METAMORPHISM AND METAMORPHIC ROCKS
COMMON METAMORPHIC ROCKS
Classified on the presence or absence of FOLIATION.
FOLIATION is the parallel alignment of tabular
minerals in the rock due to heat and pressure.
Gneiss
Foliated
Quartzite
Nonfoliated
METAMORPHISM AND METAMORPHIC ROCKS
COMMON METAMORPHIC ROCKS
Some metamorphic rocks can be produced from a
number of parent rocks, i.e., gneiss.
They are then distinguished by their appearance and
the conditions under which they form rather than
by composition.
Others form from specific parent rocks, i.e., slate.
METAMORPHISM AND METAMORPHIC ROCKS
COMMON METAMORPHIC ROCKS
METAMORPHIC GRADE describes the severity of
metamorphism.
Designated low, medium and high grade.
The higher the metamorphic grade, the less similar is
the metamorphic rock to the original parent
rock.
METAMORPHISM AND METAMORPHIC ROCKS
Foliated
Nonfoliated
Metamorphic
Rock
Parent Rock
GrainSize
Metamorphic
Grade
Slate
Shale
Fine
Low
Phyllite
Shale
Fine
Low
Schist
Basalt, greywacke,
Shale, impure ls.
Medium
Medium to High
Gneiss
As in schist, but also
felsic igneous rocks
Coarse
High
Marble
Limestone or
Dolomite
Varies
Low
Quartzite
Quartz
Sandstone
Varies
Low to High
Anthracite
Coal
Peat
Varies
Low to High
Greenstone
Basalt
Varies
Low to High
METAMORPHISM AND METAMORPHIC ROCKS
COMMON METAMORPHIC ROCKS
METAMORPHISM AND METAMORPHIC ROCKS
USES OF METAMORPHIC ROCKS
Building and foundations
Road and railroad construction (slate and quartzite)
Art - Sculpture (marble)
Fire protection (asbestos)
Sinks and counters (soapstone)
Powder (talc)
Metal ores (from metamorphism)
TELLING TIME GEOLOGICALLY
Time is a critical ingredient to most geologic processes.
Geologic time has an immense vastness.
We have 4.6 BILLION years to work with.
(info comes from the moon and meteorite)
To us a decade is a long time!
Lots of events happen in this interval of time.
How big is a billion?
Consider 1,000,000,000 seconds.
1,000,000,000 seconds equals 31.71 YEARS!
TELLING TIME GEOLOGICALLY
Earth processes do occur rapidly.
Earthquakes, landslides, volcanic eruptions, floods
and meteorite impacts.
Most take place slowly.
Opening of ocean basins, mountain building,
differentiation of the Earth.
This section will focus on the timing of geologic events
and the ages of Earth materials.
This process is called GEOCHRONOLOGY.
TELLING TIME GEOLOGICALLY
GEOCHRONOLOGY generally can’t be done to
precise instants in time.
Dates will typically have errors applied (+/-).
Geologists use a variety of clocks for geochronology.
They include trees, fossils, and radioactive isotopes to
start with.
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
Appearance of first Hominids
Demise of the Dinosaurs
First Land Plants
First Fish
First Shelled Invertebrates
First Appearance of Life
Oldest Known Earth Rocks
Age of the Earth
3-4,000,000 yBP
65,000,000 yBP
483,000,000 yBP
505,000,000 yBP
570,000,000 yBP
3,770,000,000 yBP
3,960,000,000 yBP
4,600,000,000 yBP
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
Appearance of first Hominids
Demise of the Dinosaurs
First Land Plants
First Fish
First Shelled Invertebrates
First Appearance of Life
Oldest Known Earth Rocks
Age of the Earth
4 mm
65 mm
483 mm
505 mm
570 mm
3,770 mm
3,960 mm
4,600 mm
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
Appearance of first Hominids
Demise of the Dinosaurs
First Land Plants
First Fish
First Shelled Invertebrates
First Appearance of Life
Oldest Known Earth Rocks
Age of the Earth
3-4,000,000 yBP
65,000,000 yBP
483,000,000 yBP
505,000,000 yBP
570,000,000 yBP
3,770,000,000 yBP
3,960,000,000 yBP
4,600,000,000 yBP
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
We have the dates of these events carefully calculated,
or so we want you to believe.
How was it done?
At first it was done by RELATIVE DATING.
RELATIVE DATING compares two or more features
and/or events to determine relative age, i.e.,
which came first, which is youngest.
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
RELATIVE DATING
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
As science and technology advanced we were able to
put actual dates on these features and events.
This process is termed NUMERICAL or ABSOLUTE
DATING.
Requires care in application.
ABSOLUTE would indicate that we know
absolutely when events occurred.
WE DON’T!!!
Numerical dates always represent a range within a
margin of error.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
A REVIEW
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
PRINCIPLE OF UNIFORMITARIANISM
James Hutton discovered that Earth processes acted
over a long period of time (1830s).
Hutton proposed that the physical, chemical and
biological processes observed on a daily basis, have
also acted on the Earth over very long periods of time.
Observations of current geological processes could
be used to interpret the rock record of very old
geologic events.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
Superposition - The scientific law stating that in any
unaltered sequence of rock strata, each stratum is younger
than the one beneath it and older than the one above it, so
that the youngest stratum will be at the top of the sequence
and the oldest at the bottom.
Superposition, con’t.
With tilted beds, we need
to consider other
sedimentary
structures, like
ripple marks,
mudcracks,
graded bedding, or
cross-bedding to
help determine
up direction.
Can also use vesicles in
lava flows.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
Principle of Original Horizontality - The scientific law stating
that sediments settling out of a fluid (air and water) are
deposited horizontally or nearly horizontally in layers that lie
parallel or nearly parallel to the Earth’s surface.
Photo by Kevin Hefferan
Badlands National Park
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
Principle of Cross-Cutting Relationships - The scientific law
that states an igneous intrusion or geologic structure is always
younger than the rock that surrounds it.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
Principle of Inclusions - The scientific law stating that rock
fragments contained within a larger body of rock are
always older than the surrounding body of rock.
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
We have the dates of these events carefully calculated,
or so we want you to believe.
How was it done?
At first it was done by RELATIVE DATING.
RELATIVE DATING compares two or more features
and/or events to determine relative age, i.e.,
which came first, which is youngest.
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
RELATIVE DATING
TELLING TIME GEOLOGICALLY
GEOLOGIC TIME in PERSPECTIVE
As science and technology advanced we were able to
put actual dates on these features and events.
This process is termed NUMERICAL or ABSOLUTE
DATING.
Requires care in application.
ABSOLUTE would indicate that we know
absolutely when events occurred.
WE DON’T!!!
Numerical dates always represent a range within a
margin of error.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
FOSSILS
FOSSILS are any evidence of past life.
Used with other principles to figure out the order in
which life developed, changed and went extinct.
Very hard to accomplish.
Hard to establish life history for organisms.
Only 1% of all species that ever lived have been
preserved as fossils.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
FOSSILS
To be preserved, fossils require special conditions.
Include:
Rapid burial
Hard parts
Most organisms don’t have hard parts.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
PRINCIPLE OF FAUNAL SUCCESSION
If we know the position of each rock layer in a
sequence, we know their relative age.
Then, if we look a the fossils in the layers, we can
determine the relative ages of them.
This sequence will occur again and again in the
geologic record.
Can then be used to provide relative age of the rocks.
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
PRINCIPLE OF FAUNAL SUCCESSION
TELLING TIME GEOLOGICALLY
PRINCIPLES OF RELATIVE DATING
PRINCIPLE OF FAUNAL SUCCESSION
Generally we use INDEX FOSSILS for this approach.
INDEX FOSSILS have
short lifespan (rapid evolution)
widespread geographic distribution
and are easy to recognize
TELLING TIME GEOLOGICALLY
RELATIVE DATING BY WEATHERING RINDS
Relative ages of rocks can be determined by comparing
the extent of weathering preserved in them.
Weathering will produce a RIND of weathered material
at or near the rock’s surface.
Produced in a variety of ways.
TELLING TIME GEOLOGICALLY
RELATIVE DATING BY WEATHERING RINDS
The longer the length of exposure to weathering, the
thicker the rind.
Thickness of soil can also be used in this manner.
(Need to use similar soils produced from similar
parent rocks)