Igneous Rocks Research Paper
Igneous Textures
Cooling rates influence igneous rock textures. Rapid cooling at the earth's surface results in tiny mineral crystals that can only be seen under a microscope. The resulting extrusive igneous rocks have an aphanitic texture. Rocks that form as magma cools slowly underground (intrusive igneous rocks) develop phaneritic textures, in which individual mineral crystals can be seen without a microscope.
Some igneous rocks exhibit visible crystals (phenocrysts) in a matrix of tiny crystals (groundmass), or a porphyritic texture. The magma cooled slowly underground (allowing the larger crystals to form) and then quickly at the Earth's surface (where the tiny crystals formed). When magma cools too quickly for any crystals to grow, the resulting texture is glassy.
Some magma contains large …show more content…
amounts of gas. During volcanic eruptions, these gasses expand rapidly from the lava leaving voids called vesicles. A rock containing vesicles has a spongy or vesicular texture. Expanding gasses often contribute to explosive volcanic eruptions, during which rocks are blown into smaller, angular pieces. Tremendous heat from the eruption may weld these pieces together, creating a pyroclastic texture. A pyroclastic texture typically contains welded rock fragments and ash.
If you find rocks with vesicular or pyroclastic textures near a volcano, you can infer that the volcano erupted violently in the past. If it still active, the volcano may erupt violently again, posing a substantial threat to its surrounding environment.
Mineral Composition of Igneous Rocks
Magmas have a wide variety of chemical compositions, depending upon the rocks and conditions from which they formed. Geologists chemically classify magmas and the igneous rocks they form in terms of silica (SiO2) content. Recall that quartz has the chemical formula SiO2; thus it is 100% silica. Other silicate minerals in igneous rocks contain less silica as a percentage of their total chemical makeup. Magmas with high (about 70%), intermediate (about 60%), or low (about 50%) silica content are termed felsic, intermediate, or mafic, respectively. Table’s 3-1A and 3-1B list minerals commonly found in rocks derived from these magmas. Note that all of the minerals in these tables are silicates, which we covered in Chapter 2.
In the 1920s and 1930s, Norman Bowen conducted a series of laboratory experiments with basaltic magma. As the magma cooled, silica-deficient (mafic) minerals crystallized first, at relatively high temperatures. The remaining liquid thus became more silica enriched. With cooling, progressively more felsic minerals formed. In nature, minerals forming early may react with remaining magma or settle out in a magma chamber. Olivine (most mafic) generally forms first, and quartz (most felsic) forms last. The complete sequence of mineral crystallization, discussed in detail in your textbook, is known as Bowen’s Reaction Series.
The predominant minerals in felsic, intermediate, and mafic rocks strongly influence the colors of these rocks. Generally light in color, felsic rocks may be speckled with black, white, pink, and clear spots (if they are phaneritic), or predominantly white, light gray, or pink (if they are aphanitic). Intermediate rocks may have a black and white speckled appearance (if phaneritic), or a predominantly dark gray, greenish, or brown color (if aphanitic). Abundant ferromagnesian minerals in mafic rocks give them a generally dark, blackish color. Obsidian is a notable exception to the general trends above. Typically, obsidian is felsic in composition, but dark in color due to ash impurities.
Identifying Igneous Rocks
Q3-1: Examine Specimens 40 through 52 (the igneous rocks in your kit). For some specimens, it may be helpful to use the binocular microscopes to evaluate mineral composition. First, list the ID numbers of the specimens having each of the following textures. If a specimen has a texture “in between” aphanitic and porphyritic; you may assign it to either category.
Specimens with phaneritic texture (three total): __44__ _45___ __52__
Specimens with aphanitic texture (two or three total): _40__ _47__ _42__
Specimens with porphyritic texture (two or three total): __44_ __47_ __43_
Specimen with glassy, but not vesicular texture (one total): _51__
Specimen with vesicular texture, dark in color (one total): _42__
Specimen with vesicular texture, light in color, may appear glassy under magnification (one total): __52__
Q3-2: Igneous rocks are identified as: Felsic, Intermediate or Mafic (see Table 3-1).
Organize the specimen into three piles in front of you. Then write down the ID numbers of specimens 40-52 in the three rows below. Felsic specimens: _40__ _43___ _46___ _51___
Intermediate specimens: __45__ _52___ _50___ __48__
Mafic specimens: __42__ _41___ _49__ __47__
Igneous rocks are identified by their texture and composition (see Tables 3-1 and 3-2). First identify the texture of each specimen: aphanitic, phaneritic, porphyritic, glassy, vesicular aphanetic or vesicular glassy. The next step is to identify the composition (Table 3-1 and 3-2) in each specimen. Place each specimen on Table 3-2 with respect to its texture and composition.
Q3-3: Use Table 3-2 to identify the names of specimens 40-52; then enter the names of the specimen on Table 3-3
Q3-4: Next for specimen 40-52 based on your work in Q3-2 determining whether the specimens are felsic, intermediate, or mafic - place your final answers along with the name of the specimen on table 3-3.
Table 3-1. Igneous Rock Names, Texture, and Mineral Composition Felsic Intermediate Mafic
Composition High silica content, about 70%; generally light in color; common minerals include muscovite, quartz, potassium feldspar, sodium-rich plagioclase feldspar, biotite, and amphibole) Intermediate silica content, about 60%; common minerals include quartz, plagioclase feldspar containing similar amounts of sodium and calcium, biotite, amphibole, and pyroxene) Low silica content, about 50%; generally dark in color; common minerals include calcium-rich plagioclase feldspar, amphibole, pyroxene, and olivine)
Intrusive Granite (phaneritic texture) Diorite (phaneritic texture) Gabbro (phaneritic texture)
Extrusive Rhyolite (aphanitic texture)
Pumice1 (vesicular and glassy texture)
Obsidian2 (glassy texture) Andesite (aphanitic texture) Basalt (aphanitic texture)
Scoria1 (vesicular texture)
Porphyritic Exhibits characteristics that are both intrusive and extrusive (porphyritic granite or porphyritic rhyolite) Exhibits characteristics that are both intrusive and extrusive (porphyritic diorite or porphyritic andesite) Exhibits characteristics that are both intrusive and extrusive (porphyritic gabbro or porphyritic basalt)
1. Pumice and Scoria have variable silica content, but are usually felsic and mafic, respectively. Pumice is less dense than scoria, may appear glassy under magnification and may float in water. Scoria usually does not float on water, is highly vesicular, may or may not contain crystals (phenocrysts) and is typically dark brown, black or purplish red. Composition can be either andesitic or basaltic.
2. Most obsidian is felsic, but dark in color due to ash impurities in the rock.
Cooling rates influence igneous rock textures. Rapid cooling at the earth's surface results in tiny mineral crystals that can only be seen under a microscope. The resulting extrusive igneous rocks have an aphanitic texture. Rocks that form as magma cools slowly underground (intrusive igneous rocks) develop phaneritic textures, in which individual mineral crystals can be seen without a microscope.
Some igneous rocks exhibit visible crystals (phenocrysts) in a matrix of tiny crystals (groundmass), or a porphyritic texture. The magma cooled slowly underground (allowing the larger crystals to form) and then quickly at the Earth's surface (where the tiny crystals formed). When magma cools too quickly for any crystals to grow, the resulting texture is glassy.
Some magma contains large …show more content…
amounts of gas. During volcanic eruptions, these gasses expand rapidly from the lava leaving voids called vesicles. A rock containing vesicles has a spongy or vesicular texture. Expanding gasses often contribute to explosive volcanic eruptions, during which rocks are blown into smaller, angular pieces. Tremendous heat from the eruption may weld these pieces together, creating a pyroclastic texture. A pyroclastic texture typically contains welded rock fragments and ash.
If you find rocks with vesicular or pyroclastic textures near a volcano, you can infer that the volcano erupted violently in the past. If it still active, the volcano may erupt violently again, posing a substantial threat to its surrounding environment.
Mineral Composition of Igneous Rocks
Magmas have a wide variety of chemical compositions, depending upon the rocks and conditions from which they formed. Geologists chemically classify magmas and the igneous rocks they form in terms of silica (SiO2) content. Recall that quartz has the chemical formula SiO2; thus it is 100% silica. Other silicate minerals in igneous rocks contain less silica as a percentage of their total chemical makeup. Magmas with high (about 70%), intermediate (about 60%), or low (about 50%) silica content are termed felsic, intermediate, or mafic, respectively. Table’s 3-1A and 3-1B list minerals commonly found in rocks derived from these magmas. Note that all of the minerals in these tables are silicates, which we covered in Chapter 2.
In the 1920s and 1930s, Norman Bowen conducted a series of laboratory experiments with basaltic magma. As the magma cooled, silica-deficient (mafic) minerals crystallized first, at relatively high temperatures. The remaining liquid thus became more silica enriched. With cooling, progressively more felsic minerals formed. In nature, minerals forming early may react with remaining magma or settle out in a magma chamber. Olivine (most mafic) generally forms first, and quartz (most felsic) forms last. The complete sequence of mineral crystallization, discussed in detail in your textbook, is known as Bowen’s Reaction Series.
The predominant minerals in felsic, intermediate, and mafic rocks strongly influence the colors of these rocks. Generally light in color, felsic rocks may be speckled with black, white, pink, and clear spots (if they are phaneritic), or predominantly white, light gray, or pink (if they are aphanitic). Intermediate rocks may have a black and white speckled appearance (if phaneritic), or a predominantly dark gray, greenish, or brown color (if aphanitic). Abundant ferromagnesian minerals in mafic rocks give them a generally dark, blackish color. Obsidian is a notable exception to the general trends above. Typically, obsidian is felsic in composition, but dark in color due to ash impurities.
Identifying Igneous Rocks
Q3-1: Examine Specimens 40 through 52 (the igneous rocks in your kit). For some specimens, it may be helpful to use the binocular microscopes to evaluate mineral composition. First, list the ID numbers of the specimens having each of the following textures. If a specimen has a texture “in between” aphanitic and porphyritic; you may assign it to either category.
Specimens with phaneritic texture (three total): __44__ _45___ __52__
Specimens with aphanitic texture (two or three total): _40__ _47__ _42__
Specimens with porphyritic texture (two or three total): __44_ __47_ __43_
Specimen with glassy, but not vesicular texture (one total): _51__
Specimen with vesicular texture, dark in color (one total): _42__
Specimen with vesicular texture, light in color, may appear glassy under magnification (one total): __52__
Q3-2: Igneous rocks are identified as: Felsic, Intermediate or Mafic (see Table 3-1).
Organize the specimen into three piles in front of you. Then write down the ID numbers of specimens 40-52 in the three rows below. Felsic specimens: _40__ _43___ _46___ _51___
Intermediate specimens: __45__ _52___ _50___ __48__
Mafic specimens: __42__ _41___ _49__ __47__
Igneous rocks are identified by their texture and composition (see Tables 3-1 and 3-2). First identify the texture of each specimen: aphanitic, phaneritic, porphyritic, glassy, vesicular aphanetic or vesicular glassy. The next step is to identify the composition (Table 3-1 and 3-2) in each specimen. Place each specimen on Table 3-2 with respect to its texture and composition.
Q3-3: Use Table 3-2 to identify the names of specimens 40-52; then enter the names of the specimen on Table 3-3
Q3-4: Next for specimen 40-52 based on your work in Q3-2 determining whether the specimens are felsic, intermediate, or mafic - place your final answers along with the name of the specimen on table 3-3.
Table 3-1. Igneous Rock Names, Texture, and Mineral Composition Felsic Intermediate Mafic
Composition High silica content, about 70%; generally light in color; common minerals include muscovite, quartz, potassium feldspar, sodium-rich plagioclase feldspar, biotite, and amphibole) Intermediate silica content, about 60%; common minerals include quartz, plagioclase feldspar containing similar amounts of sodium and calcium, biotite, amphibole, and pyroxene) Low silica content, about 50%; generally dark in color; common minerals include calcium-rich plagioclase feldspar, amphibole, pyroxene, and olivine)
Intrusive Granite (phaneritic texture) Diorite (phaneritic texture) Gabbro (phaneritic texture)
Extrusive Rhyolite (aphanitic texture)
Pumice1 (vesicular and glassy texture)
Obsidian2 (glassy texture) Andesite (aphanitic texture) Basalt (aphanitic texture)
Scoria1 (vesicular texture)
Porphyritic Exhibits characteristics that are both intrusive and extrusive (porphyritic granite or porphyritic rhyolite) Exhibits characteristics that are both intrusive and extrusive (porphyritic diorite or porphyritic andesite) Exhibits characteristics that are both intrusive and extrusive (porphyritic gabbro or porphyritic basalt)
1. Pumice and Scoria have variable silica content, but are usually felsic and mafic, respectively. Pumice is less dense than scoria, may appear glassy under magnification and may float in water. Scoria usually does not float on water, is highly vesicular, may or may not contain crystals (phenocrysts) and is typically dark brown, black or purplish red. Composition can be either andesitic or basaltic.
2. Most obsidian is felsic, but dark in color due to ash impurities in the rock.