Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive, lava flow or pyroclastic. Texture in this context is not whether the rock feels rough or smooth to the touch.
Igneous texture terms have objective definitions that refer only to igneous rocks. Let us start with textures associated with rocks formed by lava flows. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye.
This is sometimes referred to as a fine-grained igneous texture. Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.
This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice.
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.
What was the same about the rocks? What was different? What characteristics did they think would be useful in identifying their rocks? Make a list of their ideas to refer to during the investigation. Tell your students that they will be investigating this question and at the end of their investigations they will be able to provide reliable answers.
Here are some initial questions that your students can discuss, in pairs, groups and as a whole class:. Have your students report out their ideas and make a list of them. Give each group a new set of rocks you can switch rocks from group to group and ask the students to group the rocks based upon their characteristics.
Ask them to give a reason for why they placed each rock in its group. Complete this investigation by asking your students to reflect on this question and how their answers may have changed as a result of this investigation. Crystals in metamorphic rocks are often arranged in bands.
The following passage provides more detailed information related to this investigation that you may choose to explain to your students. Sedimentary rock is often found in layers. One way to tell if a rock sample is sedimentary is to see if it is made from grains. Some samples of sedimentary rocks include limestone, sandstone, coal and shale. Here it cools and crystallizes into rock. Look for crystals in igneous rocks. Examples of igneous rocks are gabbro, granite, pumice and obsidian.
Metamorphic rocks are rocks that have become changed by intense heat or pressure while forming. One way to tell if a rock sample is metamorphic is to see if the crystals within it are arranged in bands. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock.
Common Sedimentary Rocks Are there geologic maps or publications for where I live? Detailed geologic mapping has not been completed for the entire United States, but maps are available for most locations. Geologic maps at many scales and from many sources are listed in the National Geologic Map Database.
Download digital geologic maps for entire states Where can I find information about the geology and natural history of National Parks? Our National Parks are the showcases of our nation's geological heritage. The National Park Service has websites for most individual parks that include information about their geology and natural history. The website has listings for regions of the country What is the difference between a rock and a mineral?
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Common minerals include quartz, feldspar, mica, amphibole, olivine, and calcite.
A rock is an aggregate of one or more minerals, or a body of undifferentiated mineral What are metamorphic rocks? Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous , sedimentary , or earlier metamorphic form.
Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors.
Conditions like these are Filter Total Items: 2. View Citation. Date published: April 4, Date published: September 29, Filter Total Items: List Grid. May 25, July 20,
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