Based on the position of the red line in Figure 3. The dashed blue lines labelled a, b, c, d in Figure 3. See Appendix 3 for Exercise 3. Figure 3. You can use that when trying to estimate the ferromagnesian mineral content of actual rocks, and you can get some practice doing that by completing Exercise 3. Be warned! Geology students almost universally over-estimate the proportion of dark minerals. The four igneous rocks shown below have differing proportions of ferromagnesian silicates.
Estimate those proportions using the diagrams in Figure 3. Igneous rocks are also classified according to their textures. Almost all intrusive igneous rocks have crystals that are large enough to see with the naked eye, and we use the term phaneritic from the Greek word phaneros meaning visible to describe that.
Typically that means they are larger than about 0. If the crystals are too small to distinguish, which is typical of most volcanic rocks, we use the term aphanitic from the Greek word aphanos — unseen The intrusive rocks shown in Figure 3.
In general, the size of crystals is proportional to the rate of cooling. The longer it takes for a body of magma to cool, the larger the crystals can grow. It is not uncommon to see an intrusive igneous rock with crystals up to 1 centimetre cm long. In some situations, especially toward the end of the cooling stage, the magma can become water rich. The presence of liquid water still liquid at high temperatures because it is under pressure promotes the relatively easy movement of ions, and this allows crystals to grow large, sometimes to several centimetres Figure 3.
Finally, as already described, if an igneous rock goes through a two-stage cooling process, its texture will be porphyritic Figure 3. Some coastal areas are dominated by erosion, an example being the Pacific coast of Canada and the United States, while others are dominated by deposition, examples being the Atlantic and Caribbean coasts of the United States. But on almost all coasts, both deposition and erosion are happening to varying degrees most of the time, although in different places.
These are both made up words used to indicate the chemical composition of silicate minerals , magmas, and igneous rocks. Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements. The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but mafic magmas also are relatively enriched in calcium and sodium.
The most common felsic rock is granite , which represents the purified end product of the earth's internal differentiation process. It is important to note that there are many intermediate steps in the purification process, and many intermediate magmas which are produced during the conversion from mafic to felsic. We call the magmas associated with these intermediate stages "intermediate.
Click here to ask GeoMan a question. Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks.
A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in.
An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff. A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture.
However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic.
Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture. Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it.
A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter and sometimes is much larger.
The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite. The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic. Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg.
Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions. Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray.
Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black. The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown. The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition.
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