GEOL 214
Petrology Lab 2: Volcanic processes, textures, and field relations, part 1
Overview: The tasks in today’s lab are two-fold. First, you will be using Google Earth to learn how to
recognize various volcanic field relations and textures. Second, you will be using Google Earth and some
simple mathematical equations to learn about one of the primary factors that influence how magma will
travel – viscosity. You will be learning all of this by examining real volcanic rocks (albeit virtually) that
have erupted within a caldera on the island of Tenerife, Spain.
Due: This lab is due at the beginning of next week’s lab. You will be graded based on the completeness
and accuracy of your work. Please come see me or flag me down if you have questions.
To do: Perform the following tasks and, based on your work, answer the questions at the end of this
packet.
Part 1: Examining lava flows:
1) Using overhead transparencies, geologically map the three lava flows that are in the Google Earth
handouts. Overlay the transparency on the Google Earth sheet and use Sharpies to do your mapping.
Please use as many various colors as you would like.
In your mapping, identify the following:
a) flow boundaries
b) levees
c) source vent
d) flow structures (i.e., ogives or pressure ridges, a’a’, etc…)
e) flow direction (draw arrows to indicate flow direction)
2) Once you have completed your map, put the different flow lobes you have mapped into their proper
stratigraphic order. You will have to label each of the lobes on your map; place you orders in the space
provided below:
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3) Assume that you were able to collect samples from each of the three flows and were able to obtain
the major oxide chemistry for each of the flows –presented below in the table.
Major oxides (wt. %)
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O
CO2
S
F
Eruption temperature
(degrees C)
Flow A
55.74
0.85
18.26
1.51
4.63
0.25
1.01
2.57
8.53
4.82
0.41
0.36
0.02
0.04
0.15
~900
Flow B
46.75
2.30
13.93
3.20
8.08
0.19
9.75
10.08
2.70
0.80
0.35
~1200
Based on these compositions,
a) What is the composition of each flow (as plotted on a TAS diagram)?
Flow A:________________________________________
Flow B:________________________________________
Flow C:________________________________________
b) What minerals and textures would you predict each flow to have?
Flow A:________________________________________
Flow B:________________________________________
Flow C:________________________________________
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Flow C
65.02
0.36
14.88
1.75
3.48
0.13
0.04
1.34
5.90
4.30
0.04
~1000
Part 2: Bringing things together
4) Go back to your map and bring up Google Earth on your computer.
a) Using the ruler function in Google Earth, calculate the volume of each of your three flows and list
them in the space below.
b) Assuming that flows a and c each had a volumetric effusion rate of ~7 m3/s, how long did it take to
emplace each of the flows? Be sure to show your work below.
c) Assuming that flow b had a volumetric effusion rate of ~100 m3/s, how long did it take to emplace this
flow? Be sure to show your work below.
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5) Using the following equation, calculate the viscosity of each of the flows:
 = ( g sin d2) / (3 v)
This equation is known as Jeffrey’s Equation and is used for calculating the viscosity of a fluid in a
channel. Be sure to show your work below.
6) Based on your work from above, what is the greatest factor in controlling a magma’s viscosity?
7) What are some other factors that may also play a role in controlling a magma’s viscosity?
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