Lab_5_Fall_23-1
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Geology
Date
Dec 6, 2023
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9
Uploaded by LieutenantDoveMaster1013
Name: __________________
Lab day & time: ________________
TA: _______________________
Lab 5: Sedimentary and Metamorphic Rocks
Purpose of the lab:
●
Learn about sedimentary rocks and common sedimentary structures
●
Learn to identify several new metamorphic minerals
●
Study metamorphic rocks formed from a variety of protoliths
Sedimentary Rocks
PART 1: Clastic Sedimentary Rocks
Sedimentary rocks form by cementation of sediment derived from older igneous, metamorphic or
sedimentary rocks, accumulation of material produced by living organisms, or direct precipitation from
aqueous solutions. They can be broadly divided into clastic, biochemical, organic, and chemical subgroups.
In this section, we will focus on clastic sedimentary rocks, which are produced by cementation of sediment.
Individual grains within clastic sedimentary rocks are referred to as clasts. Clastic sedimentary rocks are
classified based on several properties, including the size of the clasts, clast shape, and the clast compositions.
Clast size:
The size of clasts is related to the amount of energy available to transport the
clasts—transport and deposition of larger clasts require high energy environments, such as mountain
rivers, whereas deposition of very fine-grained material requires very low energy environments.
For this
lab, use the provided grain size cards to determine the clast size and report an average size of the
largest clasts.
Clast shape:
Clast shape reflects how far the clasts have been transported. As clasts are transported by
wind or water, they become progressively more rounded. A sediment with very rounded clasts, therefore,
suggests a longer transport distance for the sediment making up the rock. In sandstone, rocks with
rounded clasts are called mature, whereas rocks with angular clasts are called immature.
Clast composition:
Clast composition, like clast shape, can reflect both the distance sediment has been
transported and also how many times it has been eroded and redeposited. Quartz is much stronger than
most other minerals, so as a sediment is transported and reworked, other minerals break down and the
percentage of quartz increases.
For the purposes of this lab, we will just look at whether the clast
composition is homogenous (primarily one rock or mineral) or variable. In summary, clast
composition refers to the mineralogy of the clast. If clasts are all the same mineral composition, then
the composition would be “homogeneous.”
Name
Clast size
Clast shape
Breccia
> 2 mm
angular
Conglomerate
> 2 mm
rounded
Sandstone
1/16 mm to 2 mm
Mature = rounded
Immature = angular
Siltstone
1/256 mm to 1/16 mm
Too small to see
Shale
<1/256 mm
Too small to see
1
Name: __________________
Lab day & time: ________________
TA: _______________________
Use the table and descriptions above, to name the different clastic sedimentary rocks in this section.
For
shale or siltstone samples, you can write N/A for the clast shape and clast composition.
A.
Describe and name each of the sedimentary rocks below:
Sample A1
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
Sample A5:
This is a sample collected from the Santa Ynez Mountains, behind Santa Barbara.
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
Sample E2-5.6:
This sample is from the 98 million year old Dakota Formation, which formed along the
margin of the Western Interior Seaway, an inland sea that split North America in half during the
Cretaceous.
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
Sample A2
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
2
Name: __________________
Lab day & time: ________________
TA: _______________________
Sample 4:
This sample is from the white beach cliffs that line the Santa Barbara coast.
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
Sample E2-5.7:
This is another rock type from the Santa Ynez Mountains, behind Santa Barbara.
Clast size [1]: ______________________
Clast shape (rounded or angular) [1]: _____________________________
Clast composition (homogenous or variable) [1]: _____________________________
Rock name [1]: _____________________________
PART 2: Biochemical, Organic and Chemical Sedimentary Rocks
In this section, you will study biochemical, organic and chemical sedimentary rocks. As with the clastic
sedimentary rocks, rocks in these subgroups form in distinct environments on Earth, making these rocks
useful for determining the setting and climate during rock formation.
Name
hardness
Reacts with HCl
Other
Limestone (calcite)
3
Yes
Rough weathering
Dolomite
3
When powdered
Rough weathering
Gypsum
2
No
Fibrous habit
Halite
2.5
No
3 cleavages at 90º
Chert
6.5–7
No
Conchoidal fracture, variable colors
Coal
< 3
No
Black, low density
A.
Describe how each of the different subgroups of sedimentary rocks form. Include likely settings for
formation of these rocks:
Biochemical: _____________________________________________________________________________________________________
____________________________________________________________________________________________________________________
____________________________________________________________________________________________________________________
________________________________________________________________________________________________________________ [2]
3
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Name: __________________
Lab day & time: ________________
TA: _______________________
Organic: _____________________________________________________________________________________________________
____________________________________________________________________________________________________________________
____________________________________________________________________________________________________________________
________________________________________________________________________________________________________________ [2]
Chemical: _____________________________________________________________________________________________________
____________________________________________________________________________________________________________________
____________________________________________________________________________________________________________________
________________________________________________________________________________________________________________ [2]
B.
For each sample, determine the rock name, list the properties you used to uniquely identify it (color,
hardness, reaction with HCl, appearance) and describe a likely setting for the formation of this
sample.
Sample E2-5.1
Rock name [1]:
Properties [1]:
Setting [1]:
Sample E2-5.2
Rock name [1]:
Properties [1]:
Setting [1]:
Sample E2-5.3
Note that this sample includes two types of this rock type.
Rock name [1]:
Properties [1]:
Setting [1]:
4
Name: __________________
Lab day & time: ________________
TA: _______________________
Sample E2-5.4
Rock name [1]:
Properties [1]:
Setting [1]:
Sample MR1807-01
Rock name [1]:
Properties [1]:
Setting [1]:
PART 3: Sedimentary Structures
Sedimentary structures preserved in the rock record can be useful for inferring the environment a sample
formed in. Ripples can form when water or wind move silt or sand sized sediment. You have likely seen ripples
walking on the beach. Asymmetric ripples form and migrate by sediment moving up the shallow side of the
ripple, then avalanching over the steep side (see figure below). As the sediment grains avalanche over the
steeper side, they make concave upward surfaces. The orientation of the concave surfaces can be used to
determine the direction the water or wind were moving and which direction was up when the ripples formed
(tectonic activity can lead to packages of rocks being completely overturned!).
Figure from Marshak, 2016
A.
Ripples formed by flow of water or wind in one direction (e.g., in a river) tend to be asymmetrical, with
a shallow and step side. In contrast, ripples formed by waves action can be either asymmetrical or
symmetrical. Observe samples
E2-5.5
and
SS37.7
. Determine whether the ripples are symmetrical or
asymmetrical for each sample. If they are asymmetrical, use the shape of the ripples to determine the
direction of flow, relative to the north arrow on the sample.
E2-5.5:
Symmetrical or asymmetrical (circle one) [1]
Flow direction (if asymmetric) [1]:
SS37.7:
Symmetrical or asymmetrical (circle one) [1]
Flow direction (if asymmetric) [1]:
5
Name: __________________
Lab day & time: ________________
TA: _______________________
B.
Ripples often appear in the geologic record as cross beds. Cross bedding represents a cross section
through a series of ripples (see the figure above). Cross beds are often used to determine which way
was up when a sedimentary rock formed. Observe the polished slab of rock at this station. Based on
the figure above, determine which way is up on the polished sample.
Make a sketch of the polished
sample, oriented so that it is in an upright position. Indicate the flow direction when these cross beds
formed on your sketch.
[2]
Metamorphic rocks
PART 4: Common metamorphic minerals
In additional to the minerals you have already learned, metamorphic rocks can include a range of other
interesting minerals (these minerals can also appear in igneous rocks, but are less common)
A.
Describe the four common metamorphic minerals at this station. To save time, we will just focus on a
few key properties (color, habit and hardness).
Kyanite
Color [1]:
Habit [1]:
Hardness [1]:
Hint: Kyanite has different hardness along versus across the mineral. Be sure to measure and record both.
Andalusite
Color [1]:
Habit [1]:
Hardness [1]:
Hint: Notice the black X in the middle of the andalusite. This is a common characteristic.
6
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Name: __________________
Lab day & time: ________________
TA: _______________________
Staurolite
Color [1]:
Habit [1]:
Hardness [1]:
Hint: Be sure to notice the intersecting grains of staurolite. This is a common texture, which you should note under habit.
Garnet
Color [1]:
Habit [1]:
Hardness [1]:
B.
Andalusite, kyanite and sillimanite have the same chemical formula (Al
2
SiO
5
), but different mineral
structure. Which mineral forms is dependent of the pressure and temperature during metamorphism
(see above).
Based on your observations above, do the minerals look similar? Explain why you might
expect them to look similar or different.
[2]
PART 5: Metamorphic Minerals in Hand Sample
In this section, you will practice identifying metamorphic minerals in hand samples. It is common in
metamorphic rocks to have larger, well-formed grains of one or more minerals in a fine-grained
groundmass. We call the larger grains
porphyroblasts
(these are similar to phenocrysts in volcanic
rocks).
A.
Identify the most abundant porphyroblast in each sample and list the properties that allow you to
uniquely identify the mineral.
Sample S70
7
Name: __________________
Lab day & time: ________________
TA: _______________________
Porphyroblast mineral [1]: _______________________________
Porphyroblast properties [1]:
Sample 3
Porphyroblast mineral [1]: _______________________________
Porphyroblast properties [1]:
Sample E2-5.8
Porphyroblast mineral [1]: _______________________________
Porphyroblast properties [1]:
B.
What fine-grained mineral in the groundmass leads to the shiny, silver appearance of these samples?
Hint: Look back at your Lab 3 mineral identification notes.
[1]
PART 6: Metapelites
Metapelites are metamorphic rocks formed from a shale or mudstone (pelite) protolith. Shales are
principally composed of fine-grained clay minerals. Clays are aluminum-rich, which leads to the
formation of Al-rich minerals. Metapelites progress through a series of different textures as they are
metamorphosed at higher pressures and temperatures (i.e., higher grades).
A.
Study the metapelite samples at this station. Record the sample number corresponding to each rock
name in the table below..
Name
Description
Grade
Sample
(fill in below)
Shale
Fine grained, soft
Unmetamorphosed
[1]
Slate
Fine grained, harder, platy appearance
Low
[1]
Phyllite
Silver sheen, but no large porphyroblasts
Low-medium
[1]
Schist
Formation of larger porphyroblasts
medium
[1]
Gneiss
Formation of composition (black-white)
bands
High
[1]
8
Name: __________________
Lab day & time: ________________
TA: _______________________
PART 7: Other Protoliths
The mineralogy of a metamorphic rocks depends on the pressures and temperatures of metamorphism
and
the composition of the starting rock (protolith).
A.
Use your knowledge of mineral properties to match each of the samples at this station to the starting
composition of the rock. Record your answers in the table below.
Protolith
Rock type
Mineralogy
Sample
(fill in)
basalt
amphibolite
Amphibole + plagioclase
[1]
quartz sandstone
quartzite
quartz
[1]
limestone
marble
calcite
[1]
shale
schist
See previous sections
[1]
9
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