THE HISTORY OF THE EARTH

EARTH SCIENCE

CHAPTER 21

FOSSILS Paleontology is the study of life that existed in

prehistoric times A fossil is any evidence of earlier life preserved in

rock. Shells Bones Petrified trees Footprints Impressions made by leaves Burrows made by worms

FORMATION OF FOSSILS Fossils may form in several ways:

From original remains From replaced remains Being preserved in molds and casts Being preserved as trace fossils Being preserved in carbonaceous film

ORIGINAL REMAINS In rare cases, organisms may be preserved in

their entirety. Frozen wooly mammoths in Siberia and Alaska Prehistoric insects captured in resin

Usually, soft body parts decay and only the hard parts are preserved. Bones teeth

REPLACED REMAINS The soft parts of plants and animals have decayed

and the hard parts have been replaced by minerals Circulating groundwater removes the original

organic material and replaces it with minerals such as calcite, silica, and pyrite.

This results in an exact copy of the original plant or animal.

Ex. Petrified wood

MOLDS AND CASTS Organisms can be buried in mud or other

sediments. As the sediments become rock, the hard body parts become fossils.

If the fossil dissolves out of the rock, a hollow depression in the rock called a mold results.

Minerals may then seep into the mold and fill it, forming a cast, or copy, of the original fossil.

TRACE FOSSILS Indirect evidence of life that is left behind

without being actual remains of the organism or plant is called a trace fossil.

Trails, footprints, bite marks, burrows

CARBONACEOUS FILMS Decomposing organisms may be exposed to

intense heat and/or pressure. Their tissues will undergo chemical reactions

that sometimes leave behind a very thin film of carbon that resembles a silhouette of the organism. This is called a carbonaceous film.

RELATIVE TIME Most often, scientists have no way of

knowing the exact dates of geological events on Earth.

Therefore, they often use a process called relative dating.

Relative dating does not assign exact dates to events but places events in a sequence, thus saying when they occurred in relation to other events.

THREE RULES OF RELATIVE DATING

The Principle of Superposition In an undisturbed sequence of sedimentary strata, the

oldest rock layer will be at the bottom and the youngest layer will be on the top.

The Principle of Cross-Cutting Relationships An igneous intrusion is always younger than the rock it

has intruded or cut across. Embedded Fragments

Rocks that are embedded in another rock must be older than the rocks in which they are embedded.

Example #1

Example #2

Example #3

1. Oldest layer

2. Youngest layer

3. Oldest organism

4. Place the following actions in the correct order of formation. (1-oldest, 5-youngest)

-Granite dike intrusion, -folding of gray shale, -deposition of tan sandstone, -erosion of tan sandstone, -deposition of red sandstone

Grand Canyon

Cross Section

GAPS IN RELATIVE TIME: UNCONFORMITIES

An unconformity indicates where layers of rock are missing in the strata sequence—means evidence is missing

Angular unconformity Younger, flat strata are deposited on top of older strata…older strata

could have been tilted during uplift Disconformity

All layers are horizontal, but some are missing due to erosion before the deposition of new layers

Nonconformity Occurs when sedimentary layers are deposited on top of igneous or

metamorphic rock

ROCK LAYER CORRELATION Correlation is the matching of rock layers from one area to

those in another area Several methods are used to correlate rock layers:

Walking the outcrop—outcrop is the part of the rock layer that can be seen at Earth’s surface

Matching rock characteristics—see if the rocks have the same appearance, color, and composition

Using index fossils--(next slide) Fossils as Environmental indicators—the presence of certain fossils

can indicate climate and/or sea level changes Matching key beds—a single rock layer that is unique, easily

recognizable, and widespread Stratigraphic matching—when the sequence of three of more layers

or “strata” is repeated in two different areas

INDEX FOSSILS Index fossils are the remains of animals that lived and died

within a particular time segment of Earth’s history. Remains are buried and animals become extinct, so those

rock layers can become distinctly tied to that time period. Index fossils have four characteristics:

The fossils are easily recognizable. The fossils are abundant. The fossils are widespread in occurrence. The fossils occur only in a few rock layers because the organisms

only existed for a brief time period

ABSOLUTE TIME Absolute time identifies the actual dates of events. Historical methods for measuring absolute time:

Estimating rates of erosion and sedimentation Not very accurate because rates are not constant

Counting tree rings Still used today One ring roughly equals one year Have dated trees back to 2000 BC

Counting varves A varve is any sediment that is deposited on a yearly cylce Used most accurately in glacial lakes Have dated sediments back 15 000 years

RADIOACTIVITY Recall that isotopes are atoms of the same element

with different numbers of neutrons. Many elements have radioactive isotopes. Radioactive isotopes are those that exhibit

radioactive decay by emitting or capturing tiny particles in order to try and achieve stability.

Whenever, particles are emitted or captured, it can change the atomic mass and even the atomic number (identity) of the atom.

THREE TYPES OF RADIOACTIVE DECAY

Alpha decay Two protons and neutrons are emitted So, atomic number of original isotope decreases by two and atomic

mass decreases by four Beta decay

A neutron splits into a proton (+) and an electron(-). The electron, called a beta particle, is emitted.

The atomic number of the original isotope increases by one and the atomic mass does not change.

Electron capture A proton captures an electron and becomes a neutron. The atomic number of the original isotope decreases by one and the

atomic mass does not change.

CONTINUAL DECAY The original element in the decay process is

called the parent isotope. After each decay process, a daughter isotope

is formed. If the daughter isotope is also radioactive,

decay continues until a stable, nonradioactive isotope is formed as the daughter.

HALF-LIFE Radioactive isotopes decay at a constant rate. This

rate is not affected by changes in temperature or pressure, or by the passage of time

The amount of time it takes for exactly one-half of the radioactive atoms in a sample to decay to a stable product is called the half-life.

After two half-lives, 25% of the original sample will remain. ( ½ x ½ = ¼).

After three half-lives, only 12.5 % of the original sample will remain. ( ½ x ½ x ½ = 1/8

RADIOMETRIC DATING Scientists measure the amounts of a parent

and a daughter isotope within a rock or mineral and use the ratio to find the age of the rock

Carbon-14 is used to date organic material. Uranium-lead, Rubidium-strontium, and

potassium-argon are used to date rocks and minerals.

RADIOCARBON DATING Radioactive carbon-14 and stable carbon-12 both

exist in the tissues of living organisms. After death, the carbon-14 begins to decay.

Scientists use the ratio of carbon-12 to carbon-14 to determine how long ago the plant or animal died.

Two serious limitations: Can only be used to date things that were once alive Carbon-14’s half-live is 5730 years (short amount of

time), so it can only be accurately used to date items that are 70 000 years old or younger.

URANIUM-LEAD DATING Radioactive Uranium-238 decays to form

stable lead-206. Uranium-238 is 4.5 billion years, so it is

possible to date the oldest rocks of Earth’s crust using this isotope.

Is naturally found in zircon, which is in some igneous rocks—very rarely found in sedimentary or metamorphic rocks

RUBIDIUM-STRONTIUM DATING Rubidium-87 decays to form strontium-87. The half-life of Rubidium-87 is 47 billion

years (over 10x the age of Earth) Is the best method for dating the oldest of

Earth’s rocks Is found in common minerals like feldspars

and micas.

POTASSIUM-ARGON DATING Potassium-40 decays to form Argon-40. The half-life of potassium-40 is 1.3 billion

years. Are found in micas, feldspars, and

amphiboles. Can be used to date sedimentary and

metamorphic rocks as well as igneous ones.

Geologic Time

Dinosaurs

Mesozoic Era

Title: Geologic Timeline • The Earth is about 4.6 byo based on the absolute dating of

rock samples. The Earth’s history is broken down into Eons, eras, periods and epochs. Each time frame does not last an even amount of time and the division of time is based on geologic and biologic events or evolutions.

Precambrian Eon Today, Cenozoic Era

Geologic TimeDefined by changes in species or major geologic events. Time frames are NOT uniform (NOT evenly divided!)

Earth is 4.6 BYO

Fossil evidence shows life at 3.5 BYA

Life evolved between 3.9-3.5 BYA

Precambrian ERA 4,600-544mya

• 87% of the Earth’s History• Seas formed• Mountains grew• Oxygen built up in the

atmosphere– Cyanobacteria

• Life : bacteria, jellyfish, algae (Prokaryotes and Eukaryotes)

• No life on land yet, only in seas!

Precambrian Life

Stromatalites!

Ediacara

Paleozoic ERA 544-248 mya• Age of Fishes• Appalachian

Mountains form• Warm, shallow seas

cover the majority of North America

Paleozoic Era

• Periods• 1. Cambrian Period

– “Explosion” of life– Trilobites (skeletons, hard

parts evolved)• 2. Ordovician Period

– corals, sponges, first fish– Mass extinction- 57% of

marine species (glaciation)• 3. Silurian Period

– first land plants

Paleozoic ERA• Periods• 4. Devonian

– age of fishes (armor-plated fish and amphibians) lungfish, coelacanths

– Ferns (seeds!)– mass extinction! 50%

of marine species- (glaciation)

• 5. Mississippian– Crinoids

(Invertebrates)– Many amphibians

Paleozoic Era

• 6. Pennsylvanian Period– Insects (cockroach,

centipede, dragonfly)– first reptiles– Coal deposits

• 7. Permian – abundant reptiles– Mass extinction- 95% of

land and marine species (regression)

Mesozoic Era 225-65my

• Age of Reptiles• Mass extinctions at

the beginning and end of this era

• Mississippi River System forms

• Rocky Mountains • Atlantic Ocean

Mesozoic Era 225-65my• Periods• 1.Triassic

– Pangaea breaks up• 2. Jurassic

– First birds• 3. Cretaceous

– first flowering plants• Life Forms of Mesozoic Era

– Dinosaurs, First Mammals, and Modern Fish

• Mass extinction– Meteorite, volcanism– Wipes out dinos

Cenozoic ERA 65mya- today

• Age of Mammals• Himalayan Mountains • Alps• Grand Canyon• Great Lakes• Chesapeake Bay

Cenozoic Era• Periods• 1. Tertiary

– Mastodons, Mammoths, Primates

– Early humans • Homonids (walk upright)• Neanderthals

– extinction (45% of species)• 2. Quaternary

– Civilization begins10,000 years ago with end of most recent ice age and invention of agriculture