HISTORICAL GEOLOGY
Relative & Absolute age, fossils and geologic time
Historical Geology
A. Measuring Time
1. Relative Time (Relative Age) –
Measurement of time using comparison. No
numbers used; uses words like younger and
older.
a. Example – Joe is older than Mike
2. Absolute Time (Absolute Age) –
Measurement of time using actual numbers
a. Example – Mike is 15 years old
Historical Geology continued
B. Measuring Earth’s Age (Relative Time)
1. Many Paleontologists use rock layers
a. Fossils are mainly found in sedimentary
rock.
b. The layers help tell the relative age of
rocks
c. The LAW OF SUPERPOSITION states that
oldest layers of rock are on the bottom and
younger layers are on the top.
Historical Geology continued… Layers of
rock
for
determining
Law
of
Superposition
• Unconformity – eroded rock surface pushed up from deeper
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within earth that is much older than the new rock layers
above it.
Igneous Intrusion – magma that pushes up into existing rock
layers (always younger than the rocks around it)
Igneous Extrusion – lava that builds up on top of existing rock
(always younger than the layer below it)
Rock Layers – Earth’s crust that is laid down initially in parallel
layers, most often classified as sedimentary
Glacial Till – large buildup of rock pieces carried away in the
bottom of a glacier
Kettle Lake – lake formed when a large block of glacial ice
melts leaving a depression in the ground.
Drumlins – long, smooth, canoe shaped hills made of glacial
till
Peat – substance made of decayed plant fibers, softest type of
coal
Historical Geology continued
2. Faults can be used to help determine the relative
age of a rock layers and the order in which events
happened.
a. A fault is a break or crack in part of Earth’s crust.
b. A rock layer had to be laid down before a fault can occur.
c. Younger features cut across older features – this is called
CROSSCUTTING RELATIONSHIPS
3. UNCONFORMITY – a break/gap in the rock record that
separates series or layers of deposits.
a. Layers that are exposed above sea level are often times
eroded away with brand new layers being deposited
above the erosion mark.
Historical Geology continued
C. Determining Absolute Age
1. Radioactive Dating
a. The breaking down of atoms to form
different isotopes of the same element or a
completely new element
Carbon-14
Carbon-12
b. Energy is given off in the form of
- Gamma Rays
- Alpha Particles
- Beta Particles
Historical Geology continued
2. Half-Life
a. The amount of time it takes for half of the
original atom to decay
1. Uranium-238 = 4.5 billion years
2. Carbon-14 = 5,700 years
3. Oxygen = 19 seconds
b. In the amount of time given (half-life
time), half of the original substance will be
gone.
Historical Geology continued
c. Example: If we start with 20 grams of carbon14, how long will it take to break down to 5
grams
Start of 0 years
5700 years
11, 400 years
=
=
=
20 grams
10 grams
5 grams
As you move down the left hand side of these
equations, keep ADDING THE HALF-LIFE
AMOUNT. As you move down the right hand
side of these equations, divide by 2 (take
half!)
Historical Geology continued
Fossils
A. Description - Any remains or evidence of a
living thing
B. Types
1.
2.
3.
4.
5.
6.
Shells
Bones
Pollen/Grains
Seeds
Imprints
Molds & Casts
Historical Geology continued
C. Formation of Fossils
1. Preservation without change
a. Frozen plants or animals/objects
stuck in amber (pine tree sap/resin) or
tar
2. Replacement by a mineral
a. Material decays and is slowly
replaced by a silicate (petrified wood)
Historical Geology continued
3. Formation of a carbon film
a. As material decays a carbon film is left
behind
1. Used only with living things
2. Used with half-life
4. Formation of an imprint – mold
5. Filling of an imprint – cast
D. Index Fossils – special marker used by geologists
to identify layers of rock that are separated far apart.
Geologic Time
A. Divisions
1. Eras – largest division of geologic time
a. Pre-Cambrian
*** can be broken down into eons***
b. Paleozoic
c. Mesozoic
d. Cenozoic
Geologic Time
2. Periods – subdivision of an era
a. Paleozoic Era – age of the invertebrates
1. Cambrian Period
2. Ordovician Period
3. Silurian Period
4. Devonian Period
5. Carboniferous Period – broken down into
Pennsylvanian & Mississippian in North
American only
6. Permian Period - ** The trilobite died out at
the end of the Permian Period, marking the end
of the Paleozoic Era
Geologic Time
b. Mesozoic Era – age of the reptiles/dinosaurs
a. Triassic Period
b. Jurassic Period
c. Cretaceous Period
** The dinosaurs died out at the end of the
Cretaceous Period, marking the end of the
Mesozoic Era
Geologic Time
c. Cenozoic Era – age of the mammals
1. Tertiary Period
2. Quaternary Period
** Epoch – division of geologic time that occurs
only in the Cenozoic Era; breaks down
periods into smaller chunks of time
***CENOZOIC ERA, QUATERNARY PERIOD ARE
STILL GOING ON NOW!!!!