Geology 2 Spring 1997
Name:
Lab day & time:
PRELAB #7. IGNEOUS AND METAMORPHIC ROCKS
This prelab assignment is due at the beginning of your lab session. You will not be allowed to participate in the lab if you have not completed this prelab exercise. In this lab you will explore two of the three major rock types. Read/ review in Chernicoff Chapters 3, 6 and 7, especially sections about identifying igneous rocks (pages 65 - 72, 75- 78, and 99 - 104) and metamorphic rocks (pages 185 - 189). Answer the following questions.
(The glossary in the back of the book may also help.)
1. Define an igneous rock (page 45).
2. Igneous textures. If you are examining a piece of igneous rock, what would you look for in order to describe its texture?
Describe three different textures and tell what each implies about the origin of the rock.
3. Igneous rock names. When magmas solidify at depth in the earth, they form
plutonic or
intrusive rocks such as those listed below. The same magmas, when they erupt onto the earth's surface cool quickly and form
volcanic or
extrusive rocks. List the names of the volcanic rocks that would form from the magmas that formed these plutonic rocks:
| plutonic rock name | volcanic rock name |
|---|---|
| gabbro | ? |
| diorite | ? |
| granite | ? |
5. Volcanic Eruption Styles. What factors cause some magmas to erupt and form fluid lava flows while others explode to form volcanic ashes and pyroclastic rocks? (pages 69, 72, 78)
6. Define metamorphism (page 127).
7. Describe the
four conditions that promote changes in rocks during metamorphism?
8. Define foliation.
9. Foliated metamorphic rocks derived from shale. When a shale (very fine
grained, clay-rich sedimentary rock) is gradually buried and metamorphosed to higher and higher grades of metamorphism until it becomes a gneiss, what metamorphic rocks are formed along the way (Table 7-1).
10. Unfoliated Metamorphic Rocks. Name the rocks that form when
the following protoliths are metamorphosed:
Limestone
Pure Sandstone
11. Mineral Review Question (Use your mineral ID Tables from Mineral Prelab #1.)
: In each of the following pairs of common rock-forming minerals, the minerals are similar to each other but can be easily distinguished by one or more of the tests or observations that we investigated in the minerals lab. For each pair, consult your mineral ID tables from last week and describe one or more distinguishing propertyes The first one is done for you.
| Minerals | Distinguishing Property |
|
a.
Quartz and Feldspar
(Both light colored. Together, they, make up the bulk of an igneous rock such as granite) |
Feldspar has good cleavage planes; Quartz does not. (so feldspar breaks to form shiny flat surfaces while quartz breaks along irregular, conchoidal fractures.) |
|
b.
Biotite and Muscovite
(These are both micas, common as accessory minerals in many rock types.) |
|
| c.
Calcite and Quartz
(Light colored, either can form the cement and vein fillings in sedimentary and metamorphic rocks.) |
|
| d.
Olivine and Quartz
(Both can be glassy, with conchoidal fractures) |
|
When you examine a piece of igneous rock, one of its most obvious features is its texture. Igneous textures are quite informative since they largely reflect the cooling history of the magma.
Aphanitic (fine-grained) texture. Extrusive rocks cooled quickly and often have a fine
grained texture where individual minerals cannot be seen without a handlens or microscope.
Phaneritic (coarse-grained) texture. Deep-level plutonic rocks generally cooled slowly and have large, visible crystals.
Porphyritic texture. These Igneous rocks have larger crystals set in a finer-grained matrix. This term can be used to describe medium-sized crystals in an aphanitic groundmass or coarse crystals in a phaneritic rock.
Glassy texture. When a magma cools so quickly that no minerals are formed, the rock that results has a glassy texture.
Vesicular texture. When a gas-rich lava cools, some gas bubbles may be trapped in the rocks forming vesicles.
Pyroclastic texture. "Pyro" means fire in Latin and "clastic" means broken, so pyroclastic means a fragmented volcanic rock. These are assemblages of volcanic matter resulting from an explosive eruption.
B. Mineral Composition
The chemical composition of a given magma determines what minerals will crystallize from it as it cools. For example, a melt which is low in silica and relatively rich in iron (Fe) and magnesium (Mg) will favor the crystallization of olivine, pyroxene, plagioclase and possibly amphibole. Note that these are the early forming, high
temperature minerals in the Bowen Reaction Series. A melt rich in silica (SiO2), water, sodium (Na), and potassium (K) will commonly form a rock which has abundant quartz, K-feldspar, plagioclase, biotite and amphibole. These minerals are the late forming, lower
temperature minerals of the Bowen Reaction Series.
To classify an igneous rock you need to identify any minerals that are visible in the rock. The common silicate minerals can be sorted out using the following characteristics:
1. Mafic (dark-colored) minerals; this group of minerals is rich in Fe and Mg and includes:
biotite mica is black and has 1 perfect cleavage and flakes into thin, shiny sheets;
olivine is light olive green to brown and has no cleavage;
hornblende is black-brown and forms elongate grains with diamond cross sections
pyroxene is black and forms stubby grains with square cross sections.
2. Felsic (light-colored) minerals:
quartz has a conchoidal fracture and no cleavage;
feldspar has 2 cleavages intersecting at right angles (but plagioclase cleavage meets at nearly right angles)
plagioclase feldspar has striated surfaces and is often tabular in cross section,
K-feldspar, or orthoclase, is sometimes pink, has no striations and is generally blockier;
muscovite mica has one perfect cleavage and flakes into thin, shiny sheets.
C. Classification
To classify a given igneous rock, note the following:
Texture 1. the size (or sizes) of the mineral grains
2. the presence of vesicles
3. the presence of a fragmental structure
Mineralogy 4. the presence or absence of quartz
5. the type of feldspar present (plagioclase or K-feldspar) if any
6. the types of dark minerals and their proportions
Once you have noted these features you can classify the rock using Table 1. First determine the mineralogy of the rock and find the appropriate column. Then determine the texture of the rock and find the appropriate row. You should be able to pigeon-hole the rock into one of the categories.
TABLE 1: Classification of Igneous Rocks
| Felsic (light) | Intermediate | Mafic (dark) | Ultramafic | K-feldspar >
plagioclase 10-20% quartz 0-10% dark minerals (biotite, hornblende) 0-10% muscovite |
plagioclase>
K-feldspar; 0-10% quartz; 25-40% dark min. (hornblende, pyroxene, biotite) |
30-50% plagioclase
no K-feldspar no quartz 50-70% dark min. (pyroxene > olivine+ hornblende) |
100% dark minerals:
(mostly olivine; some pyroxene or plagioclase) |
|---|---|---|---|---|
| Coarse grained | Granite | Diorite | Gabbro | Peridotite |
| Fine grained | Rhyolite | Andesite | Basalt | Komatiite |
| Glassy | Obsidian | |||
| Vesicular | Pumice | Pumice or Scoria | Scoria | |
| Pyroclastic | Tuff or Breccia | Tuff or Breccia | Tuff or Breccia |
II. Metamorphic Rocks
A. Texture
In
foliated metamorphic rocks, the minerals (especially platy or elongate ones such as mica or hornblende) are oriented in a parallel or subparallel arrangement. Foliated rocks are associated with regional metamorphism although not all regionally metamorphosed rocks are foliated.
Unfoliated metamorphic rocks have no preferred mineral orientation and are often associated with contact metamorphism.
B. Mineral Composition
Metamorphic rocks contain many of the same minerals as described for igneous rocks, above, but they often contain additional minerals that can be used to tell the particular history of that rock. These minerals are typically called
index minerals and are used to describe the
metamorphic grade, or degree of metamorphism, of that rock. Some common index minerals are (from lower to higher grade): chlorite, biotite, garnet, staurolite, kyanite, and sillimanite. In the minerals lab, you saw most of these except for sillimanite. Chlorite can appear in many shades of green; it is a fairly "soft" mineral (hardness of 3) with no obvious cleavage (NOTE: chlorite does have cleavage, but the cleavage planes are generally crushed and difficult to see). Sillimanite is a brownish to greenish mineral, with long, slender crystals (often fibrous crystals); sillimanite will scratch glass (hardness 6-7).
C. Classification
A classification scheme for metamorphic rocks is given in Table 2. First determine if the rock is foliated or unfoliated. If it is foliated, look for color banding, crystal size and visible mica flakes. Second, determine its mineral composition.
Table 2: Classification of Metamorphic Rocks
I. Foliated Rocks
| Crystal Size | Characteristics | Rock Name |
|---|---|---|
| Very fine, not visible | Slaty cleavage, may cross pre-existing layering. Usually dark colored. | Slate |
| Fine-grained, not visible | Well-developed foliation. Rock has "sheen." | Phyllite |
| Medium to coarse, visible with naked eye. | Common minerals include the micas, garnet, and staurolite, kyanite. | Schist |
| Coarse, visible with the naked eye | Well-developed color banding and/or compositional layering. Common minerals include quartz, feldspars, amphibole, and pyroxene. | Gneiss |
II. Unfoliated Rocks
| Mineralogy/Texture | Rock Name | Protolith |
|---|---|---|
| Interlocking mosaic of qtz crystals. Original sand grains commonly still visible. | Quartzite | Quartz Sandstone |
| Interlocking mosaic of calcite crystals. | Marble | Conglomerate |
| Original grains distinguishable. Matrix commonly deformed around pebbles. | Metaconglomerate | Quartz Sandstone |
| Composed of chlorite, amphibole, plagioclase, and Ca-Al-silicates | Greenstone | Basalt or Gabbro |