Etna presents us not merely with an image of the power of subterranean heat, but a record also of the vast period of time during which that power has been exerted. A majestic mountain has been produced by volcanic action, yet the time of which the volcanic forms the register, however vast, is found by the geologist to be of inconsiderable amount, even in the modem annals of the earth's history. In like manner, the Falls of Niagara teach us not merely to appreciate the power of moving water, but furnish us at the same time with data for estimating the enormous lapse of ages during which that force has operated. A deep and long ravine has been excavated, and the river has required ages to accomplish the task, yet the same region affords evidence that the sum of these ages is as nothing, and as the work of yesterday, when compared to the antecedent periods, of which there are monuments in the same district. -- Sir Charles Lyell, 1845. Travels in North America, Vol. 1, pp. 28-29.

BIG QUESTION: How do we determine the age of fossils?

Two different aspects of time to consider:

• Relative Time: sequence of events without consideration of amount of time
• Numerical Time: (sometimes called "absolute time"), dates or durations of events in terms of seconds, years, millions of years, etc.
• "The Wright Brothers flight at Kitty Hawk came after the Signing of the Declaration of Independence, but before the Apollo 11 moon landing" is a statement of relative time.
• "The Signing of the Declaration was in 1776, the flight at Kitty Hawk was in 1903, and the Apollo 11 landing was in 1969"

Relative time was determined LONG before absolute time.

• Principle of Original Horizontality: because strata are deposited under gravity, they form horizontal layers. If the strata are no longer horizontal, something has disturbed the sediments AFTER they became rocks.
• Principle of Superposition: unless they have been disturbed, the strata at the bottom of a stack were deposited first, the ones on top of that are next oldest, and so on, with the youngest strata being the ones on top.
• Principle of Lateral Continuity: sediment extends laterally in all directions until it thins and pinches out or terminates against the edge of a depositional basin.

As Steno and others mapped out strata, they found that sometimes there were types of breaks (discontinuities) in the layers. These are called unconformities, and represent gaps in the rock record (periods of erosion and/or non-deposition). James Hutton studied these and recognized that they represented aspects of relative time.

From unconformities, Hutton added additional Principles of Stratigraphy:

• Principle of Cross-cutting Relationships: any structure (fold, fault, weathering surface, igneous rock intrusion, etc.) which cuts across or otherwise deforms strata is necessarily younger than the rocks and structures it cuts across or deforms.
• Principle of Inclusions: any rock fragments included as sediment or xenoliths in a unit are from an older rock unit than the one in which they are included (really, a special case of cross-cutting)

(Here is a great blogpost about Siccar Point and Hutton's discoveries.)

Use these principles to figure out time sequence in any particular section of rock. BUT, how to extrapolate the sequence at one section with the sequence at another? That is, how could one correlate between locations?

Recognizing and defining formations is one of the main tasks of the discipline of lihthostratigraphy. Formations are given formal names (e.g., the Morrison Formation, the Hell Creek Formation, the Solnhofen Limestone, etc.). Sometimes groups of formations which lie directly on top of or next to each other are catalogued together as formal Groups, and sometimes groups which lie directly on top of or next to each other are placed into formal Supergroups. In the other direction, formations may be subdivided into members and beds.

Mapping out formations, groups, supergroups, members, and beds, geologists could connect sequences of rocks across regions. By the principle of lateral continuity, the formation will extend out to the edge of the depositional basin (or at least as far as that set of environmental conditions extend.) This allows for regional correlations, but what about across continents and oceans?

William "Strata" Smith, creating first geologic maps of southern England (and expanded out to include the Continent) observed that the pattern of fossils through the strata was consistent from location to location. Developed this into a new stratigraphic principle:

• Principle of Fossil Succession: there is a unique, non-repeating pattern (history) of fossils through stratigraphic time.
  • All rocks containing fossils of the same species were deposited during the duration of that species on Earth.

In order to be an index (or guide) fossil, the organism used must have certain desirable features:

• Have been VERY common, so chances of individuals being buried is good
• Have hard parts, so chances of fossilization are good
• Have a wide geographic range, so that correlation over wide region is possible
• Lived in (or could be deposited in) different environments, so can be found in different formations
• Have some distinctive features, so it can be recognized from closely related forms
• Have a short geological duration (a few million years at most), so finding a fossil of the species in a rock means it had to be deposited in those few million years

The method of using index fossils to correlate rocks is called biostratigraphy. Here is an excellent summary of biostratigraphic correlation.

In combination, the principles of stratigraphy were useful for determining a global relative time scale, but questions of numerical time were still unresolved.

Geologic Column divided into a series of units: from largest to smallest eons, eras, periods, epochs, ages (or Stages). No initial understanding of the scale of numerical time for these different units.

Animal and plant fossils are mostly restricted to the last (most recent) Phanerozoic Eon ("visible life eon"). The Phanerozoic Eon is comprised of three Eras:

• The Paleozoic Era ("ancient animal life")
• The Mesozoic Era ("middle animal life"): the "Age of Dinosaurs"
• The Cenozoic Era ("recent animal life"): the "Age of Mammals". We are still in the Cenozoic Era.

Expanding out the Phanerozoic, we can see the different Periods within these three Eras:

We are currently in the Quaternary Period of the Cenozoic Era, and the Holocene Epoch of the Quaternary Period.

No one region has a continuous sequence of time. Any given location has likely had periods of non-deposition or erosion, which would leave gaps in the geological and fossil record at any given spot.

An interactive project on geologic time, for those who want to explore in more detail.

Various early scientific attempts to determine the age of the Earth,included such approaches as:

• Compared ocean salinity to known amount of salt in rivers; assuming fresh water proto-ocean, how long to salinate the seas? (tens of million years)
• Calculated current rates of sedimentation, count total thickness of sedimentary rocks, determine total age (a few billion years): standard for Geology at beginning of 20th Century
• Calculate cooling rates of molten iron, determined known surface temperature of Earth & its thermal gradients, assume no additional source of energy (90 million years or so maximum, possibly less): standard for Physics at beginning of 20th Century, most famously argued by William Thomson, Lord Kelvin

Additionally, attempts were made to date particular rocks (i.e., by comparing the sedimentation-rate based time scale or the cooling-based time scale, and estimating what percentage far back a given rock came from), or the duration over which deposits were generated (for environments with annual deposition, like some lakes and glaciers, this was a matter of counting.)

How to reconcile the sedimentation-based scale (needing 100s of millions of years) with the cooling-based scale (limited to <90 million years?. The discovery of radioactive decay at the dawn of the 20th Century gave the key:

• A way around the problem of Lord Kelvin's short physical estimate of Earth's age, because a new natural heat source for keeping Earth's interior molten was now known
• Also, radioactive decay itself forms a "clock" usable for determining age of rocks.

Radiometric Dating: the single most important method of determining numerical rock ages:

• Radioactive materials decay at predictable, experimentally determined rate, known as the half-life
  • Atoms decay from one form (radioactive parent) to another (daughter product), releasing energy and particles
  • After one half-life has passed, half the original parents in the material will have decayed into the daughter product; after two half-lives, only one-quarter of the parent material remains, with three quarters daughter product; after three half-lives, 1/8 to 7/8; after four half-lives, 1/16 to 15/16; and so on
• Can thus date rocks:
  • Compare the ratio of parent product to daughter product
  • Radiometric dates will only be effective for igneous rocks, since those are the ones that form by cooling and locking atoms into place
    • In sedimentary rocks, can date the individual grains of sediment: tells you age of source rock, but not deposition
    • In metamorphic rock, recrystallization redistributes atoms and obscures signal
  • Since only igneous can best be dated radiometrically, use principles of superposition, cross-cutting relationships, etc., to determine ages of sedimentary rocks (and their fossils) relative to numerical dates, and tie dates into Geologic Column by correlating with index fossils.

Traditional radiometric dating needs some special conditions, however:

• Daughter product should only be produced naturally by decay from the parent
• Neither parent nor daughter should be able to leave the sample naturally
• Only useful for determining ages of formation of mineral grains (and thus really best for igneous rocks)

When possible, radiometric dates of different isotopes with different decay rates are calculated for same sample. If these converge, good support for that age.

In order to get around the issue the requirement of zero initial daughter product, mid-20th Century geochemists developed the actual version of radiometric dating that is currently used by geologists: the isochron method. We won't go into great detail for this in this particular course, but in brief it requires sampling not just the parent (P) and daughter (D) products of a radioactive decay series, but also a stable (non-radioactively-generated) isotope of the same element as D (D_i). By comparing the ratios D/D_i vs. P/D_i, different minerals in the same rock will plot along a particular line (the isochron), the slope of which is scaled to the number of half-lives the rock has gone through. Scatter of plots around the line is a measure of how much contamination or loss there has been of materials, and thus the degree of confidence we have in the measure.

Using the radiometric methods of dating, geologists have estimated the ages of the various boundaries of the Geologic Timescale.

Living things take in ¹⁴C during life, but not when they die: from that point on, it will simply decay away.

Of special note with radiocarbon: studies showed that the amount of ¹⁴C in the atmosphere (and thus the amount that organisms take up) has varied over time, requiring a calibration curve to convert from the amount of radiocarbon to the number of calendar years.

• Eustatic (global) sea level changes: Matching up the rise and fall of sea level worldwide based on the patterns of distribution of coastal deposits (and erosion). This most accurate in environments (shorelines, mainly) where sea-level changes are recorded.
• Marker beds: One-time events (volcanic eruptions, asteroid impacts, etc.) may send particular types of material over wide region (even globally). These record EXTREMELY short periods of time: essentially instantaneous!
• Stable Isotope Stratigraphy: Some stable isotopes of various elements (carbon, oxygen, strontium) vary over time relative to each other due to a number of factors (productivity, glaciers, temperature, erosion, etc.). When examined against a time scale, these form irregular curves. Individual samples or series of samples can be compared to the known-curve of these isotopes to see where they fall.
• Astrochronology: a relatively new field. Looks at the varying thickness of strata within sedimentary packages to calibrate their changing thicknesses to some astronomically-controlled cycles (such as tidal, daily, monthly, annual, or longer term cyclicity.) Not good for calibrating against a time scale as such, nor for correlation, but very useful for determining the duration in numerical time of packages of sedimentary rock.
• Magnetostratigraphy
  • The magnetic (but NOT the geographic) poles have "flip-flopped" throughout geologic time, so that sometimes a magnet's north pole points towards geographic North, and sometimes toward geographic South.
  • Magnetic polarity can be recovered by some iron-bearing rocks (sedimentary and igneous).
  • Because based on the Earth's magnetic field, the changes occur everywhere on the planet at the same time.
  • Can use the particular "bar code"-like pattern of flip-flops to match any section to known global pattern (based on continuous record of lava on ocean floor)
  • Only really good back to Jurassic: before that, lack the continuous magnetic record of the seafloor
  • Here is the Phanerozoic magnetochronological chart as of February 2010.

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Last modified: 23 January 2020