ES1022B_lab2_igneousrocks_student_2024-1
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Apr 3, 2024
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Background Pages 1 EARTH SCIENCES 1022B LAB 2: IGNEOUS ROCKS (Note: Hand-in sheets due at the end of this lab period)
Igneous rock:
A rock that solidified from mainly molten material (magma). General Interest: Igneous rocks have commercial value as ornamental facing stone on banks and office buildings, headstones etc., especially coarse grained (phaneritic) intrusive rocks. Granite, diorite, and gabbro and porphyritic varieties of these rocks are quarried in Canada. In London, grey diorite facing stone can be seen in the TD Bank building at Dundas and Talbot. Gabbro (specifically, anorthosite, featuring iridescent blue crystals of plagioclase) can be seen in the Bank of Montreal building at Dundas and Wellington. ORIGIN OF IGNEOUS ROCKS
Intrusive (plutonic):
Magma crystallizes slowly, deep below ground surface; rocks tend to have a phaneritic texture due to slow cooling (see below). Extrusive (volcanic):
Magma is extruded onto the ground surface where it becomes lava. Lava rocks usually have an aphanitic to glassy texture (due to rapid cooling). Explosive eruptions can produce large amounts of pyroclastic material - fragments of volcanic rock and fine-grained ash (produced by the pulverization of glassy material such as pumice). TEXTURES OF IGNEOUS ROCKS
Main Textures: There are two main textures observed in igneous rocks, as defined by the average size of their constituent crystals – phaneritic and aphanitic. Glassy texture can be considered as sort of an extreme version of aphanitic. Phaneritic:
Interlocking crystals of about equal size that are easily visible to the unaided eye. Aphanitic:
Interlocking crystals, the overwhelming majority of which, are too small to see without a microscope; rock appears massive (i.e. structureless). Additional Textures: Glassy:
Non-crystalline (amorphous) structure like glass. Obvious in solid, massive form (e.g. in obsidian). May be difficult or even impossible to be directly observed in rocks showing vesicular or pyroclastic texture (in which the glassy material forms a fine meshwork or a solid mass of fine-grained particles (see below). Special Textures: In addition to the main textures described above, igneous rocks can have additional distinctive textures that provide information of how they formed. The following textures generally occur in combination with aphanitic or glassy textures. Porphyritic:
>10% of the rock volume comprises large crystals (called phenocrysts) surrounded by a (usually) aphanitic matrix of smaller crystals (groundmass). Vesicular:
Containing lots of holes (made by trapped gas bubbles). This texture commonly occurs in rocks also characterized by glassy or aphanitic texture. Pyroclastic (fragmental):
Composed of angular fragments of broken volcanic rock and fine-grained (dust-sized) ash particles that may be fused (welded) together. The poorly sorted, jumbled, nature of the particles can impart an “earthy” look that resembles concrete or clay brick. If the majority of the largest particles are < 4mm in diameter, the rock is called tuff If the majority of the largest particles are > 4mm in diameter, the rock is called volcanic breccia.
Background Pages 2 COMPOSITION OF IGNEOUS ROCKS
Mineralogy:
Orthoclase, plagioclase, olivine, pyroxene, amphibole, quartz, and mica (biotite and muscovite) constitute over 95% (by volume) of all igneous rocks. As magma cools, these minerals crystallize in order according to Bowen's Reaction Series (see also Figure 1 below). . Figure 1: A summary depiction of Bowen’s Reaction Series, showing approximate temperatures at which common minerals of igneous rocks crystallize from magma, and rock compositions, corresponding to the minerals present. Note that each pair of rock types (in brackets) have the same composition, but differ in texture phaneritic/aphanitic). See Figure 2 for rock names incorporating specific textures. Parent Magma Composition:
a) Felsic
- magmas rich in silica (and low Fe and Mg content) produce a high proportion of light-coloured minerals such as quartz, orthoclase (potassium feldspar) and Na-plagioclase feldspar, and tend to produce rocks that are light-coloured overall
. b) Intermediate
- magma composition between felsic and mafic, producing rocks that are mid-way between light and dark overall (see tips below for rocks of phaneritic vs. aphanitic texture). c) Mafic
- magmas rich in Fe and Mg (and lower silica content) produce a high proportion of dark-coloured minerals such as pyroxene, amphibole and Ca-plagioclase feldspar, and tend to produce rocks that are dark-coloured overall
. d) Ultramafic (rare) – magmas directly sourced from Fe-Mg-rich mantle with very little modification in composition, producing nearly exclusively ferromagnesian (Fe,Mg-rich) minerals such as olivine (with olive green colour) and pyroxene (dark green to black colour). Some tips: In phaneritic rocks
, composition can be determined by the relative proportions of light vs. dark-coloured mineral crystals. Felsic: light mineral crystals > dark mineral crystals (note that pink to light red is still considered “light-coloured.”) Intermediate: light mineral crystals approx. equal to dark mineral crystals
Background Pages 3 Mafic: dark mineral crystals
> light mineral crystals In aphanitic rocks
, composition can be determined by the overall “tone” of the rock. In general: Felsic = white to light grey or pink Intermediate =
medium-grey Mafic aphanitic rocks =
dark-grey to black One notable exception
to this colour-composition relationship is obsidian
, a natural volcanic glass that tends to be dark-coloured regardless of its composition (obsidian is usually black, even though it is almost always forms from felsic lava). Figure 2:
Classification chart for igneous rocks, showing relationships between overall rock colour (light to dark), composition of parent magma (felsic, intermediate, mafic), specific mineral content, and names of igneous rocks according to these criteria.
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HAND-IN PAGES Student Name:______________________ Student Number: _____________________ 1 of 3 Lab Section (indicate yours) Tuesday Wednesday Thursday Friday Sec. 002 (10:30-12:30) Sec. 003 (1:30- 3:30 ) Sec. 005 (1:30 - 3:30 ) Sec. 007 (9:30 -11:30) Sec. 004 (3:30 - 5:30) Sec. 006 (3:30 - 5:30) Earth Sciences 1022B Lab 2: Igneous Rocks Hand-In Sheets The following list of igneous rocks are in your sample study trays, for which questions are to be answered below: I-1, I-2, I-3
, I-6, I-7, I-8, I-9, I-10
, I-11, I-12 and 25 QUESTIONS 1. Identifying Igneous Rock Textures.
In each of the blank boxes, write the name of the texture type (main or special texture as indicated), represented in the igneous rock samples indicated. Sample number Name of Igneous Texture (1 mark each, 10 marks total)
I-2 (main texture): I-6 (main texture): I-8 (main texture): I-9 (main texture): I-7 (main texture): I-11 (special texture): I-3 (special texture): I-1 (main texture): I-12 (special texture): I-10 (special texture): 2. Identifying Igneous Rock Compositions. In each of the blank boxes, write the name of the composition represented in the igneous rock samples indicated (using the compositional terms mafic, intermediate or felsic). Sample number Name of Igneous Composition (1 mark each, 6 marks total)
I-2
I-6 I-8 I-9
I-7
I-11 Continued on next page -
HAND-IN PAGES Student Name:______________________ Student Number: _____________________ 2 of 3 3. Naming Igneous Rocks. In each of the blank boxes, write the name of the rock represented in the igneous rock samples indicated (based on their combined characteristics of texture and composition. Sample number Name of Igneous Rock Type (1 mark each, 6 marks total)
I-2
I-6 I-8 I-9
I-7
I-11 4.
The general rule of “mafic = darker” breaks down when we consider rocks that have even more ferromagnesian silicate mineral content than “normal” mafic rocks (i.e. an ultramafic composition). Examine sample 25, refer to Figures 1 and 2 (above), and answer the following questions (1 mark each, 4 marks total)
: a) Describe the colour of Sample 25? Answer:
______________________ b) This rock is composed of nearly 100% of which ferromagnesian mineral (which is why it has this distinctive colour)? Answer: ______________________ c) What is the texture of this igneous rock? Answer: ______________________ d) What is the name of this igneous rock? Answer: ______________________ 5.
The Skaergaard intrusion is an enormous, funnel-shaped igneous intrusion (10 x 6 km in size), exposed on the east coast of Greenland. Intruded into older rocks (specifically gneiss (metamorphic) and basalt (mafic volcanic rock)), magma entered the magma chamber via fractures (now preserved as “feeder dykes”). The intrusion has an unusual, layered internal structure (see Figure 3, below), due to different minerals having crystallized at different temperatures within the magma chamber. Use Figures 1 and 2 in the background pages (above) and Figure 3 (below) to help you answer the questions below (
1 mark / blank, 10 marks total)
. Figure 3 (left):
Schematic cross-section view of the Skaergaard Intrusion of Greenland.
HAND-IN PAGES Student Name:______________________ Student Number: _____________________ 3 of 3 a) If magma entering the feeder dykes and lower zone of the intrusion had an estimated average temperature of about 1100-1200°C, what are the three main minerals that would you expect to have crystallized in the lower zone of the intrusion?
(Use Figure 1 in outline.) Answers: _______, _____________ and ________________ b) What is the type of igneous texture (one of the main types) you would expect to see in rocks of the lower zone of the intrusion
?
Answer:
_____________________
c) What is the name of the igneous rock would you expect to find in the lower zone of the intrusion?
Answer:
__________________
d) Assuming the magma that produced the middle zone of the intrusion had an estimated average temperature of about 800-900°C, what are the three main minerals you would expect to find in the resulting rock? (Use Figure 1 in outline.) Answer:
________________, ________________
, and ________________
e) What would be the texture of the rock of the middle zone of the intrusion? Answer:
__________________
f) What would be the name of the rock forming the middle zone of the intrusion? Answer:
__________________
Total Possible Marks for Lab: 36
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