Themes > Science > Earth Sciences > Geology > Earth > Histories of the Earth

1. Introduction to Historical Science

Science is "the systematic observation of natural events and conditions in order to discover facts about them and to formulate laws and principles based on these facts." (Academic Press Dictionary of Science and Technology)

(Note that many definitions of "science" are really about one type of science.  E.g., The Penguin Dictionary of Science says science is "the study of the physical universe and its contents by means of reproducible observations, measurements, and experiments to establish, verify, or modify general laws to explain its nature and behavior."  But this definition really only reflects the experimental sciences...)

Experimental versus historical sciences:

Experimental sciences are quantitative, simplifying, and predictive.
Historical sciences are narrative, deal with complex, unpredictable systems.

Historical sciences (like geology and paleontology) are just as rigorous as the experimental sciences; both use the same fundamental assumptions.

The historical sciences are relatively new. Up to the 1600s, the Neoplatonic viewpoint was popular. Neoplatonism held that forms on Earth are universal and unchanging, and corresponding forms can be found in different "kingdoms" on Earth. Hence, a clam in a rock was created when the rock formed; its resemblance to a living clam only shows the harmony of the Created world.

Nicolaus Steno, Danish physician working in Italy, countered this viewpoint with an historical one. His 1669 work "An introduction to a dissertation on a solid body naturally contained within another solid body" showed convincingly that a clam or shark's tooth in a rock was once a living thing, which subsequently became incorporated into the rock.  Steno's work provided geology with two fundamental principles of history:

1. By careful study, the order of events in time can be determined.
2. The causal processes that made an object can be inferred by studying that object.

The historical sciences contribute three important things:

1. The unique perspective of "deep time" - Earth is very old, and much older than humans.
2. A method of studying complex, historically contingent systems.
3. Pedestal-smashing—the restriction of human history and our removal from a central position in history.

 2. Deep Time–James Hutton and his Cyclic Theory of the Earth

"Deep time" -- the idea that the Earth is very old, and much older than humans -- may be the greatest contribution of geology to human knowledge.

James Hutton, a Scottish intellectual, first convincingly proved the Earth is very old, in 1795 book Theory of the Earth.

He started with an 18^th century problem, the Paradox of the Soil. Soil is needed for human life. But soil is produced by the process of erosion, which will eventually destroy the land humans live on. A benevolent Creator would not have a process which we need to live also be the cause of our destruction. Therefore, a restorative force must exist that creates new rocks.

Hutton demonstrated that granites, which make up the bulk of the continents, were igneous rocks, that is, they formed from cooling molten pre-existing rock.   Previously, many thought that all rocks formed all at once early in Earth's history.  But igneous rocks could form at any time.  So continents could be added to over time by melting and uplifting preexisting rocks—a restorative force does exist.

Hutton also demonstrated that unconformities (breaks separating different sets of rock layers) are ancient erosion surfaces. Hence, a set of rocks separated by an unconformity must have formed by deposition of the first layers, uplift and erosion of those layers, then renewed deposition of the second set of layers. This interpretation demonstrates that the restorative force has indeed operated, that new rocks can form to replace those that have eroded away.

The presence of many unconformities in rock sequences also proves that Earth must be millions of years old.  We can observe how slowly erosion and deposition happen today, so the amount of time needed for each one of these multiple cycles of deposition-uplift-erosion is very large.  Clearly, the Earth must be very old.   In this way, Hutton convinced most educated people that Earth was very, very old.

Hutton focused on the cyclic nature of this worldview - his "World Machine". He had no interest in a narrative history of the Earth, with a unique sequence of events through time.

3. Catastrophism and Uniformitarianism—Georges Cuvier and Charles Lyell

Baron Georges Cuvier (1769-1832) was a brilliant French naturalist.  He essentially invented the modern fields of comparative anatomy and paleontology.

Unlike Hutton, Cuvier did care about reconstructing a narrative history of the Earth.   Cuvier felt that geologists needed an empirical criterion for working out Earth history, a clock, something in the geological record that changed over time to produce a unique sequence of events. He recognized that rocks would not work, because a very old rock looks just like a very young rock. But fossils, he argued, do change throughout the geological record.

Cuvier noted that particular fossils are only found in certain strata, and was able to construct a sequence of fossil species through time. This sequence is unique and unvarying, and so provided a criterion for determining the relative ages of different rock layers.

Cuvier championed the idea that species could become extinct, and carefully documented many examples of fossil animals clearly unlike anything alive today.

Cuvier also tried to explain what caused the extinction of these organisms. Since the fossil record shows suites of organisms that appear and disappear abruptly, he concluded that many times in the past, great catastrophes must have killed all life on Earth. The Earth was then repopulated by a new Creation event each time. These catastrophes were unlike anything humans have ever experienced. This idea became known as catastrophism.

Scottish geologist Charles Lyell (1797-1875) was a great opponent of catastrophism. His ideas became fundamental to modern geology.

Lyell’s great work was the 3-volume Principles of Geology, first published in 1830. The full title, "Principles of geology, being an attempt to explain the former changes of the Earth’s surface, by reference to causes now in operation," indicates that the book was a critique of the catastrophist viewpoint. Instead, Lyell proposed a concept that has been called uniformitarianism.

The principle of uniformity is really made up of four distinct concepts:

1. Uniformity of law – natural laws are invariant in space and time
2. Uniformity of process – the present is the key to the past; if possible, past patterns should be explained by presently observable processes (not, for instance, terrible catastrophes)
3. Uniformity of rate – all geological change is in small, steady, slowing accumulating steps (no sudden large events)
4. Uniformity of state – no long-term directionality exists in Earth history

Law and process are fundamental assumptions of all science.  Even the catastrophists would accept these, although they would argue that it is not always possible to explain past patterns by presently observable processes.

Rate and state are testable statements about how the world works (and have been found to not always be true).  But Lyell succeeded in convincing geologists that to be good scientists they must accept these latter two uniformities as well (esp'ly u. of rate).

Lyell argued that you cannot read the fossil record literally, as Cuvier and others did, but must "probe behind appearances." If a change appears abrupt, that only indicates a piece of the rock or fossil record is missing.  If you had the missing rocks, you would see that the change was really slow and gradual.  Both Lyell and Cuvier were "right" - the rock record is imperfect, but sometimes catastrophes and non-gradual changes really do happen.

4. Development of the Geological Time Scale

(See also the exercises on Relative Time and the Grand Canyon on the textbook's CD-ROM.)

Most rocks that contain information about Earth history are sedimentary rocks.   These rocks are usually deposited in layers or strata.

Stratigraphy = Study of layered or stratified rocks, especially their origins and age relations. The basic unit of stratigraphic study is the formation = a distinctive definable package of rock.  Examples are the Tapeats Sandstone and Moenkopi Formation, exposed in the Grand Canyon.

In particular, stratigraphy is concerned with working out the sequence in which a group of rocks was deposited; that is, the relative ages of rocks. Three basic principles of stratigraphy were formulated by Nicolaus Steno in the late 1600s.

Steno’s Principles:

• Superposition - older strata on bottom, younger strata on top
• Original Horizontality - All strata start out horizontal; any deformation must happen after the strata form
• Original Lateral Continuity - All strata start out as a continuous expanse; any breaks must form after the strata form

In the late 1700s, the English canal engineer William Smith observed that various types of fossils are always found in the same invariant sequence…"Fossil Succession"…this permits a biostratigraphy – using fossils to work out the relative ages of rocks.

Biostratigraphy is based on index fossils -- abundant, easily-recognized, widespread fossils that are found in a narrow stratigraphic interval, or zone.   The shorter the lifespan of the group, the narrower the zone.

Index fossils are useful tools for correlation - determining correspondences between geographically separated rocks.  Correlation matches up rocks deposited at the same time in different places.

Index fossils, marker beds (like ash beds, glacial deposits), chemical changes, and sea level changes are used to correlate rocks.

These stratigraphic techniques were used in the 1820s-1840s to work out most of the Geological Time Scale. 

5.  Radiometric Dating

Radiometric dating is used to determine the absolute, numerical ages of rocks.

Atoms are made of a nucleus of protons and neutrons, with electrons surrounding the nucleus. The number of protons defines the element.

Isotopes – atoms with same # protons (so same element) but different # neutrons

Some isotopes are stable (don't change over time) and some are radioactive.

Radioactivity = the spontaneous decay or change of an atom into a different element by releasing particles and energy from its nucleus

For some elements, like uranium, all the different isotopes are radioactive; for others, like potassium, only one or a few isotopes are radioactive, while the rest are stable.

Parent isotopes decay into daughter isotopes, like Uranium-238 to Lead-206 or Carbon-14 to Nitrogen-14.

Radioactive decay occurs at a fixed rate.  If we take a collection of atoms of a radioactive isotope, the probability of any individual atoms decaying is constant, so a rock would lose a constant proportion of atoms per unit time. On a plot of # of atoms versus time, this decay produces a curved line, specifically an exponential curve.

The equation of the line is:

P = P_oe^-lt where

P = # parent atoms in rock now
P_o = # parent atoms originally
l = decay constant for this particular isotope
t = how long decay has been happening = age of rock

The decay constant is determined in the lab for the various isotopes.

Over time, one daughter atom is created for every parent atom that decays, so P_o = P + D.  This allows us to rewrite the equation as:

D/P = e^lt-1

So, in principle, if we measure the amount of parent and daughter isotopes in a rock, and look up the decay constant of the system, we can solve for t, the age of the rock.

We measure P and D using a mass spectrometer, an instrument that can separate out and count atoms of different masses in a sample.

Worked example:

Let’s say your rock has a D/P ratio of 1/9. That is, for every 9 parent atoms in the rock, there is one daughter atom. The decay constant for these particular isotopes is 1.54x10^-10 per year (note: decay constants are usually very small numbers). Then we have:

Several key assumptions underlie this seemingly straightforward equation. First is the assumption that the decay constant really is constant and hasn’t changed over time. A lack of constancy has never been observed in the laboratory, so scientists are content with this assumption.

An assumption that can be violated is that the rock has remained a closed system, that is, no parent or daughter atoms have escaped or entered the rock since it formed.

Melting a rock "opens" it, so a radiometric date is always the time the rock last cooled from a melt.

Note that sedimentary rocks cannot typically be dated radiometrically.

Heat, pressure, chemical reactions can also open the system, allowing atoms to come or go. This changes the counts of parent and daughter atoms so that the equation no longer holds. Dating the rock will then give you an age that may be very precise but not at all accurate.

The half-life of a radioactive isotope is the time it takes for half the parent atoms in a system to convert to daughters.  It is related to the decay constant:

        t_½ = ln(2)/ l

For instance, the half-life of uranium 238 = 4.5 billion years, and the half-life of carbon 14 = 5370 years

Longer t½ - good for dating older rocks
Shorter t½ - good for dating younger rocks

Some Useful Isotope Systems:

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