Standard 8-2.6 Notes: Infer the relative age of rocks and fossils from index fossils and the
ordering of rock layers.
The ______________________ means the age of one object compared to the age of another object.
Relative age does not tell the exact age of an object. The relative age of rocks and fossils can be determined
using two basic methods: __________________________________________________________:
RELATIVE AGE: The ______________________ compared to the ages of rock ___________.
ABSOLUTE AGE: The ________________________ given as the number of years since the rock formed.
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LAW OF SUPERPOSITION: in horizontal ___________________________, the oldest layer is at the
_______________. Each higher layer is ________________ than the layers below it. Each rock layer is older
than the one above it.
Clues from Igneous Rocks:
1. Lava that hardens on the surface is called an _____________________.
(example – an eruption would put a layer of igneous rock on top of sedimentary rocks.
Rock layers ____________an extrusion are ________________________than the extrusion.
The Extrusive is in BLACK
Now the extrusive is the youngest layer
2. Magma that cools and pushes into bodies of rock and hardens is called an ____________________. An
intrusion is always _________________than the rock layers around and beneath it.
The intrusion (in red) is now
younger than the surrounding
rocks.
Understand that these are all rocks that are millions of years old.
Clues from Faults:
Fault: is a break in the Earth’s crust.
• Forces inside the Earth _____________________________ of the rock on __________________sides of a
fault.
• Fault is ________________________ than the rock it cuts through.
GAPS IN THE GEOLOGIC RECORD
- Record of sedimentary rock layers is not ___________________________.
- Deposition slowly builds layers upon layer of _________________________ rock, BUT some of these layers may
____________ away, exposing an __________ rock surface.
Unconformity – is a _________ in the geologic record. An unconformity shows where some
____________________have been _________ because of _____________.
USING FOSSILS TO DATE ROCKS
To date rock layers, geologists first give a relative age to a layer of rock at one location. THEN they can give the
same age to matching layers of rock at other locations.
Certain fossils, called Index Fossils help geologist match rock layers.
Index Fossils
Certain fossils, called _____________________, can be used to help find the ______________________ of rock
layers. To be an index fossil –
 an organism must have lived only during a ___________________ part of Earth’s history; many fossils of the
organism must be found in ______________________; the fossil must be found over a
_________________________ of Earth; and the organism must be __________________________.
The ________________ time period a species lived, the better an index it is. A key example of an organism
used as an index fossil are __________________________, a group of hard-shelled animals whose body had
three sections, lived in shallow seas, and became extinct about 245 million years ago. Therefore, if a trilobite is
found in a particular rock layer, it can be compared with trilobites from other layers to estimate the
______________ of the layer in which it was found. Fossils that are found in many rock layers, therefore living
__________________ periods of time, _______________ qualify as index fossils.
Trilobites
• Trilobites evolved in _______________ seas more than 500 million years ago.
• They have been found in many different places.
To become an Index Fossil … a trilobite must be _________________ in some way from other trilobites.
Example – type with large eyes
These large-eyed bites survived for a time AFTER other bites became extinct. If a geologist finds large-eyed
Trilobites in a rock layer, the geologist can ____________ that those rocks are younger than rocks containing other
types of trilobites.
Paleontologist
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•
Geologist
Archaeologist
is the science comprising
studies prehistoric people
of fossils of organisms
the study of solid Earth
and their cultures
(plant or animal) that
and the processes by
once lived on the earth. It
which it evolves.
archaeology is to learn
is a specialization
Geology provides
more about past societies
belonging to the field of
primary evidence for
and the development of
geology.
plate tectonics, the
the human race.
"the study of ancient
history of life and
life".] Paleontology seeks
evolution, and past
society, primarily
information about
climates.
through the recovery and
In modern times,
analysis of the material
organisms: "their identity
geology is commercially
culture and
and origin, their
important for mineral
environmental data that
environment and
and hydrocarbon
they have left behind,
evolution, and what they
exploration and for
which includes artifacts,
can tell us about the
evaluating water
architecture, biofacts and
Earth's organic and
resources;
cultural landscapes (the
is openly important for
archaeological record).
Paleontology is the study
several aspects of past
inorganic past".
•
•
•
the prediction and
understanding of natural
hazards, the remediation
of environmental
problems, and for
providing insights into
past climate change.
•
•
The purpose of
is the study of human