The regular order of the occurrence of fossils in rock layers was discovered around 1800 by
William Smith. While digging the
Somerset Coal Canal in Southwest England, he found that fossils were always in the same order in the rock layers. As he continued his job as a
surveyor, he found the same patterns across England. He also found that certain animals were in only certain layers, and that they were in the same layers all across England. Due to that discovery, Smith was able to recognize the order that the rocks were formed. Sixteen years after his discovery, he published a
geological map of England showing the rocks of different
geologic time eras.
Principles of relative dating Methods for relative dating were developed when geology first emerged as a
natural science in the 18th century. Geologists still use the following principles today as a means to provide information about geologic history and the timing of geologic events.
Uniformitarianism The
principle of Uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist
James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."
Intrusive relationships The
principle of intrusive relationships concerns crosscutting intrusions. In geology, when an
igneous intrusion cuts across a formation of
sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks,
laccoliths,
batholiths,
sills and
dikes.
Cross-cutting relationships can be used to determine the relative ages of
rock strata and other geological structures. Explanations: A –
folded rock strata cut by a
thrust fault; B – large
intrusion (cutting through A); C –
erosional
angular unconformity (cutting off A & B) on which rock strata were deposited; D –
volcanic dyke (cutting through A, B & C); E – even younger rock strata (overlying C & D); F –
normal fault (cutting through A, B, C & E). The
principle of cross-cutting relationships pertains to the formation of
faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a
normal fault or a
thrust fault.
Inclusions and components The
principle of inclusions and components explains that, with sedimentary rocks, if inclusions (or
clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when
xenoliths are found. These foreign bodies are picked up as
magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.
Original horizontality The
principle of original horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although
cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).
Lateral continuity The
principle of lateral continuity states that layers of
sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a
valley or other
erosional feature, can be assumed to be originally continuous. Layers of sediment do not extend indefinitely; rather, the limits can be recognized and are controlled by the amount and type of
sediment available and the size and shape of the
sedimentary basin. Sediment will continue to be
transported to an area and it will eventually be
deposited. However, the layer of that material will become thinner as the amount of material lessens away from the source. Often, coarser-grained material can no longer be transported to an area because the transporting medium has insufficient energy to carry it to that location. In its place, the particles that settle from the transporting medium will be finer-grained, and there will be a lateral transition from coarser- to finer-grained material. The lateral variation in sediment within a
stratum is known as
sedimentary facies. If sufficient
sedimentary material is available, it will be deposited up to the limits of the sedimentary basin. Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.
Inclusions of igneous rocks Melt inclusions are small parcels or "blobs" of molten rock that are trapped within crystals that grow in the
magmas that form
igneous rocks. In many respects they are analogous to
fluid inclusions. Melt inclusions are generally small – most are less than 100
micrometres across (a micrometre is one thousandth of a millimeter, or about 0.00004 inches). Nevertheless, they can provide an abundance of useful information. Using microscopic observations and a range of chemical
microanalysis techniques
geochemists and
igneous petrologists can obtain a range of useful information from melt inclusions. Two of the most common uses of melt inclusions are to study the compositions of magmas present early in the history of specific magma systems. This is because inclusions can act like "fossils" – trapping and preserving these early melts before they are modified by later igneous processes. In addition, because they are trapped at high pressures many melt inclusions also provide important information about the contents of volatile elements (such as H2O, CO2, S and Cl) that drive explosive
volcanic eruptions.
Sorby (1858) was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after
World War II (Sobolev and Kostyuk, 1975), and developed methods for heating melt inclusions under a microscope, so changes could be directly observed. Although they are small, melt inclusions may contain a number of different constituents, including glass (which represents magma that has been quenched by rapid cooling), small crystals and a separate vapour-rich bubble. They occur in most of the crystals found in igneous rocks and are common in the minerals
quartz,
feldspar,
olivine and
pyroxene. The formation of melt inclusions appears to be a normal part of the crystallization of minerals within magmas, and they can be found in both
volcanic and
plutonic rocks.
Included fragments The
law of included fragments is a method of relative dating in
geology. Essentially, this law states that
clasts in a rock are older than the rock itself. One example of this is a
xenolith, which is a fragment of
country rock that fell into passing
magma as a result of
stoping. Another example is a
derived fossil, which is a
fossil that has been eroded from an older
bed and redeposited into a younger one. This is a restatement of
Charles Lyell's original
principle of inclusions and components from his 1830 to 1833 multi-volume
Principles of Geology, which states that, with
sedimentary rocks, if
inclusions (or clasts) are found in a
formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for
gravel from an older formation to be ripped up and included in a newer layer. A similar situation with
igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as
magma or
lava flows and are incorporated later to cool in the
matrix. As a result, xenoliths are older than the rock which contains them.
Planetology Relative dating is used to determine the order of events on
Solar System objects other than Earth; for decades,
planetary scientists have used it to decipher the development of bodies in the
Solar System, particularly in the vast majority of cases for which we have no surface samples. Many of the same principles are applied. For example, if a valley is formed inside an
impact crater, the valley must be younger than the crater. Craters are very useful in relative dating; as a general rule, the younger a planetary surface is, the fewer craters it has. If long-term cratering rates are known to enough precision, crude absolute dates can be applied based on craters alone; however, cratering rates outside the Earth-Moon system are poorly known. == Ecology ==