Nicolas Steno introduced basic principles of stratigraphythe study of layered rocks, in William Smithworking with the strata of English coal Former swamp-derived plant material that is part of the rock record. The figure of this geologic time scale shows the names Relative dating and stratigraphic principles quiz the units and subunits. Using this time scale, geologists can place all events of Earth history in order without ever knowing their numerical ages. The specific events within Earth history are discussed in Chapter 8.
A Geologic Time Scale Relative dating is the process of determining if one rock or geologic event is older or younger than another, without knowing their specific ages—i. The principles of relative time are simple, even obvious now, but were not generally accepted by scholars until the scientific revolution of the 17th and 18th centuries.
James Hutton see Chapter 1 realized geologic processes are slow and his ideas on uniformitarianism i. Stratigraphy is the study of layered sedimentary rocks.
This section discusses principles of relative time used in all of geology, but are especially useful in stratigraphy. Lower strata are older than those lying on top of them. Principle of Superposition : In an otherwise undisturbed sequence of sedimentary strataor rock layers, the layers on the bottom are the oldest and layers above them are younger.
Principle of Original Horizontality : Layers of rocks deposited from above, such as sediments and lava Liquid rock on the surface of the Earth. The exception to this principle is at the margins of basins, where the strata can slope slightly downward into the basin. Principle of Lateral Relative dating and stratigraphic principles quiz : Within the depositional basinstrata are continuous in all directions until they thin out at the edge of that basin.
Of course, all strata eventually end, either by hitting a geographic barrier, such as a ridge, or when the depositional process extends too far from its source, either a sediment source or a volcano.
Strata that are cut by a canyon later remain continuous on either side of the canyon. Dark dike cutting across older rocks, the lighter of which is younger than the grey rock.
Grand canyon example
Principle of Cross-Cutting Relationships : Deformation events like fold A rock layer that has been bent in a ductile way instead of breaking as with faulting. Principle of I nclusions: When one rock formation contains pieces or inclusions of another rock, the included rock is older than the host rock.
Principle of Fossil Succession: Evolution has produced a succession of unique fossils that correlate to the units of the geologic time scale. Assemblages of fossils contained in strata are unique to the time they lived, and can be used to correlate rocks of the same age across a wide geographic distribution. Assemblages of fossils refers to groups of several unique fossils occurring together. The Grand Canyon of Arizona illustrates the stratigraphic principles. The photo shows layers of rock on top of one another in order, from the oldest at the bottom to the youngest at the top, based on the principle of superposition.
The predominant white layer just below the canyon rim is the Coconino Sandstone. This layer is laterally continuous, even though the intervening canyon separates its outcrops. The rock layers exhibit the principle of lateral continuityas they are found on both sides of the Grand Canyon which has been carved by the Colorado River. In Relative dating and stratigraphic principles quiz lowest parts of the Grand Canyon are the oldest sedimentary formationswith igneous and metamorphic rocks at the bottom.
The principle of cross-cutting relationships shows the sequence of these events.
The metamorphic schist 16 is the oldest rock formation and the cross-cutting granite intrusion 17 is younger. As seen in the figure, the other layers on the walls of the Grand Canyon are ed in reverse order with 15 being the oldest and 1 the youngest.
This illustrates the principle of superposition. The Grand Canyon region lies in Colorado Plateau, which is characterized by horizontal or nearly horizontal stratawhich follows the principle of original horizontality. These rock strata have been barely disturbed from their original depositionexcept by a broad regional uplift.
The red, layered rocks of the Grand Canyon Supergroup overlying the dark-colored rocks of the Vishnu schist represents a type of unconformity called a nonconformity. Because the formation of the basement rocks and the deposition of the overlying strata is not continuous but broken by events of metamorphismintrusion, and erosionthe contact between the strata and the older basement is termed an unconformity.
An unconformity represents a period during which deposition did not occur or erosion removed rock that had been deposited, so there are no rocks that represent events of Earth history during that span of time at that place. Unconformities appear in cross sections and stratigraphic columns as wavy lines between formations. Unconformities are discussed in the next section.
7 geologic time
There are three types of unconformitiesnonconformitydisconformityand angular unconformity. A nonconformity occurs when sedimentary rock is deposited on top of igneous and metamorphic rocks as is the case with the contact between the strata and basement rocks at the bottom of the Grand Canyon. The strata in the Grand Canyon represent alternating marine transgressions and regressions where sea level rose and fell over millions of years.
When the sea level was high marine strata formed. When sea-level fell, the land was exposed to erosion creating an unconformity. In the Grand Canyon cross-section, this erosion is shown as heavy wavy lines between the various ed strata. This is a type of unconformity called a disconformitywhere either non- deposition or erosion took place. In other words, layers of rock that could have been present, are absent. The time that could have been represented by such layers is instead represented by the disconformity.
Disconformities are unconformities that occur between parallel layers of strata indicating either a period of no deposition or erosion.
In the lower part of the picture is an angular unconformity in the Grand Canyon known as the Great Unconformity. Notice flat lying strata over dipping strata Source: Doug Dolde. The Phanerozoic strata in most of the Grand Canyon are horizontal.
Relative dating principles
However, near the bottom horizontal strata overlie tilted strata. This is known as the Great Unconformity and is an example of an angular unconformity. The lower strata were tilted by tectonic processes that disturbed their original horizontality and caused the strata to be eroded. Later, horizontal strata were deposited on top of the tilted strata creating the angular unconformity.
Here are three graphical illustrations of the three types of unconformity. Disconformitywhere is a break or stratigraphic absence between strata in an otherwise parallel sequence of strata. Nonconformitywhere sedimentary strata are deposited on crystalline igneous or metamorphic rocks.
Block diagram to apply relative dating principles.
The wavy rock is a old metamorphic gneiss, A and F are faults, B is an igneous granite, D is a basaltic dike, and C and E are sedimentary strata. In the block diagram, the sequence of geological events can be determined by using the relative-dating principles and known properties of igneoussedimentary, metamorphic rock see Chapter 4Chapter 5and Chapter 6. The sequence begins with the folded metamorphic gneiss on the bottom. Next, the gneiss is cut and displaced by the fault labeled A.
Both the gneiss and fault A are cut by the igneous granitic intrusion called batholith B; its irregular outline suggests it is an igneous granitic intrusion emplaced as magma into the gneiss. Since batholith B cuts both the gneiss and fault A, batholith B is younger than the other two rock formations.
Next, the gneissfault A, and batholith B were eroded forming a nonconformity as shown with the wavy line. This unconformity was actually an ancient landscape surface on which sedimentary rock C was subsequently deposited perhaps by a marine transgression. Next, igneous basaltic dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes. This shows that there is a disconformity between sedimentary rocks C and E. The top of dike A narrow igneous intrusion that cuts through existing rock, not along bedding planes.
Fault F cuts across all of the older rocks B, C and E, producing a fault scarpwhich is the low ridge on the upper-left side of the diagram. The final events affecting this area are current erosion processes working on the land surface, rounding off the edge of the fault scarpand producing the modern landscape at the top of the diagram. Relative time allows scientists to tell the Relative dating and stratigraphic principles quiz of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate.
Relative dating principles was how scientists interpreted Earth history until the end of the 19th Century.
Because science advances as technology advances, the discovery of radioactivity in the late s provided scientists with a new scientific tool called radioisotopic dating. Using this new technology, they could as specific time units, in this case years, to mineral grains within a rock. These numerical values are not dependent on comparisons with other rocks such as with relative datingso this dating method is called absolute dating.
There are several types of absolute dating discussed in this section but radioisotopic dating is the most common and therefore is the focus on this section. All elements on the Periodic Table of Elements see Chapter 3 contain isotopes.
An isotope is an atom of an element with a different of neutrons. For example, hydrogen H always has 1 proton in its nucleus the atomicbut the of neutrons can vary among the isotopes 0, 1, 2. Recall that the of neutrons added to the atomic gives the atomic mass. When hydrogen has 1 proton and 0 neutrons it is sometimes called protium 1 Hwhen hydrogen has 1 proton and 1 neutron it is called deuterium 2 Hand when hydrogen has 1 proton and 2 neutrons it is called tritium 3 H.
Many elements have both stable and unstable isotopes. For the hydrogen example, 1 H and 2 H are stable, but 3 H is unstable.
Unstable isotopescalled radioactive isotopesspontaneously decay over time releasing subatomic particles or energy in a process called radioactive decay. When this occurs, an unstable isotope becomes a more stable isotope of another element. For example, carbon 14 C decays to nitrogen 14 N. Simulation of half-life. On the left, 4 simulations with only a few atoms. On the right, 4 simulations with many atoms. The radioactive decay of any individual atom is a completely unpredictable and random event.