UNIT 10A. Mini Field Trip to Study Granitic Structures and Textures
7/14/14
1. RATIONALE: Igneous minerals, textures and structures are best studied in the field.
ll. Topics, Objectives, and Learning Activities
Topic A: Igneous Minerals and Textures that are commonly found in Granite: (tentative 25 points):
Objective for Topic A: Recognize textures and structures commonly found in granite
Equipment and Location A: Ingleside on the Bay Breakwater, field books, camera; wear shoes that will
not slip on angled rock surfaces; sunglasses, hat, dress appropriate for the weather
Introduction A: This mini-field trip will be scheduled during a lab period, in which the students will
meet at 1:00 PM at Ingleside of the Bay, TX to study these textures
- Ingleside On The Bay Breakwater – maximum 35 minute drive from DMC East
- GPS (27o 49’ 33”N; 97o 13’ 27”W)
- Location contains a large number of granite blocks used as a “breakwater” located by Ingleside on
the Bay – Bayshore Drive alongside the Ingleside Beach Club (Maps below).
- Each of the many, large granite blocks here seem to have their own unique geological history and
cooling and crystallization environments.
Learning Activities A:
- Below are a list of terms that all working geologists know by heart, and so should you
- Except for the first two terms, we will see examples of all the remaining terms
Batholith = intrusive whose area is larger than 100 square kilometers (36 Sq. Mi.)
Roof pendent = is a mass of “country rock” that projects downward and is entirely surrounded by
an igneous intrusion (but the upper portion of the country rock may be exposed). The pendent
usually is subjected to contact metamorphism.
Xenoliths = (foreign rock) rock fragment within an intrusive igneous body that is unrelated to the
igneous body itself and is usually greatly altered; (question: are the xenoliths older or younger
than the intrusive?)
Dike = is a sheet of rock that formed in a crack in a pre-existing rock
Pegmatite = is an intrusive igneous rock with very large crystals that forms in the later stages of a
magma chamber’s crystallization, usually associated with mineral-rich hydrothermal fluids
Phaneritic = crystal sizes large enough to be distinguished with the unaided eye which is formed
by the slow cooling of a magma deep underground
Phenocrysts = a relatively large and usually conspicuous crystal distinctly larger than the grains
of the rock groundmass of an igneous rock
Porphyritic = characterized by large crystals surrounded by a background of material with much
smaller crystals; indicates the magma moved from a deep, hotter setting to a shallower, cooler
setting before the magma solidified.
Poikilitic = larger anhedral crystals that encloses multiple subhedral minerals of a different type
Overgrowth (Corona) Texture = where a crystal host has a rim of another mineral; represents a
later crystallizing phase (i.e. plagioclase crystal surrounded by orthoclase crystals
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- The following website list a lot more igneous textures:
http://www.d.umn.edu/~mille066/Teaching/2312_09/IgTexGlos.pdf
- Minerals present include: orthoclase and plagioclase feldspars, quartz, smoky quartz biotite,
hornblende, and pyrite among others.
- This exercise gives you an opportunity to see these above features in rocks instead of words on a
paper. Surprisingly, there are very few good internet pictures to explain these features.
- You should take pictures (or draw) as many of these as features as possible for your report.
- After I help you find these features, see how many more you can find by yourself or with your
group, for your report.
- This mini-field trip report will be similar the other mini-trip reports. Here I am more interested
in pictures or drawings. With the pictures and drawings, I want “arrows with names” drawn to
the various textures and features. With at least one picture, I want named arrows pointed to the
various minerals that we see in the groundmass. (With pegmatites, I want arrows to minerals
like quartz, orthoclase and pyrite or sulfides, etc.). Shared pictures are OK but I want each
student to make their own notations.
Typed Report after visit: To include:
1) Cover Page with title, author, date:
2) Conclusions of two-three sentences about your trip;
3) Your observations to include textures, minerals, structures, weathering, etc. with annotated
photographs or drawings.
4) I expect to see most of the underlined definitions above used in your report
5) Map to location
6) Bibliography to include URLs to websites that you visited before the trip
(Location maps follow)
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Ingleside On The Bay Breakwater (black arrow)
(Driving instructions from Ingleside is on the next page).
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Ingleside On The Bay Breakwater Location – 35 minute drive from DMC
From 361, turn right by traffic light and DQ onto 1069, drive about 3.4 miles to Starlight Drive, go
to the end of Starlight drive and turn left onto Sandpiper; turn to the first right on Woodcrest and
drive to the end, turn right on Bayshore for a block where you see a steel wall and the granite blocks;
park at the end of the steel wall.
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lll. Evaluation (Method) 10A : A report that counts a tentative 25 points toward mini-field trip requirement
lV. Remediation (method) 10A Repeat learning activities as needed
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