GEOLOGIC HISTORY
What the Earth’s surface features tell
us about its past.
my webpage
Really Smart Men Observed…
In the 1600’s Nicolas Steno realized that the shark
tooth he was holding looked an awful lot like
impressions in certain rocks.
The rocks, he believed, held preserved shark’s
teeth.
He was hardly the first to realize that the remains
of dead organisms could be cast in rock. But, he
was writing during the push in science as a new
way of knowing things.
Scientific Inquiry
This was the time AFTER Copernicus and Galileo
and it was the era of microscopes and telescopes
and the codifying of the ‘Scientific Method’….
Steno developed principles that identified the
earth’s surface as an ever-changing place. His
work supported future scientists in evolution
(Darwin) and the geologic history of the planet
(Hutton).
I.
Sequencing of Events
Nicholas Steno
1. The principle of Superposition: In undisturbed
sedimentary rock strata, the oldest layer is on the
bottom and the youngest on top
when sediments are deposited in water, they build
up in layers, horizontally, with the oldest on the
bottom. When these layers become rock, the oldest
rock will be on the bottom, and the sedimentary
rock layers will remain horizontal, unless tectonics
alter them.
2. The principle of original horizontality:
sedimentary rocks form in layers, so any
folding or faulting or bending must have
occurred after the rock was formed.
3. Principle of lateral continuity: sediments
are deposited in continuous layers in water.
Any gaps between layers means erosion
occurred.
James Hutton in the 1700’s
Hutton took Steno’s work and realized that
the earth must be WAY older than the 6000
yrs held as ‘fact’. He expounded on Steno’s
principles and also said:
“The present is the key to the past.”
What we see happening today, the steady
changes in season that breaks the rocks so
slowly…the streams and rivers that carry those
particles to the seas and lakes…the torturously
long period of time required to build up any
real sediments…then the sediments turned into
layers of sedimentary rock a mile high…then
these pushed up, miles above the sea, tilted,
broken, folded and eroded themselves….must
take many hundreds of millions of years….
Hutton’s contribution
4. Principle of uniformitarianism: the geologic
events that occur now have always occurred, and
at the same rate.
“THE PRESENT IS THE KEY TO THE PAST
5. Principle of cross-cutting relationships: faults
and igneous intrusions must be younger than the
rock they cut across
creating layers
6. Embedded fragments: any rock found
inside a rock must be OLDER than the rock.
Think of my chocolate chip, walnut, raisin
story….
7. Xenolith: broken rocks caught in a
magma/lava intrusion are older than the
intrusion.
Observe an animation showing the
formation of an unconformity.
8. Any unconformity represents a gap in the
rock record created by:
Uplifting and erosion of rock strata, followed
by
Submergence of that same rock strata and
renewed deposition
Caused by either the lowering and raising of
sea levels (climate change) or by actual changes
in land elevations.
9. Sequencing of Events
Geologists make use of Hutton’s and
Steno’s principles, the strata they see when
they walk the outcrop, correlation of layers,
to make a reasonable history of a geologic
profile.
sequencing of events animation.
II. Relative Age (dating)
1. Relative dating: the sequence of events can be
determined in terms of one event happening
‘before’ and ‘after’ another, without identifying
actual age through;
Fossil correlation
Unconformities
Igneous intrusions
Placement of Key beds
Rock strata/horizontality and superposition
2. Fossils
Fossils are the remains of organisms, the
imprint of the organism or trace evidence of
the life of an organism that is PRESEVED
IN ROCK….
Observe how fossils can form.
3. Index fossils
Are fossils that can be used to give the date
of a rock layer with good accuracy. The
fossils must be:
1. widespread
2. easily recognizable
3. have lived for a relatively short period of
earth’s history
4. Evolution
The study of fossils in the rock gave birth
to the theory of evolution, which states that
life changes over time. How this happens is
another debate. 2 views:
Darwinian evolution: slow, steady as a result of
the interplay of forces of nature
Punctuated equilibrium: life stays relatively
static until some HUGE upheaval (climate
change) creates pressure for change.
III. Absolute Age
1. The age of a rock in years, which is
possible due to:
Radioactive decay dating: large, unstable atoms
(parent element) change into more stable atoms
(daughter element) in a steady, predictable rate
(half-life) as the nucleus of the atom changes.
carbon and you
2. Key beds: certain profound events on earth
leave clear records in the rock strata. These events
may be:
• Volcanic ash falls from ancient eruptions. These
ash falls may be worldwide and contain
strontium (radioactive decay)
• Remnants of ash and dust from ancient asteroid
impacts and contain iridium from space. dead
dinos
Key beds have the same characteristics as index
fossils.
3. Absolute age and information about
past environments are also found using;
• Ice cores in the Arctic, etc
• Tree rings example
• Sedimentary varves
IV. Divisions in Earth’s Geologic
History
The rock record showed geologists and biologists
that more species had become extinct in the past
than were alive in the present. In addition, the
rock record shows us that life evolved in ‘phases’,
with the simplest coming first…..
Earth’s history is divided according to the
predominant life forms of a time period and the
major extinctions that closed one period and began
the next.
ESRT p 8-9
“Precambrian”. From formation 4.6 bya until
540 mya almost no fossils existed:
• We know now very simple life lived then,
and that simple life changed our
atmosphere from anoxic to oxygen-rich in
2 BILLION years of effort.
ESRT p. 8-9
The Phanerozoic includes all time since the
end of the Precambrian and is broken into
Eras:
Paleozoic: ‘ancient life’; also know as the
“Age of Fishes and Amphibians”
Mesozoic: ‘mid life’: also known as the
“Age of Reptiles” and “Age of Dinosaurs”
Cenozoic: recent life: also known as the
“Age of the Birds” and “Age of Mammals”
New York State
ESRT p. 3 gives us a simple picture of the
geology of NYS. What we see is that our state
is mostly horizontal (undisturbed) sedimentary
rock that is 100s of millions of years old!!!
The Adirondacks and Hudson Highlands are
very different……
Nowhere is the rock very recent, except where
there are glacial deposits…
We have remains of mountains, volcanoes,
oceans and asteroids…..
Vocabulary
: to compare rock strata (layers) from
one area to another. Fossils, key beds, strata
assist in this, even though the strata may be
many miles apart.
: layers of sedimentary rock
: assessment of the types
and ages of rock from the top of a strata
down.25
: walking along the
surface of the earth and noting the geology
along the route. It is ‘ground-truthing’
Are terms relating to specific unconformities.
are movements along a break in the
crust and may be diagonal, horizontal or
other. They imply crustal motion.