CPSP118G Fall Semester: Earth, Life & Time Colloquium
The Restless Earth: an Introduction to Geology
Thomas R. Holtz, Jr.
Traditional (i.e., pre-scientific) View of Earth: the origin of the Earth and its structures
(mountains, oceans, seas, rivers, etc.) are totally distinct from modern disasters (earthquakes,
floods, landslides, volcanoes, etc.). The former was thought to be a singular event at the Dawn
of Time, and completed; the latter were ongoing processes due to divine, diabolic, astrologic, etc.,
influences. (Indeed, "disaster" means "bad star"). Earthquakes, volcanoes, floods, and the like
were thought to have no more relation to the formation of the Earth and its structures than
diseases have to do with the birth of a child.
Various philosophers proposed "Theories of the Earth": several models beginning in the 17th
Century questioned a single Creation week as the entirety of the formation of the Earth and
its structure. Did the Earth itself have a history?
Two main competing philosophies among the "Theories of the Earth":
- Catastrophism: Natural laws of the past were not the same as the current natural laws
- Did not require that the transitions between phases of Earth's history were necessarily "catastrophes"
in the common use of the word.
- However, many did interpret these transitions as disasters, with Noah's Deluge being the
last major phase change.
- Uniformitarianism: Natural laws today are the same as in the past
- "The Present is the Key to the Past"
Neptunism was a leading (but not the only) catastrophist model in the 18th and early 19th
Centuries. Postulated that all rocks precipitated out of the primordial seas. As the seas evaporated
to certain depths, the type of rock deposited would change. Thus, under neptunism, rock type (lithology)
equals rock age. Under neptunism, rocks could be catagorized as:
- Primary: crystalline rocks at the cores of mountains, such as granite and gneiss
- Secondary: layered, solid rocks along the slopes of mountains, such as limestone and
sandstone. Secondary rocks commonly had fossils.
- Tertiary: layered, crumbly sediments in the lowlands, such as sand and gravel. Tertiary
rocks also commonly had fossils.
- Volcanic: were observed being formed today.
Many promoters of neptunism worked primarily in the lab rather than in the field. The switch to modern
interpretations of Geology came from a combination of field and lab observations. Some of the main
contributions leading to modern geology came from
Nicolaus Steno (1638-1686) and James Hutton (1726-1797). Steno
demonstrated that "Secondary" and "Tertiary" rocks were bits of previously existing rocks that were
stuck together (solidly stuck in "Secondary", poorly stuck in "Tertiary"): what we now call sedimentary rocks.
Fossils were just the remains of dead animals and plants that got buried with the sediment and preserved
in the rock.
Hutton proposed a uniformitarian model termed plutonism, which is the basis of modern geology.
He had observed that you could trace out huge masses of crystalline rock ("Primary" rock under
the neptunist model) grading directly into veins of volcanic rock (which under Neptunism were from
an entirely different period of time!). Hutton instead suggested that these were variations of the same
process, and the reason the rocks produced looked different was that they formed in different
The plutonist model:
Heat at depth creates molten masses of magma
Magma, because it is hot and relatively bouyant, rises
Uplifted rocks are subject to erosion, transportation, and redeposition as
layers of sediment: strata
So under plutonism, rock type does NOT equal rock age, but rather equals rock environment. Or,
to use the official ELT Geology Mantra:
- As it rises, it uplifts the rocks above it and crumbles, folds, bakes and
melts the rocks around it
- Magma that cools before it reaches the surface forms crystalline rock masses called plutons
- Magma that reaches the surface erupts as lava and forms volcanic rocks
Hutton perceive the word as an "Earth Engine", where the processes around us today are responsible
for the formation of its structures. These include both slow gradual processes (streams, wind,
rain, etc.) and dramatic "disasters" (floods, earthquakes, volcanoes, etc.). Indeed, the latter
were consequences of the normal operation of the Earth!
Hutton's arguments were good, but he was not a good writer. His 1798 Theory of the Earth was
not as well read as it might have been, due to the style of the prose. In the next generation, geologist
Sir Charles Lyell (1797-1875) would
be more successful in promoting uniformitarianism and plutonism with his well-written three-volume
Prinicples of Geology.
A major point of modern geoloy: the Rock Cycle:
Rocks (naturally occurring cohesive solids comprised of one or more minerals or
mineraloids) are generated in one of three primary manners (basis of rock classification):
- Formed by the cooling of molten material
- Igneous formed by lava cooling on the surface are volcanic rocks:
- Also called extrusive because they extrude onto the surface
- Because they cool so quickly, their crystals are too small to see easily with the naked eye
- Igneous rocks formed by magma cooling at depth are plutonic rocks:
- Also called intrusive because they intrude into previously-existing rocks
- Because it takes a lot longer to cool while insulated by other rocks, their crystals
are typically large enough to see
- Formed by transformation of previously existing rocks due to intense heat
and/or pressure (i.e., they are baked and/or squashed)
- Will often contain unusual minerals that only form at incredible temperatures and/or pressures
- Formed from transported fragments of previously existing rocks
- Bits of previous rock and/or organic matter are called sediment
- Sediment might be in the form of:
- Solid chunks (gravel, sand, silt, mud), which form detrital sedimentary rocks
- Ions in solution, which can
- Be precipitated out as chemical sedimentary rocks
- or be taken up into the living tissues of organisms, made into hard parts like shell or
wood, and then deposited when the organism dies to form biogenic sedimentary rocks
- Fossils are common in many sedimentary rocks
- Because they are deposited, sedimentary rocks naturally form horizontal strata.
- Sediment is transported by water, wind, glacial ice, etc.
- Sediments are deposited in deserts, flood plains, rivers, lakes, swamps,
coastlines, continental shelves, etc.: depositional environments. The different
environments of deposition of sedimentary rocks can preserve different sorts of
informationin the form of sedimentary structures such as mud cracks, ripple marks,
raindrop marks, etc.
The modern view of geology shows that environments have changed dramatically over the face
of the Earth, but that we can use the clues in the rocks to interpret those changes.
The Stenonian-Huttonian-Lyellian view actually gave geologists a way of understanding the history of the Earth, and
in particular the fact that it changed through time.
"Deep Time": analogy to "deep space"; the vast expanse of time in the (geologically
Many attempts at calculating age of the Earth:
- Different cultures using religious texts/beliefs come up with different ages (thousands of years to hundreds of billions of years)
- Examples: counted up "Begats" and lifespans of Biblical figures (yields thousands of years)
- Early attempts by physicists and geologists predicted a few million to hundreds of millions of years
- 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
Two different aspects of time to consider:
- Relative Time: sequence of events without consideration of amount of time (A
came before B, B before C, etc.)
- Numerical Time: (sometimes called "absolute time"), dates or durations
of events in terms of seconds, years, millions of years, etc.
- "The Apollo 11 moon landing came after the Signing of the Declaration of Independence,
but before attacks of September 11" is a statement of relative time.
- "The Signing of the Declaration was in 1776, the Apollo 11 landing was in 1969, and
the attacks of September 11 were in 2001" is a statement of numerical time.
In the history of geology and paleontology, relative time was determined LONG before
Sedimentary rocks, because they are deposited, naturally form horizontal layers
singular stratum). Because of their layered form, strata allow geologists to
determine relative time (that is, sequence of deposition of each layer, and thus the
relative age of the fossils in each layer). These form the basic Principles of
Stratigraphy. The first three principles were developed by Niels Steinsen (better
known as Nicholas Steno):
- 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.
- Can determine way up from surface sedimentary structures: mudcracks, ripple marks,
footprints, raindrop marks, etc.)
- 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). 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. Hutton, of
Uniformitarianism fame, studied these and recognized that they represented aspects of
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.
Use these principles to figure out
time sequence in any particular section of rock. Using these principles, early geologists were able to figure out the sequence of events of
deposition, the changing local environments, and the folding, faulting, igneous
intrusions, etc. for any particular section of rock. BUT,
how to extrapolate the sequence at one section with the sequence at another? And how could they tell numerical time?
We'll see those questions answered in the next few weeks.
Last modified: 10 August 2007