestige of a beginning – no prospect of an end” Hutton, 1788.

Principles of Relative Dating Placing rocks in their proper sequence of formation (which ones formed first) but cannot tell us how long ago the event took place. Relative dating is still widely used today. It is not replaced by absolute dating, but supplemented it. Law of Superposition Most basic principle of relative dating. In an undeformed sequence of sedimentary rock, each bed is older than the one above it and younger than the one below Also applicable to other surface deposited materials such as lava flows, volcanic ash etc.

Principles of Relative Dating Principle of Original Horizontality Layers of sediment are generally deposited in a horizontal position. Rock layers that are flat means that they have not been disturbed and thus still have their original horizontality. If they are inclined or folded, they must have been moved into that position by crustal disturbances after their deposition.

Principles of Relative Dating Principle of Cross-cutting Relationships When a sequence of sedimentary rocks is cut by faults or intruded by igneous rocks, the faults and igneous rocks must be younger and occurred after the sedimentary layers were deposited.

Principles of Relative Dating Inclusions Inclusions are pieces of rock that are contained within another. The rock mass adjacent to the one containing the inclusions must have been formed first in order to provide the rock fragments. The rock containing the inclusions is the younger of the two.

Principles of Relative Dating Unconformities Conformable: when layers of rocks were deposited without interuption. No place on earth has a complete set of conformable sedimentary rock layers. Interuption of deposition – break in the rock record is call unconformity Unconformity represents a long period during which deposition has ceased, erosion removed previously formed rocks and then deposition resumed. There are 3 types of unconformities: Angular unconformity, disconformity & nonconformity

Angular Unconformity Tilted or folded sedimentary rocks that are overlain by younger, more flat lying strata. There is a pause in deposition, period of deformation and erosion.

Disconformity Not easily identified because both sides are near parallel. Easier to identify if the older rocks are deeply cut by erosion.

Nonconformity A break separating the older igneous or metamorphic rocks from the younger sedimentary layers.

Principle of faunal succession “Fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognised by its fossil content” Fossils are arranged according to their age and do not present randomly or haphazardly Basic principle of historical geology Index fossil

Correlation of Rock Layers To develop a stratigraphic time scale for the entire Earth, rocks of similar age in different regions must be matched up by CORRELATION. To correlate rocks over great distances, fossil records are required.

Succession in geologic development

Geologic Time Scale Most of the original geologic time scale was based on studies of strata in Europe in mid 1900s Major units are generally named after a geographic area where they are well exposed The rock units are separated by major changes in rock types, unconformities or fossil groups Rocks in other parts of the world that contain the same fossil groups are considered as having the same age. Absolute age for the standard geologic column is added in the 20th century with data from radiometric dating.