Tiempo Geologico.pdf

The Great Unconformity Thermopolis, Wyoming

GEOLOGICAL EVOLUTION

The Great Unconformity Thermopolis, Wyoming

3 Billion Year old Granite

550 Million Year old Sandstone

Geologic Time Scale

The Geologic Timescale is the “Earth’s calendar” that divides the Earth’s

history into different time segments.

The Geologic Timescale was developed by relative dating. Later, absolute

dating allowed actual years to be placed on the scale.

The Geologic Timescale is divided (not evenly) into 4 “Eons”

From oldest to youngest, the Eons are:

Hadean

Archaen

Proterozoic

Phanerozoic

The first 3 Eons are referred to as the Precambrian

Precambrian

The Phanerozoic Eon is the most recent and best understood

Eon. It spans from 545 million years to the present day.

The Phanerozoic Eon is divided into 3 time sections called

“Eras”.

Paleozoic Era (“old life”) 545-245 million years ago

Mesozoic Era (“middle life”) 245-65 million years ago

Cenozoic Era (“recent life”) 65 million years ago until

now!

The Phanerozoic Eras are subdivided into “Periods.”

The boundaries between periods are typically determined by

extinctions in the fossil record.

The Hadean Eon 4.6 – 3.8 Billion Years ago

The Hadean Eon is the Eon which lacks

a rock record!

The age of the Earth is determined by

meteorites to be 4.6 Billion years old.

Oldest minerals found on the Earth are

4.1 Billion years old.

It is hypothesized by geologists that

during this Eon, the Earth was extremely

hot due to meteorite bombardment.

The surface was covered by a

magma ocean.

There was no liquid water.

The Archean Eon 3.8 – 2.5 Billion Years ago

At the beginning of the Archean, the magma ocean cooled enough to

form a solid crust on the Earth’s surface. In other words, rocks start to

form! Oceans also formed at this time.

The Archean marks the time when the Earth begins recording geologic

history in the rock record. It also marks the time when life began.

The first evidence of life comes from organic carbon and fossil bacteria

in 3.8 Billion year old rocks.



Stromatolites are fossilized mounds of layered bacteria.

Stromatolites are observed in the rock record 3.2 Billion years ago.

The Proterozoic Eon 2.5 Billion Years to 545 million years ago

The Proterozoic Eon marks the time when the Earth cooled enough for

plate tectonics to work as it does today.


About 2 Billion years ago, a type of rock called a banded iron formation

(BIF) began to appear.

The presence of BIFs indicates that oxygen appeared in the atmosphere,

most likely due to photosynthesis of single-celled organisms.

Without oxygen, iron ions exist as Fe2+ that is easily dissolved in

water.

The early oceans (before 2 Billion years ago) had lots of dissolved

Fe2+ in them.

Once oxygen appeared in the atmosphere, Fe2+ started to

‘oxidize’, turning into Fe3+.

Fe3+ is not soluble in water (cannot be dissolved), so it precipitates

and is deposited.


BIFs have alternating layers of hematite and chert

Hematite

Chert


Keep in mind that during all this time, there is lots of life in the oceans,

but it is only in the form of simple single-celled organisms that lack a

nucleus (for example, bacteria).

1.5 Billion years ago, the first complex single-celled organisms are

observed.

Contain a nucleus

Produce energy by respiration (breathing oxygen)


670 Million years ago - the first evidence for multi-cellular life.

These creatures are called the Ediacaran fauna

They were soft-shelled animals similar to jellyfish or worms (no shells)

The Phanerozoic Eon 545 million years ago until today

Paleozoic Era (545-245 million years ago)

545 million years ago (at the Precambrian-Cambrian boundary), an

abundant amount of hard-shelled multi-cellular organisms appear in

the fossil record. This is often referred to as “The Explosion of Life.”

The diversity of fossils dramatically increases!

During much of the Paleozoic, shallow seas covered most of the

continents, providing a rich environment for sea plants and animals.


Cambrian Period (545-510 million years ago)

Trilobites, Brachiopods, and Molluscs (snails, clams, and squids) appear in

the fossil record.

Trilobites make up 50% of all species.

trilobite brachiopod

mollusc


Ordovician Period (510-440 million years ago)

Armored fishes, corals, and graptolites (floating colonies of organisms)

appear in the fossil record.

armored fish

coral

graptolite


Silurian Period (440-408 million years ago)

Coral reefs develop.

First land plants (Before this time the continents were barren)

Devonian Period (408-360 million years ago)

Also called ‘the age of fishes.’ Fish species become diverse.

Land becomes forested

First land animals (Insects and amphibians)

Pangea is about to form!


Carboniferous Period (360-286 million years ago)

You don’t need to know this, but the Carboniferous is often called the Mississippian

and Pennsylvanian Periods instead.

Many swamps existed in North America and Europe (forming much

of the coal that we mine today).

Appalachian mountains begin to form. Very large mountain range!

First reptiles (can lay eggs on land, unlike amphibians)

Pangea has formed – the Appalachians are a huge mountain range!


Permian Period (286-245 million years ago)

Dimetrodons (sail backed reptiles) and Therapsids (mammal-like reptiles)

are first observed in the fossil record.

The supercontinent Pangea has formed, drastically changing the

environment

dimetrodon

therapsid

The Great Permian-Triassic Extinction (245 Million years ago)

90-95% of all species disappeared from the fossil record!

This is the greatest extinction known; much larger than that

associated with the death of the dinosaurs.

Geologists are unsure of what caused this extinction, but it is thought

to have been caused by either:

Formation of Pangea

Reduced continental shelf (shallow sea regions off the

coast)

Large landmass leads to harsh climates in interior regions

Massive flood basalt volcanism that occurred in Siberia

A meteorite impact?

Mesozoic Era (245-65 million years ago) – Age of the Dinosaurs

Triassic Period (245-208 million years ago)

Dinosaurs first appear

First mammals – small and probably nocturnal


Jurassic Period (208-144 million years ago)

Stegosaurus and Sauropods appear

The first feathered bird-like reptile Archaeopteryx appears

(perhaps the ancestor of birds!)

Pangea begins to break apart – the Atlantic

ocean is first formed!


Cretaceous Period (144-65 million years ago)

Tyrannosaurus Rex and Triceratops appear

The first flowering plants appear

The Cretaceous-Tertiary Extinction

(also called the K-T Extinction)

Dinosaurs, and most plankton and plant species became extinct

In recent years, there has been an abundance of evidence supporting

the hypothesis that a large asteroid caused this extinction.

An impact site of the same age as the extinction was found on the

Yucatan Peninsula in Mexico. This is called the Chicxulub crator.

Geophysical evidence of impact crator

Cenozoic Era (65 million years ago until today) – Age of the Mammals

Tertiary and Quaternary Periods

Some reptiles survived the K-T extinction (crocodiles, alligators, turtles,

and lizards).

Mammals rapidly diversify

Increase in size

Increase in brain capacity

Specialization of teeth and limbs

Grasses become widespread plants, and large grazing mammals

appear

First Hominids (human-like apes) (4 million years ago)

(don’t worry about these animal names)

But do know that Wooly Mammoths were around

in the Cenozoic.

India drifts toward Asia

Africa and Italy drift towards Europe

The Rocky Mountains are Forming

The Alps and Himalayas begin to Form

The last ice ages

Based on current plate velocities, we can guess

at possible future arrangement of the plates.

(This is only a rough guess)

For the exam, you must know all of the information listed on the slides

(unless otherwise stated), but not all of the dates. Here are the dates

that you need to know:

4.6 Byr – age of the Earth, beginning of the Hadean Eon

3.8 Byr – first evidence for bacteria

3.2 Byr – stromatolites

2.0 Byr – BIFs, presence of oxygen in the atmosphere

670 Myr – first evidence of multi-cellular life

545 Myr – Precambrian-Cambrian boundary – Explosion of Life

245 Myr – The Great Permian-Triassic Extinction

65 Myr – The KT (Cretaceous-Tertiary) Extinction

4 Myr – First hominids

You do NOT need to know the details listed on

paleomaps.

Tiempo Geologico.pdf

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