The Great Unconformity at the Grand Canyon: the Cambrian Tonto Group overlaying the tilted beds of the Proterozoic Grand Canyon Supergroup
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
Physical Stratigraphy
Sedimentary rocks, because they are deposited, naturally form horizontal layers (strata,
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. he first three principles were developed by Niels Stensen (better
known as Nicholas Steno):
If the strata are no longer horizontal, something has disturbed the sediments AFTER they became rocks.
Can determine way up from geopetal (or "way up") indicators: surface sedimentary structures such as mudcracks, ripple marks,
footprints, raindrop marks, etc.)
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.
Richard Alley (PSU) gives some advice concerning geopetal indicators:
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). Hutton, of
Uniformitarianism fame, studied these and recognized that they represented aspects of
relative time.
Disconformity: Surface of erosion/non-deposition parallel with bedding planes
Angular Unconformity: Surface of erosion which cuts across the bedding plane of
lower strata, indicating these were tilted prior to erosion.
Nonconformity: Erosional surface cut into crystalline (non-sedimentary) rocks
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.)
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. However, how could they extrapolate
the sequence at one section with the sequence at another location, far away?
A brief video about the human and geologic history of Siccar Point and unconformities:
