GEOLOGIC HISTORY

What the Earth’s surface features tell us about its past.

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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 bySubmergence of that same rock strata and renewed depositionCaused 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 naturePunctuated 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. 8-9The 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.