Basic principles for relative geologic dating

Geology Online Subchapter

basic principles for relative geologic dating

Using relative and radiometric dating methods, geologists are able to answer the and using a few basic principles, it is possible to work out the relative ages of. Start studying 5 Geologic Principles/ Relative Dating. Learn vocabulary, terms, and more with flashcards, games, and other study tools. I. Principles of Stratigraphy and Relative Time “present is the key to the past” d. radiometric dating of fossil-bearing rocks using cross- cutting igneous rocks.

Gaps In the Geologic Record Following on the Law of Original Horizontality and Law of Superposition, both Hutton and Lyell recognized erosional boundaries preserved between rock layers representing gaps in the geologic record. They named these gaps unconformities examples in Figure An unconformity is a surface between successive strata that represents a missing interval in the geologic record of time.

basic principles for relative geologic dating

Unconformities show evidence that the Earth's surface may have been exposed to erosion for periods of time. Unconformities are produced either by an interruption in deposition or by the erosion of per-existing strata followed by renewed deposition. Note that unconformities can be complex. For example, may erosion may be taking place in one location in geologic time, where nearby or elsewhere sediments may have continued to be deposited and preserved. As a result, an unconformity in one location may span a different about of time in another location.

Several types of unconformity boundaries are recognized: There is typically little evidence to support a significant passage of time occurred at a conformable boundary. Types of unconformities boundaries between layered rocks representing "gaps" in the geologic record in a locality. Examples of unconformities and conformable boundaries in the Grand Canyon of Arizona Fig. Nonconformity in the Grand Canyon known as the "Great Unconformity" between Precambrian-age metamorphic rocks and overlying Cambrian-age sedimentary rocks.

Disconformities between layers in the Grand Canyon. Disconformities occur between sedimentary rock formations of different ages and represent gaps of time spanning millions of years. An angular unconformity occurs between sedimentary rocks of different ages. Uplift and erosion of a mountainous landscape occurred before the overlying sediments were deposited. Conformable or gradational contact between sedimentary layers.

This type of boundary only reflect a change in how sediments were depositied—not necessarily representing a long gap in time. Unconformities are caused by relative changes in sea level over time. Wave erosion wears away materials exposed along coastlines, scouring surfaces smooth. On scales of thousands to millions of years, shorelines may move across entire regions. Erosion strips away materials exposed to waves and currents. New younger material can be deposited on the scoured surface.

Shallow seas may flood in and then withdrawal repeatedly. Long-lasting transgressions can erode away entire mountain ranges with enough time.

A transgression occurs when a shoreline migrates landward as sea level or lake level rises. A regression occurs when a shoreline migrates seaward as sea level or lake level falls Figure Sea level changes may be caused by region uplift or global changes in sea level, such at the formation or melting of continental glaciers.

Whatever the cause of sea level change, when sea level falls, sediments are eroded from exposed land. When sea level rises, sediments are typically deposited in quiet water settings, such as on shallow continental shelves or in low, swampy areas on coastal plains.

Some unconformities represent great gaps in time. For example, the Great Unconformity in the lower Grand Canyon illustrates where a great mountain range existed in the region during Precambrian before erosion completely stripped the landscape away back down, eventually allowing seas to flood over the region again in Cambrian time.

The "gap in the geologic record" in some locations along the Great Unconformity represents billions of years see Figure Unconformities can form by the rise and fall of sea level. A rise in sea level causes a transgression which creates space underwater for sediments to be deposited. New younger material is deposited on the scoured surface.

When sea level falls it causes a regression, and sediments are not deposited or are eroded away. Geochronology is the branch of earth sciences concerned with determining the age of earth materials and events through geologic time. How do geoscientists determine the age of rocks or fossils?

How do they figure out how long ago and in what order did geologic processes or events take place? For instance how do they know how often a volcano erupts or how often earthquakes take place?

Geologists now have many ways to determine the age of materials using relative dating and absolute dating methods. Relative Dating Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age see below. Relative dating involved the study of fossils and the correlation or comparison of fossils of similar ages but from different regions where their age is known.

Microfossils derived from sediments and cores from wells help in the subsurface exploration for oil and gas. When studying an area where layered rocks are exposed, the Law of Original Horizontality dictates that the sedimentary layers were originally deposited as flat layers.

The Law of Superposition dictates that if a series of rock layers are exposed, the oldest are on the bottom of the stack. That is true, unless the sequence of rock layers have been disturbed by some later geologic even.

It that case, the Law of Cross-Cutting Relationships dictates that the rocks will be older than the forces that later changed them. Examples of forces that change landscapes include movement of faults, or the tectonic folding of rock layers, or an intrusion of igneous material, such as the formation of a volcano.

Use these rules to interpret this general vertical cross section of a landscape Figure The diagram could represent a road cut along a highway or a wall on the side of a canyon. The Law of Original Horizontality and Law of Superposition suggest that layer C layers of shale was deposited before layer B beds of limestonewhich was deposited before layer A beds of sandstone.

basic principles for relative geologic dating

The Law of Cross-Cutting Relationships dictates that next thing to happen was that feature D an igneous intrusion cut across the sedimentary layers. After that a fault feature E broke through all the older materials. The final thing to happen was erosion of the landscape down to partially expose some of the features on the surface. Note also that the boundaries between layers C and B, and A and B may represent unconformities possible gaps in time.

Relative dating only can be used to sort the exposed visible features in the order that they formed example in Figure Fossils and Relative Dating Sedimentary rock layers of different ages often look very similar. Conversely, sedimentary rock layers of similar ages may appear very different in other locations.

Sometimes fossils preserved in sedimentary layers are very useful for correlating rock layers from one area or region to another. Paleontologists have extensively studied fossil found in sedimentary rock formation of all geologic ages around the world. Certain "index fossils" are both abundant and widely distributed through sedimentary rocks of relatively limited geologic time ranges.

Unfortunately, many rocks formation do not contain fossils, or the fossils they may contain are very rare or poorly preserved. Metamorphic and igneous rocks do not contain fossils.

In many cases, the exact age of each of the rock units is unknown until it can be confirmed by other means involving absolute dating methods discussed below. Applying basic geologic principles: Laws of Original Horizontality, Superposition, and Cross-Cutting Relationships explain the order of this diagram with the order of formation: Rock layers like these in Utah record information about million years of Earth history exposed in the region.

These rocks were originally deposited horizontally but then were later folded upward: Relative Dating Exercise 1. Tracks left in the mud along a river bank include a bear, bird, deer, dog, bobcat, and human and blood.

What happened, and in what order? Try Relative Dating sort out the visually available clues in a map view. The basic geologic principles have many applications to interpreting the order of events on many scales, ranging from very large like the Grand Canyon or the geologic history of parts of the Rocky Mountainsto small scale, like interpreting the order of footprints along a lake shore as illustrated in this exercise. Geologists use basic geologic principles to study geology map as illustrated belowand use them to unravel the complex history of an area, such as planning tunnels in large underground mining operations, or for exploring for oil and gas, and deep-water resources.

Try interpreting the order of events in the map-view diagrams involving animal trackways Figures and Notice that some tracks overly are superimposed on other tracks. Sometimes things aren't as clear as they seem, but inferences can be made. Basic geologic principles are used to interpret the geologic history of an area.

See examples in Figures to Relative Dating Exercise 2. Tracks left in the mud along a river bank include a bear, bird, deer, bobcat, duck, and human. Can you figure out the chronology of events in this nature scene? Minor faults cut though layers of volcanic ash beds and sedimentary rocks, exposed along I near Kingman, AZ. In Figurethe Laws of Original Horizontality and Superposition show a series of sedimentary layers and volcanic ash bed deposited from oldest on the bottom to a lava flow on the top of the cliff.

The Law of Cross-Cutting Relationships shows that the layers were offset by faulting after they were deposited as layers. Therefore, the San Andreas Fault must be younger than the sedimentary layers. In Figurethe Law of Cross-Cutting Relationships suggests that the fault in the picture is younger than the sedimentary rock formation on either side of the fault.

However, the age of the two rock formation on either side would have to be determined by some other means such as by the fossils they may contain. In Figurethe Law of Cross-Cutting Relationships suggest that the dark, basalt igneous dike is younger than the pink volcanic rocks on either side, however, the exact age of the volcanic rocks would have to be determined by other means.


A geological cross-section is a graphic representation of the intersection of the geological features in the subsurface with a vertical plane. Where the vertical plane intersects the surface is typically shown as a line on a map. Like geologic maps, cross sections show different types of rocks, their structure, and the geometric relationship between them are represented. Note that geologic cross sections are made by using available mappable features found on the surface or interpreted from data about the subsurface.

Natural cross sectional views are sometimes possible along canyon high walls or along steep mountain range front Figure How most subsurface data derive or imaged through geophysical methods, such as by seismic data by earthquakes or man-made explosionsby measurements of gravity, magnetism, electrical resistivity, or information derived from wells such as core sample, radiation measurements, or other geophysical methods.

A wall of rocks exposed in a cliff illustrate the vertical view illustrated by a cross section. This cliff shows igneous intrusions exposed in Wind River Canyon, Wyoming. For example, geologists use use sound shock waves produced by explosives or machines making sounds to penetrate the ground.

Other geophysical methods collect measurements using electricity and radar to penetrate the ground, and measurements of variations in Earth's gravity and magnetism. In addition, geologists use data from wells drilled in an area to add information to cross sections. Seismic Reflection Techniques Produce Cross Sections Used For Subsurface Exploration Seabed exploration produces cross-sectional seismic profiles, raw data that are converted to cross-section diagrams Figure Modern systems produce views that are in three dimensions.

Geologists study cross sections created by geophysical exploration methods.

Introduction to Geology

Figure is an example of a seismic profile showing the location of exploratory wells. A marine seismic reflection profile expedition. A seismic reflection profile data is a cross section. In most places, this is not an easy task because the landscape feature often cover or disguise geologic features. In most places, soil and vegetation cover the bedrock, and private property designations often prevent geologists from gaining access to suitable outcrops where fresh rock material is exposed.

Geologist have to work with regional government and land management companies in order to gain access to land and construction sites, and this is often quite time consuming.

However, in many regions, only a few good outcrops, road cuts, or stone quarries provide access to enough detail for field geologists to reconstruct the geologic history of a region, and make assessments of potential geologic hazards, such a active fault zones and earthquake historylandslide hazard-prone areas, and the history of volcanic eruptions in a region.

Ideal locations for study geology include exposures in canyon walls or fresh road cuts, or sea cliffs where wave erosion create fresh exposures of bedrock geology. Figure illustrates a sea cliff in Encinitas, California where an angular unconformity is locally exposed between massive layers of sedimentary rock. The sea cliff exposure provides a natural cross section view of rocks hidden in the subsurface.

Sites like this can be used to interpret the geologic history of this part of the coastline. Sedimentary rocks and an unconformity exposed in a sea cliff at Fletcher Cove in Encinitas.

Beneath the surface in this landscape are a mix of layered and non-layered rocks. Letters correspond to different features including a fault, unconformities, and the active magma chamber feeding lava to the volcano erupting on the surface. Examine the cross section to see the chronology of the formation of geologic features, from oldest to youngest represented by letters A to Q. These ancient rocks may have been an ancient mountain range that later wore down by erosion over time.

Letter B represent a unconformity an erosional gap in time between the ancient rocks below and layer C, a layered sedimentary bed on of the older rocks A. The letter B is technically a nonconformity between stratified and non-stratified rocks. Sedimentary layers D and E were deposited originally horizontal beds on top of layer C.

Letter F point to the unconformity that is actually younger than the unconformity at letter B! This unconformity represents a period of uplift and erosion before sedimentary layer G was deposited.

This unconformity is a nonconformity or an angular unconformity depending where you examine it. Another period of uplift occurred.

During this time, a fault formed Letter L. Letter M represents another unconformity. This unconformity beneath layer N tells us that the fault L formed before sedimentary layer N was deposited on the eroded surface and younger than layer K. Without more information we can only know that the fault L is younger than layer I but is younger than layer N.

Sedimentary layer O horizontally rests on top of layer N. Finally, igneous activity produced volcano P. Molten material is still flowing from the magma chamber Q to produce the active eruption on the volcano. A hypothetical cross section showing layers beneath a landscape with an active volcano in a region where mountain building has occurred in the ancient past. Then, work things out in reverse older to tell the geologic history of a region.

Be sure to examine the geologic time scale to understand the ages of the rocks in this cross section. Figure is a satellite image map of northwestern Wyoming.

Relative dating

Using this principle any fault or igneous intrusion must be younger than all material it or layers it crosses. Once a rock is lithified no other material can be incorporated within its internal structure. In order for any material to be included within in the rock it must have been present at the time the rock was lithified.

For example, in order to get a pebble inside an igneous rock it must be incorporated when the igneous rock is still molten-- such as when lava flows over the surface. Therefore, the piece, or inclusion, must be older than the material it is included in.

basic principles for relative geologic dating

Lastly the Principle of Fossil Succession. Aside from single-celled bacteria, most living organism reside at or very near the Earth's surface either in continental or oceanic environments.

As these organisms die they are deposited on the surface along with all other sediments. If conditions are right the remains of the dying organisms can then be preserved as fossils within the rock that formed from sediments that covered the remains. Since, all sedimentary rock is formed through the gradual accumulation of sediment at the surface over time, and since the principle of superposition tells us that newer sediment is deposited on top of older sediment, the same must also be true for fossils contained within the sediment.