To understand what you’ll observe as you explore the geology of the Grand Canyon, central Oregon Cascades, and elsewhere using this website; the why, the how, and the when of it, a brief introduction to the discipline of geology is in order.  To begin, geology is itself a science devoted to the study of the earth’s physio-chemical systems, and to a geologist the concept of a “system” implies two things: form and process.  That is, geologists are interested not only in describing observable features of the earth system at all scales, but they are interested in understanding the physical, chemical, and sometimes biological processes responsible for their creation.  The earth system is also very large and very old, so by necessity, the study of geology must incorporate space and time, in many cases, lots of it.  That is, many earth processes and the forms they manifest occur over vast regions of the planet, and require a great deal of time to develop, and indeed, many processes are cyclical (consistently repeated), so that no process ever truly ends (or begins), and no form ever lasts indefinitely.

Practicing Geology

The Scientific Method

Geologists apply a logical methodology to the study of the earth.  This “way of knowing” involves certain ordered steps, which when followed to their conclusion, result in a fundamental inference or “truth” that is acceptable to their peers, other geoscientists, as having been “proven beyond reasonable doubt”.  This generality does not imply that scientific inferences are absolute and unchangeable; all scientific concepts are continually subjected to the scrutiny of the scientific method and our understanding of them is modified as necessary for their truthfulness to remain sound.  Figure 1 provides a schematic flow chart describing the process.

Figure 1.  The Scientific Method.

In geological studies, an initial body of related observations are collected and integrated into a hypothesis, a testable statement or model about a geological feature (or features) and the process (or processes) responsible for its (or their) formation.  Further geological observations are systematically obtained in the field and/or laboratory in an attempt to verify and solidify the validity of the hypothesis; eventually, a more thorough understanding leads to the formulation of a theory, its falsification leads to modification of the original hypothesis and more testing.  Theories are those concepts accepted as being truthful by a vast majority of the scientists studying the problem, after having gone through the rigorous “rite of passage” that is the scientific method; certain universally accepted concepts may even become scientific principles or laws.

Descriptions of geologic phenomena and their interpretations are made with qualitative and quantitative data; qualitative data uses descriptive words, sketches, photographs, etc., while quantitative data is numerical (it expresses distance, volume, concentration, density, time, rate of change, etc.).  Geologists use a variety of “tools” to describe observable features and processes such as simple sketches of landforms and rock outcrops, maps and photographic imagery, as well as sophisticated field and laboratory technology like seismic stratigraphy and scanning electron microscopy; but because geology is fundamentally a field science, nothing beats the power of direct observation, such as walking the landscape, sampling an outcrop or streambed, measuring a stratigraphic section or soil profile, etc.  General observations can begin an investigation, but soon, they become more detailed and systematic. 

One important goal of geological research is classification, placing phenomena into categories based on similar characteristics (form), but another is understanding how those phenomena come about (process).  Geology uses physical (morphological) and chemical characteristics to classify a variety of earth’s features: minerals, rocks, geologic structures, and landforms (to name a few).  Geologists also use their observations and classifications to build systems models to explain the relationships between phenomena and to make predictions of future events.  A “model” could be a mathematical formula, a graph, a topographic or geologic map, a diagram, or sequence of evolutionary diagrams. 

      In simple terms, earth’s systems involve the transfer of matter and energy from place to place (even from one system to another).  For example, think of a stream transferring water from its headwaters to its outlet; water that may escape the stream as floodwaters, or pass through an outlet that may empty into a larger stream, a lake, or even the ocean.  The stream’s water and the sediment load it carries would be its “matter,”, while the streamflow (its velocity and turbulence) exhibit the “energy” of the system.  Of course, the end goal of this geological research is to conceptualize how earth systems work, how they operate internally, how they interact with other systems, and how they evolve over time.  Again, using steams as an example; what factors determine its volume and rate of flow, its ability to transport sediment, when, where, and how much it floods, etc., etc.?  How do stream erode the landscape over which they flow?  How are stream systems influenced by climate, by uplift, or by changes in sea level? All of these questions, and many more, have been, or can be, answered through geological investigation.


Uniformitarianism is a fundamental inference in geological investigations that has reached the lofty status of a principle.  In essence, this concept can be summarized as “the present is the key to the past.”  In other words, when a geologist observes a form resulting from a process, it can be surmised that that form has always resulted from that process (or that process always results in that form), and thus, geological features preserved at the earth’s surface or within the earth’s crust can be used to infer past geological events and/or their environments of formation (Figure 2).  This concept could be applied to innumerable active geological processes: tectonics, volcanism, crustal deformation, formation of sedimentary rock, and glaciation to name a few.  These processes have resulted in the present-day configuration of the earth’s crust and landscapes and the multitude of geological features preserved in the crust and on those landscapes.  Uniformitarianism is an accepted concept in geology; however, certain changes to the earth system can occur so rapidly, so catastrophically, and/or so infrequently, such as meteorite impacts and mass extinction events, that a strict use of the concept breaks down.  Certain processes and their resultant forms occurred so long ago that prevailing physical, chemical, and most certainly biological conditions were not the same as today, and in these cases, uniformitarianism also breaks down.  However, these latter problems are really nonissues because most geological processes and their resulting forms are quantifiable; that is, they occur at observable, measurable rates, frequencies, and associations.

Figure 2.  The geological concept of uniformitarianism demonstrated: (A) shows oscillation ripple marks recently formed on a sandy beach in northwest Scotland; (B) offers a similar scene formed long ago, oscillation ripple marks preserved on a slab of billion-year-old Dox Sandstone from the Grand Canyon.

Unraveling Geologic Time

This then begs the dual questions: what is geologically old versus young, and how do geologists determine this?  The answer to the first question is that Earth has gotten older, and its age less approximate, the longer geologists have studied it (we now believe the earth is about 4.6 billion years old); the answer to the second question is the same for any science, qualitatively and quantitatively (or more accurately, by use of relative and absolute dating techniques).  In relative dating, the goal is to unravel a sequence of geological “events” that may include the formation of a rock body, or its alteration (removal by erosion; deformation by faulting, folding, or intrusion of magma; and/or metamorphism by heat and pressure), but the actual age of the event is unknown.  In absolute dating, the objective is to actually date the timing of the event which helps make interpretation of the sequence of events it is related to more accurate.

Relative Dating

Determining the relative age of a geological “event” requires the determination of its proper place or position within a chronological sequence that includes all of the events found at a given locality, even though the actual ages of events are unknown.  The order of events and the relative passage of time can be deduced from several universal stratigraphic principles related to rock bodies and geological structures (formulated by geologists Nicholaus Steno and William Smith):

a) Principle of Original Horizontality – Sedimentary rocks (and more broadly, the volcanic lava flows and pyroclastic deposits often discussed in this guidebook) are initially deposited in subhorizontal layers conforming to the topography they are laid down upon, therefore tilted strata indicate deformation and the passage of time necessary to complete that deformation (Figure 3).

b) Principle of Superposition – In a sequence of undeformed sedimentary and/or volcanic rocks, the oldest rocks are at the base, becoming progressively younger toward the top (Figure 3).

c) Principle of Lateral Continuity – A layer of sedimentary rock, lava flows, and/or pyroclastic material is initially deposited as a broad, continuous sheet, its pattern disrupted only by obstructions (landforms) that occur at higher positions, where it grades laterally into a different type of rock, and/or by subsequent erosion (Figure 4).

d) Principle of Cross-Cutting Relations – A rock body must exist before it can be altered, therefore strata altered by another intruding rock body or disrupting geological structure must be older than that altering event (Figure 5).

e) Principle of Inclusions – A rock body that contains fragments of another rock type must be younger than those fragments (Figure 6).

f) Principle of Faunal Succession – In a sequence of sedimentary rocks, changes in fossil content (known as fossil assemblages) occur systematically upward even though the rock type may not change; the changing fossil content implying the passage of time (Figure 6). This same concept can be applied to plant fossils as well.

Figure 3.  This diagram illustrates the principle of Original Horizontality by showing a sequence of horizontally deposited sedimentary rocks (A) subjected to folding and uplift (B), then planed-off by erosion (C), only to be buried by the deposition of  a younger sequence of horizontally deposited sedimentary rocks (D); the principle of Superposition is also illustrated by showing a sequence of sedimentary rocks in which the oldest rock unit lies at the base of the sequence (T# denotes time of deposition, lowest number = oldest rock).

Figure 4.  This diagram illustrates the principle of Lateral Continuity by showing a layer of sedimentary rock or volcanic material deposited within a basin as a broad, continuous sheet, only terminating where it pinches out at the edge of a basin or where it grades laterally into a different type of rock (A); however, subsequent erosion may remove part of the unit necessitating a correlation across the gaps (B).

Figure 5.  This diagram demonstrates the principle of cross-cutting relations by showing a body of sedimentary rock layers (A) subjected to two events, intrusion of magma (B), and strike-slip faulting (C); the body of rock had to exist before it could be altered, therefore the strata altered by another intruding rock body or disrupted by faulting must be older than those altering events.

Figure 6.  This diagram illustrates the principle of Faunal Succession by showing that changes in the fossil content of sedimentary rocks occur systematically upward and imply the passage of time; the lowest sedimentary rock layer in the sequence contains inclusions of crystalline basement from below and illustrates the Principle of Inclusions.

Sedimentary (and often volcanic) rocks are initially deposited in subhorizontal layers.  When an interruption in the process of deposition occurs, geologists can infer that some amount of time is missing from the rock record, presumably induced by some geological event or events that affected the area where the rocks where forming.  This event or events could involve simple nondeposition, or involve much more extensive alteration related to intrusion, uplift and deformation, accompanied by a possible marine recession, subaerial exposure, and erosion.  The break in the sequence, and its implied gap in time, is referred to as an unconformity.  The passage of time and degree of alteration indicated by the interruption in the deposition of sediment can be interpreted from a characterization of the type of unconformity (Figure 7):

a) Disconformity – Layers of subhorizontal sedimentary or volcanic rock separated by a nondeposition or erosion surface; implies the shortest time gap and the least alteration. Figure 7a illustrates the formation of a disconformity.

b) Angular Unconformity – Younger sedimentary or volcanic layers deposited horizontally over older strata that was initially deposited horizontally, then tilted by deformation and beveled by erosion; implies an intermediate amount of time and alteration. Figure 7b illustrates the formation of an angular unconformity.

c) Nonconformity – Younger rock layers deposited horizontally over the eroded surface of older intrusive igneous and/or metamorphic “crystalline” basement rock; implies the greatest time gap and the most significant alternation. Figure 7c illustrates the formation of a nonconformity.

Figure 7.  The formation and appearance of geological unconformities: (A) disconformities; (B) angular unconformities; and (C) nonconformities.

Absolute Dating

To measure the absolute passage of time and the precise age of a geological event, geologists must have a process that occurs at a constant rate and a means of keeping a cumulative record of that process. All elements found in minerals (minerals make up rocks) have isotopes, atoms with a variable number of neutrons in the nucleus.  Some of these isotopes are unstable under the conditions of pressure and temperature where they occur in surface soil or regolith or within the earth’s crust.  With passage of time the nuclei of such isotopes break down spontaneously, emitting subatomic particles and energy as radioactive decay, and becoming altered into new isotopes, even the isotopes of new elements.

All radioactive isotopes decay at specific rates called the half-life of the element.  Half-life is the time it takes for half of the original volume (weight %) of the unstable, radioactive isotope (called the parent isotope) to decompose to a more stable isotope (called the daughter isotope).  The decay period remains the same for each iteration of volume change (weight % change).  The volume change can be measured if the original volume of the parent and daughter isotopes can be assumed with accuracy.  Half-life for relatively large samples (it doesn’t take much when considering the size of atoms) is constant for a given element’s isotope and is only affected by geologic processes that subject the original material containing the parent isotopes to enough heat and pressure to result in the chemical alteration of the original material, thereby resetting the radioactive “clock”.  Thus, barring chemical alteration, the longer a suitable geological material containing radioactive isotopes exists (since its formation), the less parent isotopes and the more daughter isotopes there will be (that is, the ratio of parent to daughter isotopes will decrease).  This process is described in Figure 8.

Figure 8.  A graphical illustration of a typical decay curve for a radioactive isotope; the shape of the curve is determined by the isotope’s half-life, or the time it takes for half of the original volume (weight %) of the unstable, radioactive isotope (called the parent isotope) to decompose to a more stable isotope (called the daughter isotope).

Many radiometric dating techniques exist for measuring absolute time, each is applicable to dating materials of varying age depending on the half-life of the radioactive isotope contained in the sample and used in the analysis.  In general, parent isotopes with long half-lives can be used to date older geological events, but not younger events; and parent isotopes with short half-lives can be used to date younger geological events, but not older events.  This is because the longer the half-life of the isotope, the less the amount of parent isotope has decayed to daughter, and the shorter the half-life, the greater the amount of parent isotope has decayed to daughter.  In the former case, the less time that has passed, the more difficult it is to measure the change in the ratio of parent to daughter.  In the latter case, the longer the time that has passed, the more difficult it is to measure any parent isotope remaining at all.

Two absolute dating methods have been routinely applied to determine the age of various geological materials (and the events that they represent).  There are many others, but these examples will suffice to illustrate the technique.  The Carbon-14 (14C) dating technique involves the radioactive decay of the unstable 14C isotope into stable Nitrogen-14 (14N).  Since carbon is a common constituent of all living things, plants and animals accumulate a small percentage of 14C over their lifetimes.  When the organism dies, it ceases to accumulate carbon, and the 14C decays to 14N at a fixed rate known as its half-life (about 5730 years for 14C).  The relatively rapid decay rate of 14C and the miniscule amount of the isotope present (about 98.8% of the carbon stored in an organism is in its stable form, Carbon-12 or 12C), limits the usefulness of this method to dating events less than about 75,000 years in age to as little as a few hundred years.  Determining the ratio of 14C to 14N reveals the number of half-lives that have passed since the organism died.  For example, assume that a tree was buried in sediments by the advance of a glacier; long after the glacier has retreated, the tree is recovered from a streambank cut in the glacial sediments and subjected to 14C analysis.  A sample from the tree’s bark is determined to have an average ratio of 14C to 14N that indicates only 12.5% of the parent (14C) remains, and thus, three half-lives have passed since death and burial of the tree (after one half-life the ratio would be 50%, after two it would be 25%, after three it would be 12.5%, and so on……).  The passage of three half-lives indicates that the glacial advance occurred roughly 17,190 years ago (3 x 5730 years).

The Potassium-40 (40K) to Argon-40 (40Ar) radiometric dating technique can be readily applied to materials that contain minerals such as orthoclase feldspar and mica; fortunately, these minerals are abundant in lava flows and pyroclastic deposits which typically have large aerial distributions when they form.  40K simultaneously decays to 40Ar and Calcium-40 (40Ca), but this geological dating method only monitors the change to 40Ar because 40Ca is a stable isotope found abundantly in all potassium-bearing minerals even before the decay process begins (and therefore, it would be considerably more difficult to determine how much 40Ca resulted from the decay of 40K).  40K has a much longer half-life than 14C (1.25 billion years in fact!) and is more abundant in volcanic materials, thus, this method can be used to date geological events as young as about 5,000 years and as old as many billions of years.  Let’s assume that a volcano erupted at some time in the past and filled an old stream valley that had been previously carved into the landscape with a succession of lava flows.  Samples of rock containing the appropriate minerals are collected from the basal lava flow, and subjected to the 40K to 40Ar dating technique.  The mineral samples are determined to have an average ratio of 40K to 40Ar that indicates 99.994% of the parent (40K) remains, and thus, only 0.00006 half-lives have passed since the basal lava flow was emplaced.  This age indicates that the eruptive event that produced the succession of lava flows began about 75,000 years ago (0.00006 x 1.25 billion years).  It also suggests that carving of the stream valley occurred at least that many years ago, a minimum age for this erosive event.  

The age of a geological event is therefore estimated by some combination of the following: 1) material contained within the rock or sediment of interest is dated and this date is used to infer the age of the rock or sediment (and the event that produced it); 2) material contained within a rock or sediment underlying or overlying the feature of interest is dated and used to infer a maximum or minimum age, respectively, for that feature (less precise than #1); 3) material contained within a rock or sediment altered by some event can be dated and used to infer a minimum age for the altering event (the material had to exist to become altered); and/or 4) material contained within a rock or sediment underlying or overlying another unit altered by an event can be dated to provide a maximum or minimum age for that altering event (less precise than #3).

Using these stratigraphic “tools” of relative and absolute dating, geologists have been able to correlate sedimentary and volcanic rock units in isolated locations, often over vast distances.  One such correlative tool uniquely suited to the dating and correlation of volcanic and sedimentary rocks is the use of tephra (volcanic ash) units.  Each tephra unit has a unique chemical signature (no two volcanic ashes are exactly alike), and so changes in tephra upward within a volcanic or sedimentary sequence, although not systematic, also indicate the passage of time.  When a tephra unit is found at a given location, its chemistry can be determined and tied in to known tephras for that area.  Applying Lateral Continuity, when enough locations have been determined, the aerial distribution and fallout pattern of that tephra unit can be mapped and its eruptive source revealed.  Once the age of the tephra has been determined radiometrically, it can be tied into the overall regional tephra chronology, and the age of other rocks or sediments associated with it can also be inferred.   

The ultimate result of all this age-dating via relative and absolute methods has been the correlation of geological materials and events from one location to another and the construction of a global geological time scale (Figure 9).  The rock bodies and deformational events contained within the earth’s crust have been dated, correlated, and tied to specific time intervals: eras, periods, and epochs.  The principles of relative dating might be applied to the differentiation of the sequence of events represented by a series of interlayered, overlapping lava flows, pyroclastic deposits, sediments, or sedimentary rocks.  The actual age of these geologic units could be determined by a combination of 14C and 40K-40Ar dating in order to further define the age relationships of these materials.

Figure 9.  The globally standardized Geological Time Scale.