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Geology 115/History 150
Name(s):
Lab 2: Sedimentary and igneous rocks
Objective: To distinguish sedimentary and igneous rocks, and to identify some specific
varieties of each rock.
Needed:
• The Geologic Map of the United States (1974), and the Explanations sheet
• Tubs
• Rock testing kit
• Geology Lab Manual
The Geologic Map of the United States
Examine the Explanations sheet; like all such geologic map explanations, the sheet is
organized with the most recent (youngest) rocks at the top and the oldest rocks at the
bottom. Left to right, the rocks seem to be organized by their type. In any geology textbook,
rocks will be classified into three rock types: igneous, sedimentary and metamorphic.
Clearly, the furthest right column contains metamorphic rocks.
1. From Geology Prep Assignment 2 and your geology textbook, which columns in the
Explanations sheet are the igneous rocks?
2. Which columns are the sedimentary rocks?
3. Find Mesa Verde NP on the geologic map (hint: recall what state it is in, and what part of
the state; further, it is just southeast of the town of Cortez). What abbreviation(s) are used
to designate the rock(s) of this area? Look on the Explanations sheet and write the full
name(s) of the rock(s). From Geology Prep Assignment 2, what type of rock is found in
Mesa Verde NP? Is this consistent with the geologic map information?
4. Find Acadia NP on the geologic map (hint: recall what state it is in, and what part of the
state; further, “Mount Desert Island” is clearly identified on the map). The park appears to
have two kinds of rock: the area colored in green, and a circular area with a hard-to-see
color (it’s supposed to be pink) and delineated by a dotted/solid line. What abbreviation is
used to designate the green area rock? Look on the Explanations sheet and write the full
name of this rock.
5. The other kind of rock at Acadia NP (the pink area) is labeled in other parts of the map as
“Pzg2”. Find this abbreviation on the Explanations sheet, and write the full name of this
rock. From Geology Prep Assignment 2, what type of rock is found in Acadia NP? Is this
consistent with the geologic map information?
Rocks and the rock cycle
Rocks are divided into three major categories on the basis of their origin:
Igneous rocks (from the Latin word, ignis = fire) are composed of minerals which
crystallized from molten rock, called magma. This category includes rocks that formed
as a result of volcanic activity and those that crystallized from magmas cooling under
the Earth's surface.
Metamorphic rocks (from the Greek word, metamorphose = to transform) are derived
from existing rocks which have been exposed to increased pressure and/or temperature
to the point where they begin to alter, both texturally and mineralogically.
Sedimentary rocks (from the Latin word, sedimentum = settling) form by the
accumulation and consolidation of unconsolidated material from weathered and eroded
rocks.
Rocks do not remain the same throughout geologic time. They are constantly being
changed by external forces. Given time, the effect of these forces is to change any one
rock type into any rock type. These relationships constitute the rock cycle, as shown in
the following figure:
Igneous Rock
Sedimentary
Rock
Metamorphic
Rock
Tub 3
Obtain a Geology Lab Manual; this guide was written by John Figge specifically for the rocks
that North Seattle College has, but is also generally applicable to rocks anywhere. Pages 1
through 8 contain information about reading maps, similar to what you found out in Lab 1,
and you should read these pages when you have a moment.
6. Tub 3 contains three boxes labeled R1, R2 and R3; these are rocks from the three different
rock types. Using the descriptions given on page 19 of the lab manual, classify each of the
three rocks into one of the rock types and give an explanation of why you chose that type.
R1 is ________________________________________, because
R2 is ________________________________________, because
R3 is ________________________________________, because
Igneous rocks – Tubs 4 through 9
The six igneous rocks in these tubs are identified by their R number. The goal of this section
is for you to identify the specific name of each rock. In the lab manual, examine pages 22
through 26; these pages divide the igneous rocks into their two major kinds. On pages 23
and 25 are flowcharts that help you identify the specific name of a rock, given by the boldfaced name at the end of each set of arrows; this is the name you should enter into the
table in question 10.
7. What is the major difference between the two kinds of igneous rock (a good explanation
is given on page 22)?
8. Within each kind of igneous rock what distinguishes the different rock names? The chart
on page 23 (especially the arrows at the bottom of the chart) should be helpful.
9. The terms “felsic”, “intermediate”, “mafic” and “ultramafic” on the chart on page 23
refer to the chemical composition of the rock’s minerals (minerals are the crystals,
sometimes too small to see, that make up a rock). Since we don’t have access to a mineral
identifying machine (they don’t exist), what simple way, according to the lab manual, is
there of distinguishing a mafic rock from a felsic one?
10. Identify by name rocks R4 through R9 in the table below by first observing its
composition (“felsic”, “mafic” or “intermediate”; there are no ultramafic rocks in this
set), then its predominant grain size (either “coarse” or “fine”). Finally, identify the rock
name using the flowcharts on pages 24 and 26.
Rock #
Felsic, mafic or
intermediate?
Predominant
grain size
Rock name
R4
R5
R6
R7
R8
R9
Tubs 12 – 16
11. In addition to the names in the table above, some volcanic rocks are named for their
texture. Use the flowchart on page 28 to aid your identification. Under the texture
column, write the descriptive word or words from that flowchart that helps you
categorize the rock.
Rock #
R12
Texture
Rock name
R13
R14
R15
R16
Soils and soil classification — Tubs 73 and 74
Soils comprise the weathered bedrock (regolith) and organic material at the
surface of the Earth. Most soil investigations begin with an analysis of the particle
(grain) size distribution, a measurement of the percentage of each category of
primary particles by weight. Most scales used to measure particle sizes (including the
one used here) identify four categories — gravel (cobble, pebble or larger), sand, silt and
clay.
A cross-section of a soil will typically divide the soil into layers called horizons. There
are usually four horizons within a soil: the uppermost O horizon, which is the layer
with organic-rich decomposing material; the A horizon, which is mostly inorganic
particles with air in between the grains; the B horizon, which is nearly all inorganic
particles with water in between the grains; and the bottom C horizon, which is the
regolith with no organic materials, with bedrock below that.
If you classify soils by their grain size distribution, then the triangular soil texture
diagram (on the next page) can be used. This kind of classification, as shown in the
diagram, relies only on the percentages of sand, silt and clay. Note that this information
will be useful for some endeavors (construction) but less useful for others (agriculture).
12. Use the triangular soil texture diagram provided to classify the following soils; these
are not the soils in the tubs.
Soil
% sand
% silt
% clay
A
15
51
34
B
72
14
14
C
47
32
21
Texture
Fertility, drainage and stability of the soil are some very important soil properties
that are assumed to be correlated with the particle size distribution, or texture, of the
soil.
Although precise measurement of the particle size distribution is an involved laboratory
procedure, a good first approximation can be made on site with no more equipment that
your hand and a little water. Instead of determining exact percentages, this field test
(called the three-test technique) determines the textural class of the soil. The
textural classes each have specified ranges of sand, silt and clay percentages (they are the
thick-outlined areas on the triangular soil texture diagram). The boundaries between two
classes of soil are not as distinct as shown; one soil class grades into the adjacent classes.
13. Examine each of the soil samples in the tubs and determine its textural class using
the three-test technique flow chart shown on the next page.
Soil
sample
Ribbon test
Sandiness
Smoothness/
stickiness
Soil texture
1
2
3
4
THREE-TEST TECHNIQUE FOR SOIL TEXTURE ANALYSIS
This field technique comprises three tests determining the relative amounts of clay, sand and silt in a soil,
and classifying the soil TEXTURE accordingly.
Ribbon test — gather a small handful of the soil and moisten (not saturate) it. Clench that handful of
moist soil and see if a continuous ribbon of wet soil is extruded from between your thumb and bent
forefinger.
No ribbon/poor ribbon — go to A
Fair ribbon (short stretches with many breaks) — go to B
Good continuous ribbon — go to C
A. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb
and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — go to D
Negative result (not sandy) — go to E
B. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb
and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — soil is SANDY CLAY LOAM
Negative result (not sandy) — go to F
C. Next, perform the sandiness test — moisten the soil (if necessary) and roll some between your thumb
and forefingers; if the soil feels coarse (lots of graininess) then this is a positive result.
Positive result (sandy) — soil is SANDY CLAY
Negative result (not sandy) — go to G
D. Finally, perform the aggregate test — in a dry sample of the soil, look for aggregates (dirt clods).
No aggregates — soil is SAND
Some aggregates — soil is SANDY LOAM (or LOAMY SAND)
E. Finally, perform the smoothness test — moisten the soil (if necessary) and roll some between your
thumb and forefingers.
Soil feels smooth (no graininess) — soil is SILT (or SILTY LOAM)
Soil feels even slightly coarse — soil is LOAM
F. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your thumb
and forefingers.
Soil feels sticky but not slick — soil is CLAY LOAM
Soil feels sticky and slick — soil is SILTY CLAY LOAM
G. Finally, perform the stickiness test — moisten the soil (if necessary) and roll some between your thumb
and forefingers.
Soil feels sticky but not slick — soil is CLAY
Soil feels sticky and slick — soil is SILTY CLAY
14. Which of the samples (#1, 2, 3 or 4) would have the greatest permeability (= ability
to freely pass water)? Why?
15. Which of the samples (#1, 2, 3 or 4) would most likely be the best soil to grow plants
in? Why isn’t this the same answer as question 14?
Sedimentary rocks — Tubs 20, 21, 22, 23, 24, 26
Note: The tub numbers no longer are the same as the R number, so make sure you are
looking at the R number for each rock.
16. These are the rocks that are made from sediment. For each R sample, use the flowchart
on page 30 to identify its specific name, and in the “characteristics” column, write the
characteristics of the rock (given in the rectangles of the flowchart) that helped you
identify the rock.
Sample #
Characteristics
Rock name
R18
R19
R20
R21
R22
R 24
17. Are there any fossils in the sedimentary rocks? If so, note the rock number and try to
identify what type of organism is represented? Are there any fossils in the igneous rocks?
Why not?