Geology Cafe

Introduction to Geology

Chapter 2 - Basic Geologic Principles

The science of geology is founded on basic principles that are useful for making observations about the world around us. This chapter presents a mix of information that is essential (fundamental) to all following chapters. This chapter is an introduction to rocks and minerals, and the rock cycle.

Basic chemistry is important to all sciences, especially geology!
Everything around us is made of chemical compounds that have testable and identifying characteristics, allowing them to be classified, and their age determined. This also applies to rocks, minerals, and derivative materials (such as sediments and soil). The chemical composition of Earth's crust has similarities with other stony planets, with silicate-rich rocks being dominant in most locations on the surface.

In addition, basic geologic principles can be applied to resolving the order of events leading to the formation of rocks and landscape features. This section presents many basic concepts that are universal to all physical sciences.
Click on thumbnail images for a larger view.
Angular unconformity exposed along a beach cliff in Encinitas, California
Fig. 2-1. Layered rocks in a sea cliff in Encinitas, CA with an angular unconformity.
1. What are "rocks" and "minerals" - explain the differences.
2. Describe essential concepts of chemistry related to earth materials.
3. What is the chemical and mineral composition of the Earth's crust?

4. List some common silicate and nonsilicate minerals.
5. Describe and illustrate the "rock cycle"
as it relates to processes and products.
6. Describe basic geologic principles for interpreting landscape forming processes.
7. How are the ages of rocks determined?
8. Cross Sections - interpretations of vertical views of geologic features below the surface.
Keywords and Essential Concepts
1. Define what rocks and minerals are, and their significance.

First... What is a mineral?

A mineral is a naturally occurring, inorganic (never living) solid with a definite internal arrangement of atoms (crystal structure) and a chemical formula that only varies over a limited range that does not alter the crystal structure. On Earth, more than 4,000 minerals have been identified, however, of those fewer than 2 dozen are common minerals in Earth's physical environment (Figure 1-1 shows common rock-forming minerals). In contrast, minerals considered "gems" are, mostly, exceedingly rare.

What is the difference between a rock and a mineral?

A rock is a relatively hard, naturally formed mineral or petrified matter; a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.

Stone is another common term used to describe rock.

Rocks consist of one or more minerals. Figure 2-2 shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions.

The mineral composition of a rock reflects the physical environment and geologic history where a rock formed. Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space. Most of the rocks we see on the surface of the planet formed by processes that happened long ago, but we can see these processes actively taking place in many places. Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland. However, most rocks we see around us form very slowly in settings that are not visible on the land surface. Slow processes creating rocks can be inferred by observing reefs growing in the oceans, or sediments being carried by flowing water in streams or moved by waves crashing on beaches. We can see sediments being deposited, but we cannot see them turning into stone because the process may take thousand or even millions of years.

The mineral composition of a rock reflects the physical environment and geologic history where a rock formed.
Click on thumbnail images for a larger view.
Rock Forming Minerals
Fig. 2-1. Common rock-forming minerals are the most abundant minerals found on our planet Earth.
Minerals forming rocks
Fig. 2-2. Combinations of common minerals occur in different kinds of rocks. The kind of rock depends on the geologic setting where they form: igneous, sedimentary, or metamorphic.

"Every Rock Has A Story"

Rocks are composed of chemical compounds naturally occurring in nature. Rocks are composed of particles ranging from microscopic grains to full sized crystals and crystal grains of different kinds of minerals, and containing many different identifiable physical characteristics.

It is conceptually important that each rock has an origin in concepts of place, time, and physical and chemical conditions. Once rocks form, they are subject to change. These changes may be rapid (such as a volcanic explosion) or gradual, taking place over millions or billions of years, and involving movement over great distances, both at the surface or to deep within the Earth's crust below us. Trying to explain the what, how, and when of a rock's journey is fundamental to explaining why rocks are significant to resolving questions about our Earth's history and conditions within the physical environments where we live.

Gypsum crystals from Jewel Cave, South Dakota serpentinite
Fig. 2-4. Serpentinite, the "State Rock" of California, is a rock composed of serpentine minerals (of which there are many varieties).
Fig. 2-3. Gypsum crystals from a cavern wall in Jewel Cave, South Dakota
2. Describe essential concepts of chemistry related to earth materials.

Essential concepts of chemistry related to earth materials

Basic concepts of chemistry are essential to understanding the physical and chemical properties of earth materials (minerals, rocks, organic matter, etc.). The chemical characteristics of earth materials are reflect the environments how and where they are formed, they also determine their potential fate when exposed to chemical changes. For instance, rocks and minerals formed deep underground may not be stable in the surface environment where they are exposed to water, air, temperature changes, and other physical and chemical conditions.

All matter is made up of atoms, and atoms are made up of atomic particles (electrons, protons, and neutrons - see Figure 2-5). A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen. The Periodic Table is a list of 108 known elements arrange by atomic number (see Figure 2-6). Of these, 92 are naturally occurring (prior to development of artificial nuclear research and development). The lightest element, hydrogen, has one proton, whereas the heaviest naturally occurring element, uranium, has 92 protons.

Many elements have one or more isotopes. Isotopes are each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. Some isotopes are not stable and ultimately break down or change in other elements. I this case, the isotope is considered a radioactive form of an element. Many elements have both stable and radioactive isotopes. For example, the element carbon has 3 isotopes: 12C and 13C are stable, whereas 14C is unstable and will undergo radioactive decay. All there isotopes have 6 protons, but have 6, 7, and 8 neutrons, respectively.

In "nature,"here are at least 254 stable isotopes that have never been observed to decay. Of these 50 are radionuclides (unstable isotopes that undergo radioactive decay). They occur among the 80 different elements that have one or more stable nuclides. With the invention of nuclear weapons, and the numerous nuclear bomb test through the 1950s to the present, there are now many more radionuclides loose in the environment.

A molecule is a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction

chemical compound
—a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds.

All minerals are chemical compounds, but by comparison relatively few compounds are naturally occurring minerals!

mixture—solid, liquid, or gas composed of two or more substances, but each keeps its original properties. Note that earth materials (rocks and sediments), magma (molten rock), seawater in oceans, and the atmosphere are all mixtures.

An atom of lithium is composed of a nucleus with 3 protons and several nuetron, and surrounded by a cloud of 3 spinning electrons
Fig. 2-5. Structure of an atom: this example is the element lithium composed of a nucleus of 3 protons, 4 neutrons, and an outer shell of 3 electrons spinning around the nucleus.
Periodic Table
Fig. 2-6. Periodic Table of the Elements
(Lawrence Berkeley National Lab)

Chemical Bonds

Molecular compounds are held together on an atomic level by chemical bonds. Three types of chemical bonds include ionic bonds, metallic bonds, and covalent bonds. The types of chemical bond influence the physical properties of the molecular compounds they form.

Molecular compounds held together by ionic bonds are salts. Salts readily precipitate from and dissolve in water. Natural salts like halite (NaCl) and gypsum (CaSO4) are soft minerals (not suitable for gems because they scratch or fracture easily, and can dissolve in water; see Figures 2-7 and 2-8).

Metals are held together by metallic bonds. Compounds with metallic bonds transmit electricity. Metalloids are intermediate between those of metals and solid nonmetals. Although most elements are metals (all those on the left and center parts of the Periodic Table), only a few elements occur naturally in metallic form including gold, platinum, copper, iron, and mercury (in liquid form). Some minerals are metalloid compounds including pyrite (FeS2), magnetite(Fe3O4), and galena (PbS)(see Figure 2-9).

Molecular compounds held together by covalent bonds are non-metallic compounds. These materials can form crystal complexes and do not transmit electricity and tend to be durable compounds. Most gems are non-metallic compounds. The mineral quartz (SiO2) is a non-metallic crystalline compound (see Figure 2-10).

More about Minerals is discussed in the next chapter (Chapter 3).
Salt dissolves in and precipitates from water Salt deposits in Death Valley
Fig. 2-7. Salt crystals are held together by ionic bonds. Salt compounds dissolve in and precipitate from water. Fig. 2-8. This view shows salt crystals precipitating on a dry lakebed in Death Valley, California.
Metals (native copper and gold), magnetite and pyrite Quartz crystal
Fig. 2-9. Metallic bonds occur in metallic minerals (like native copper and gold) and metalloid minerals (like magnetite and pyrite). Fig. 2-10. Most minerals are non-metallic crystalline compounds held together by covalent bonds (and will not transmit electricity).

3. What is the chemical and mineral composition of the Earth's crust?

Composition of the crust Rock samples collected from around the world show that the chemical composition of the Earth's crust is not uniform, but certain elements are much more abundant than others. Silicon and oxygen are the two most abundant elements in the crust. Therefore, silicate minerals (compounds that contain some silicon and oxygen) are most abundant.

Currently there are about 4,000 known minerals of different composition and mineral arrangement. However, slightly more than a dozen are considered "common minerals."
Fig. 2-11. Composition of the crust.

General composition of the Earth's crust.

The general terms "felsic, mafic, and ultramafic" are sometimes used to describe masses of silicate rocks. In general rocks found on continents are mostly felsic in composition, whereas rocks within the crust beneath ocean basins is mostly mafic to ultramafic in composition. These general terms are important in the discussions about Plate Tectonics Theory in Chapter 5.

felsic—minerals of silica and aluminum-rich composition, and the rocks that form from them. Felsic materials are typically less dense than mafic materials. Felsic minerals include quartz, feldspars, muscovite, and clays. Rocks include granite, rhyolite, sandstone, quartzite.

mafic— A mnemonic term combining and “Ma” (for magnesium) and “Fe” (for ferric iron). The term is used to describe dark-colored igneous minerals rich in iron and magnesium, as well as the rocks that bear those minerals. Basalt is a rock of mafic composition.

ultramafic—a rock composed chiefly of mafic minerals (rich in iron and magnesium), and less than about 45 percent silica, such as the minerals olivine, pyroxene, or amphibole. Peridotite, pyroxenite and serpentinite are ultramafic rocks.

Silicate minerals are the dominant group of minerals that make up the rocky crusts of the Earth, Moon, and other stony planets (Mercury, Venus, Mars, and many other moons and asteroids within the Solar System. Silicate minerals chemically consist of compounds that contain the geometric arrangement of silicon-oxide tetrahedrons contained within simple to complex crystalline structures. Other elements combine with the silicon-oxide to form many different minerals with unique physical properties.

Most of the rock-forming minerals on earth and other stony planets are silicate minerals. The earth's crust and mantle are dominantly composed of silicate minerals, whereas the core is most likely composed of solid metallic minerals (mostly iron, nickel, and traces of other elements). Current thought is the solid inner core of the planet is composed of the same material found in iron meteorites.
granite basalt
Fig. 2-12. Granite is an igneous rock made up of light-colored felsic minerals, mostly quartz and varieties of feldspar minerals. Granite is found in abundance in the core of contiental regions. Fig. 2-13. Basalt is a dark colored igneous rock composed of mafic or ultramafic minerals. Basalt is the dominant rock found under ocean basins and exposed in places like Hawaii.

5. Describe and illustrate the "rock cycle" as it relates to processes and products.

The "Rock Cycle" (revisited)

Charles Lyell (1797-1875) compiled the first geology textbook entitled "Principles of Geology" in which he promoted concepts of the "rock cycle." The rock cycle illustrates the series of events in which a rock of one type is converted to one or more other types and then back to the original type. The "rock cycle" is a graphic and conceptual model to illustrate common rocks and earth materials and the processes that form or change them.

There are 4 classes of rocks and earth materials: igneous rocks, sediments (which are not rocks), sedimentary rocks, and metamorphic rocks.

Igneous rocks are rocks formed from the cooling and crystallization of molten materials. The word igneous applies to any rock or mineral that solidified from molten or partly molten material (referring to magma underground or lava on the surface). The word igneous also applies to the processes related to the formation of such rocks. Igneous rocks includes intrusive rocks (rocks that cooled below the surface) and volcanic rocks formed on the Earth's surface by volcanism. Igneous rocks also form from melting associated with extraterrestrial impacts. Examples of igneous rocks include granite, gabbro, and basalt. Rocks of igneous origin are discussed in Chapter 6.

Sediments are solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice.. Examples of sediment include gravel, sand, silt, clay, mud (mix of sand, silt, and clay), soil, lime mud, and ooze. Sediments are not rocks, but they may become rocks through heating, compaction, and cementation. Sediments are discussed in Chapter 7.

Sedimentary rocks
are rocks that have formed over time through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice ( glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Examples of sedimentary rocks include shale, sandstone, conglomerate, limestone, and chert. Sedimentary rocks are discussed in Chapter 8.

Metamorphic rocks are rocks that was once one form of rock but has changed to another under the influence of heat, pressure, or fluids without passing through a liquid phase (melting). Examples of metamorphic rocks include slate, schist, gneiss, marble, quartzite, and serpentinite. Metamorphic rocks are discussed in Chapter 9.

These rock cycle diagrams illustrate how earth materials form and change over time. Both products (rocks and sediments) and processes (such as melting, cooling, erosion, and deposition) are illustrated. The passage of geologic time is an essential component, although some processes are much faster than others. Note that all these types of processes are taking place simultaneously, but at different locations on and within the planet.
The Rock Cycle
Fig 2-14. The Rock Cycle: processes are in purple; products are in black and blue.
Rock Cycle Illustrated
Fig. 2-15. Rock Cycle Illustrated. This version of the rock cycle is the same as above, but showing more detail in graphic form. It is good to compare the two diagrams.
Rocks and Geology of the San Francisco Bay Region is a 64 page report (Adobe .pdf file) that contains basic information about regional geology and the Earth materials that are found in the Central Coast region of California. This guide provides a general tour of the "rock cycle" as it applies to a this region where nearly all rock types are exposed in close proximity.

igneous rocks sediments sedimentary rocks metamorphic rocks  
Fig. 2-16. Igneous rocks Fig. 2-17. Sediments Fig. 2-18. Sedimentary rocks Fig. 2-19. Metamorphic rocks

Review of Some Basic "Rock" Concepts

igneous rocks Fig. 2-20. Igneous rocks from the cooling of molten material (magma, lava) and are rich in silicate minerals.

Molten material derived from deep in the mantle is typically enriched in iron- and magnesium-rich silicate minerals (called mafic or ultramafic). In contrast, magma and lava associated with continental crustal regions are enriched in felsic silicate minerals.
water weathering and erosion Fig. 2-21. Weathering of rocks and minerals: water (H2O) occurs in rocks within the earth and is a primary chemical agent on the earth surface. Water is called the "universal solvent," dissolving and transporting material in solution, altering the chemical composition of mineral, and transporting sediment (erosion and deposition). Not all rocks and minerals behave similarly when subjected to weathering and erosion. Hard and durable minerals like quartz tend to resist weathering and erosion, and therefore can be carried long distances carried as sediments by flowing water or wind.
sediments Fig. 2-22. Durability of quartz: because quartz is an extremely durable mineral (with a Mohs hardness of 7.0) and because is is an extremely abundant mineral in the Earth's crust, quartz is concentrated by erosional processes in the form of "sand" found on beaches and in desert dune fields.
sedimentary rocks Fig. 2-23. Minerals in sedimentary rocks: most sedimentary rocks are enriched in the minerals quartz, calcite, and some clay minerals. Minerals with high hardness and low solubility are transported by erosion and deposited in sedimentary basins. Typically soft minerals that are highly soluble are dissolved and carried by surface and groundwater where most contributes to the saltiness of seawater. However, dissolved components in water can precipitate to form mineral cements, including like calcite (CaCO3), iron-rich minerals (hematite and limonite), and silica (quartz).
metamorphic rocks Fig. 2-24. Metamorphic processes cause changes in the mineral composition in rocks: changes in heat, pressure, and exposure to fluids, over time, will change the mineral composition of earth materials, such as converting sediments into sedimentary rocks, changing sedimentary and igneous rocks into metamorphic rocks. Conversely, exposing rocks to fluids—at or near the surface—degrade rocks of many kind into sediments.

6. Describe basic geologic principles for interpreting landscape forming processes.

What is bedrock?

Bedrock is the solid rock the occurs beneath soil or alluvium that coved the surface of the land in most locations. In some places the bedrock is exposed as rocky outcrops scattered across the landscape, particularly in mountainous areas or along stream canyons.

Why do landscapes in different areas have unique characteristics? Its all about the bedrock and its history!
Landscapes change from on region to the next because of the composition and character of bedrock changes from place to place.

Geologists unravel the geologic history by studying the geometry of outcrops in an area. Subtle characteristics of undisturbed landscapes often reveal the location of faults, ore deposits, and other features of geologic interest.

Layered versus non-layered rocks

Large bodies of rocks typically display recognizable characteristics, most notable are characteristics of rock "layers" called strata. Strata are layered rocks are typically formed when an accumulation of rock material is deposited is a setting, then as time passes more layers accumulate on top. If cut and exposed by erosion, the layers appear as a series of "beds." Geologists who study layers in rocks will assign them names and will described them by their composition and their ages (if possible). A rock formation is a rock unit that is distinctive enough in appearance that a geologist can distinguish it from other surrounding rock layers. A named rock formation must also be thick enough and extensive enough to plot on a geologic map. Some rocks are layered (or stratified), others are not. Sedimentary rocks and volcanic deposits (lava flows and air-fall ash deposits are examples of commonly stratified rocks. Examples of rocks that are not stratified are igneous rocks (like granite) that cooled and crystallized deep underground and did not develop layers. Also rocks that have experienced extensive metamorphism and remelting may no longer preserve "stratification." (As always, in geology, and science in general, there are exceptions to these rules!)
Layered ("stratified") rocks
Unlayered ("unstratified") rocks
Flat-lying strata at the Del Mar Dog Beach in San Diego County, CA Stacked lava flows in the Grand Coulee, Washington City of Rocks, Idaho Unstratified igneous and metamorphic rocks exposed along the Colorado River in the Inner Gorge of the Grand Canyon, Arizona
Fig. 2-25. Stratified layers of sedimentary rocks exposed along the sea cliff at Del Mar Dog Beach, San Diego County, California Fig. 2-26. Stratified layers of volcanic rocks (stacked layers of lava flows and ash beds), Grand Coulee, Washington Fig. 2-27. Unstratified intrusive igneous rocks exposed by erosion at the City of Rocks National Preserve, Idaho Fig. 2-28. Unstratified igneous and metamorphic rocks exposed along the Colorado River in the Grand Canyon, Arizona

Basic Geologic Principles

James Hutton first proposed several basic geologic principles that were later embellished by Charles Lyell. These basic principles are easily observed in geologic outcrops, but have value for any number of scientific and technical applications beyond geology. Figure 3-13 illustrates the three "laws" that are used in resolving the age of rocks and the order in which they formed or geologic events occurred. The three laws are as follows:

Law of Original Horizontality—this law states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal.

Law of Superposition—this law states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

Law of Cross-Cutting Relationships—this law states that a body of igneous rock (an intrusion), a fault, or other geologic feature must be younger than any rock across which it cuts through.
Basic geologic principles
Fig. 2-29. Basic geologic principles illustrated.

Sedimentary rock formations display features that reveal information about the environments where they formed.

sedimentary facies
—sedimentary deposits that reflect environmental conditions at the time of deposition of sediments. Examples include offshore mud facies, reef facies, beach sand facies, terrestrial facies, etc. Facies reflect the character of a rock expressed by its formation, composition, and fossil content.

Sedimentary facies preserved in rock formation reveal much information about earth history including how climates have changed, the timing of the formation of nearby mountain ranges, and the shifting of shorelines over time.
Sedimentary facies
Fig. 2-30. Sedimentary facies reveal information about past environmental conditions.

Unconformities: 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. An unconformity is a surface between successive strata that represents a missing interval in the geologic record of time, and produced either by an interruption in deposition or by the erosion of depositionally continuous strata followed by renewed deposition.
Several types of boundaries are recognized:
  • nonconformity—an unconformity between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock
  • angular unconformity—an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers
  • disconformity—an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.

  • conformable boundary—an arrangement where layers of sedimentary strata are parallel, but there is little apparent erosion and the boundary between two rock layer surfaces resemble a simple bedding plane.
Types of unconformities
Fig. 2-31. Types of unconformities (boundaries between layered rocks)

Examples of unconformities and conformable boundaries in the Grand Canyon of Arizona
nonconformity in the Grand Canyon disconformities in the Grand Canyon Angular unconformity in the Grand Canyon Conformable contacts in the Grand Canyon
Fig. 2-32. Nonconformity in the Grand Canyon (known as the "Great Unconformity") Fig. 2-33. Disconformities between sedimentary formations in the Grand Canyon Fig. 2-34. Angular unconformity between sedimentary rocks of different ages Fig. 4-35. Conformable or gradational contact between sedimentary layers

How do unconformities form?

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.

Sea level change 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.
Formation of unconformities
Figure 2-36. Unconformities can form by the rise and fall of sea level. Erosion strips away materials exposed to waves and currents. New (younger) material is deposited on the scoured surface.

7. How are the ages of rocks determined?

Geochronology, 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 and absolute dating methods.

relative dating—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.
The Waterpocket fold in Capitol Reef National Park Utah
Fig. 2-37. Rock layers like these in Utah record information about 100 million years of Earth history of the region.

trackways Try Relative Dating (unsorting the visually available clues)

Basic geologic principles are used to interpret the geologic history of the earth. These basic rules have many basic applications to interpreting the order of events from small scale activities like interpreting the order of footprints along a lakeshore, to large scale like interpreting the order of event in the formation of features like mountain ranges or the sequence of events exposed in a region where oil exploration is taking place. To illustrate try interpreting the order of events in the diagram involving animal trackways.

Relative dating of tracks
Fig. 2-38. Relative dating exercise #1
Bear, bird, deer, dog, duck, human. What happened, and in what order?
Fig. 2-39. Relative dating exercise #2
Can you figure out the chronology of events in this nature scene?

absolute dating—general term applied to a range of techniques that provide estimates of the age of objects, materials, or sites in real calendar years either directly or through a process of calibration with material of known age. There are many methods of absolute dating rocks or other ancient materials. The methods of absolute dating used depends on whether suitable sample are available for testing.

One variety of absolute dating methods involves the study of the decay of radioactive isotopes. Commonly referenced studies of absolute dating utilize the radioactive decay of 238U (unstable uranium isotope) to 206PB (stable lead isotope); or 40K (unstable potassium isotope) to 40Ar (stable argon isotope). Note that here are many other absolute dating methods. Perhaps most important is radiocarbon dating which utilizes the decay of 14C (unstable carbon isotope) to 14N (stable nitrogen isotope). Dating of materials that contain naturally-occurring radionuclides (radioactive isotopes) is possible because the rate of decay of the radionuclides are known. The radiation decay "clock" starts the moment a mineral in a rock forms (or for 14C when an organism dies). A "half-life"is when only half of a parent radionuclide remains. The next half-life is when only a quarter of the original parent radionuclide remains, and so on. Age determinations can be determined by comparing the percentage of the radionuclides in a new "fresh" sample with the percentage in the old sample material being tested.

Radiocarbon Dating

14C (isotope carbon -14) is a unstable radioactive isotope (radionuclide). Radiocarbon dating (using ratios of the isotopes of radioactive isotope14C to to stable isotopes 12C and 13C derived from buried or isolated organic or carbonate materials. The "half life" of 14C [unstable isotope carbon-14] is about 5,730 years. Radiocarbon dating has extensively used in archeological investigation and the study of climate change over the last several hundred thousand years, and precision methods now available make radiocarbon dating highly reliable. Radiocarbon dating is highly effective for extracting ages of organic materials (bone, tissues, wood, etc.) that have been isolated by burial and is effective for dating materials materials from ancient human activities going back for many thousands of years.
Absolute dating
Fig. 2-40. Absolute dating methods. Different isotopes are used to study different materials and geologic time ranges.
Radiocarbon Dating method
Fig. 2-41. The science behind the radiocarbon absolute dating method.

8. Cross Sections - interpretations of vertical views of geologic features below the surface.

A geologic cross section is an interpretation of a vertical section through the Earth's surface, most usefully a profile, for which evidence was obtained by geologic and geophysical techniques or from a geologic map.

Cross sections are important tools for relative dating!

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 mapable 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. How most subsurface data derive or imaged through geophysical methods, such as by seismic data (by earthquakes or manmade explosions), by measurements of gravity, magnetism, electrical resistivity, or information derived from wells such as core sample, radiation measurements, or other geophysical methods.
Cross section
Cross section of the South Bay region, California
Fig. 2-42. Cross section of Little Rocky Mountains, Montana Fig. 2-43. Cross section of the South Bay region, California

Geologic Principles Illustrated
Cross section Wind River Basin seismic shop Geophysical cross section
Fig. 2-44. 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 units C, B, A, D, then E. Fig. 2-45. Example of a cross section through the Wind River Mountains, Wind River Basin, and Absaroka Mountains of Wyoming. Can you interpret the chronology of geologic events illustrated in this cross section? Fig. 2-46. Seabed exploration produces cross-sectional seismic profiles, raw data that are converted to cross-section diagrams. Modern systems produce views that three dimensional Fig. 2-47. Geologists study cross sections created by geophysical exploration methods. This is an
example of a seismic profile showing the location of exploratory wells.

Cross Section - Relative Dating Interpretation Exercise
Cross Section Quiz Fig. 2-48. Using the laws of original horizontality, superposition, and cross-cutting relationships interpret the order of the formation of features illustrated in this hypothetical cross section. Create a list in the order that rock units formed or were deposited, in order from oldest to youngest. Note that features show included sedimentary layers and igneous intrusions, some are faults, and some are unconformities (lines representing periods of erosion and non-deposition.

Quiz Questions