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Geology of the Pacific Northwest

Basics -- Rocks and Minerals

Introduction

The solid earth is made of rocks, which are made of minerals. To understand geology, you need to become familiar with rocks and minerals, what they are, how they are identified, and the geologic processes that create them.

Basics Tables for minerals and each of the three rock types will assist you in examining, identifying, and thinking about minerals and rocks. Links to the appropriate table and instructions for its use are located in each rock and mineral section below. This basics page gives you the background needed to understand the terms used in the rock and mineral tables.

What are minerals?

All rocks except obsidian, coal, and amber are made of minerals. Minerals are naturally occurring, crystalline solids with specific chemical formulas. In addition, minerals are inorganic solids, which means they do not consist of molecules built around carbon atoms. Obsidian is a volcanic rock made of glass. Glass is an amorphous solid which does not qualify as a mineral because it does not have a specific chemical composition and is missing a crystal lattice structure. Coal and amber do not qualify as minerals because they are organic materials derived from plants.

Although a few rocks are made almost entirely out of a single mineral, most rocks contain several minerals in a mixture which is characteristic of that particular type of rock. To identify a rock, you must first identify the individual minerals that make up that rock. Although thousands of minerals in the earth have been identified, just ten types of mineral make up most of the volume of the earth's crust - plagioclase, quartz, orthoclase, amphibole, pyroxene, olivine, calcite, biotite, garnet, and clay.

Together, the chemical formula (the types and proportions of the chemical elements in the mineral) and the crystal lattice symmetry (the geometry of how the atoms are arranged and bonded together) determine the physical properties of each mineral.

By examining a mineral and determining several of its physical properties, you can identify the mineral. First, you need to become familiar with the physical properties of minerals and how to recognize them.

What are the physical properties of minerals?

The physical properties of minerals that we will consider are color, luster, cleavage or fracture, hardness, crystal shape, and selected special properties.

Color Color is often useful, but should not be relied upon. Some minerals come in many different colors. Quartz, for example, may be clear, white, gray, brown, yellow, pink, red, or orange. So color can help, but do not rely on color as the sole determining property.

Luster Luster is how the surface of a mineral reflects light, and is not the same thing as color. Types of luster include glassy, pearly (faint iridescence or color play), dull, and metallic. Identifying luster takes a little practice. Remember to distinguish luster from color. A mineral described as "shiny yellow" is being described in terms of luster ("shiny") and color ("yellow").

Cleavage A mineral cleavage is a direction of weakness in a mineral's crystal lattice structure, along which the mineral breaks into perfectly flat surfaces. Such surfaces can be distinguished by how they consistently reflect light, as if polished and smooth. Some minerals have no cleavage.

For those minerals that do have cleavage, it is essential to determine the number of cleavages, the number of directions to which the cleavage surfaces are parallel. The number of cleavages that are possible in crystal lattices are 1, 2, 3, 4, or 6.

For example, feldspar commonly breaks into rectangular shapes in which the top and bottom are parallel (one cleavage direction), the front and back sides are parallel (a second cleavage direction), the ends are fractured into rough surfaces (no third cleavage direction). The two cleavage directions of feldspar are one of its characteristics. A single feldspar crystal may cleave into hundreds of little surfaces, but each surface will be parallel to either one direction or else parallel to a second direction, so those hundreds of cleaved surfaces add up to two cleavage directions, or two cleavages for short.

If a mineral has more than one cleavage direction, it must be determined whether the cleavages intersect at 90° or not.

To see mineral cleavage, hold the mineral up in bright light and move it around, and move it around some more, to see how the different sides reflect light. A cleavage direction will show up as a smooth, shiny, evenly bright sheen from the flat surface (or parallel flat surfaces) that reflect light evenly and smoothly.

Fracture Fracture is irregular breakage, which occurs on surfaces with no cleavage. Some minerals, such as quartz, have no cleavage whatsoever. They fracture in an irregular way when broken apart by a hammer.

When it is broken, quartz will commonly display conchoidal fracture, like a piece of broken glass, with concentric, curving ridges on the broken surface. However, quartz crystals commonly have so many flaws that instead of exhibiting conchoidal fracture, they simply exhibit irregular fracture.

Other minerals with no cleavage have other types of fracture, with names such as irregular fracture (like it sounds) or hackly fracture (little points sticking out of the fracture surface). However, simply recognizing that a mineral has no cleavage is more important than specifying which type of fracture it displays.

Hardness Mineral hardness is specified by the Mohs hardness scale, in reference to 10 standard minerals, from talc the softest (Mohs hardness of 1), to diamond the hardest (Mohs hardness of 10).

Mohs Hardness Scale
Index Minerals Common Objects
1-talc  
2-gypsum 2.5-fingernail
3-calcite 3.5-pure copper
4-flourite
5-feldspar 5 to 5.5-stainless steel
5.5 to 6-glass
6-apatite 6 to 6.5-hard steel file
7-quartz  
8-topaz  
9-corundum  
10-diamond  

Using the Mohs hardness scale - some simple starting steps

Crystal Shape If a mineral has space while it grows, it may form natural crystals, with a crystal shape that reflects the geometry of the mineral's internal crystal lattice structure. Quartz, for example, forms six-sided crystals. Quartz has a crystal lattice with hexagonal (six-sided) symmetry.

There are two complicating factors to remember here: (1) minerals do not always form nice crystals when they grow, and (2) a crystal face is different from a cleavage surface. A crystal face forms during the growth of the mineral. A cleavage surface is formed when the mineral is broken.

Other general physical properties of minerals not listed here, such as density, are not needed for identifying the ten most common minerals.

Special Properties Some properties that only apply to a few minerals can help to distinguish those particular minerals.

One special property is how calcite reacts to a weak solution of hydrochloric acid (5% HCl). A drop of 5% HCl on calcite effervesces ("fizzes", "bubbles") as the HCl solution reacts with the calcite and creates CO2 gas. Do not be concerned if you do not have HCl solution, which must be professionally prepared and is NOT pure HCl acid. Calcite is usually easy to identify anyway, by its hardness, luster, color, and cleavage.

Note that it is not true that calcite will effervesce in vinegar, even though some web sites say that it will. However, if calcite is turned into a powder, then it may weakly effervesce in vinegar, depending on the acidity of the vinegar.

Another special property is magnetism. The only common mineral that is strongly magnetic is the mineral magnetite.

The link between physical properties and the nature of minerals The properties of a mineral are controlled by its chemical composition (which types of atoms it consists of, and in what proportions) and its crystal lattice structure (the three-dimensional geometric pattern in which those atoms are arranged and bonded together).

It is no coincidence that crystals of quartz (SiO2) are six-sided, while crystals of halite (NaCl) are cubic. This is because of the geometry of their crystal lattice structures. Quartz has a hexagonal symmetry in its crystal lattice. Halite has a cubic symmetry in its crystal lattice. These are clear examples of how macroscopic properties (things you can easily see without magnification) are directly derived from atomic-scale properties.

It is also no coincidence that quartz is hard enough to scratch glass and will not dissolve in water to any visible extent, whereas halite will not scratch glass and will easily dissolve in water. These differences are due to the different chemical compositions of the minerals. In halite, the sodium (Na) and chlorine (Cl) ions respond to the strong electrical charges on water molecules, readily break their bonds with each other, and become dissolved ions in the water. In contrast, the silicon (Si) and oxygen (O) in quartz are linked by much stronger bonds, which do not yield so easily to the dissolving force of water.

How to identify minerals

First, you need good light and a hand lens or magnifying glass. A hand lens is a small, double-lens magnifying glass that has a magnification power of at least 8X and can be purchased at some bookstores and nature stores.

Second, be sure you know the difference between a rock and a mineral. In taking this course, you have purchased the Pacific Northwest Geology Rocks set. Rocks are made of minerals. One or two of the rocks in your Pacific Northwest Geology Rocks set may consist of just one mineral, but most of them will consist of more than one mineral. Look closely at your rocks. In some of them you will see different grains with black colors, white colors, or other colors. These are probably grains of the different minerals that make up the rock. Some rocks are too fine-grained to distinguish the minerals it is composed of. However, if you can identify the rock using the rock classification systems described in other sections, then you can estimate its probable mineral content.

Minerals are identified on the basis of their physical properties, which have been described in the the previous section. To identify a mineral, you look at it closely. At a glance, calcite and quartz look similar. Both are usually colorless, with a glassy luster. However, their other properties they are completely different. Quartz is much harder, hard enough to scratch glass. Calcite is soft, and will not scratch glass. Quartz has no mineral cleavage and fractures the same irregular way glass breaks. Calcite has three cleavage directions which meet at angles other than 90°, so it breaks into solid pieces with perfectly flat, smooth, shiny sides.

When identifying a mineral, you must:

  1. Look at it closely on all visible sides to see how it reflects light
  2. Test its hardness
  3. Identify its cleavage or fracture
  4. Name its luster
  5. Evaluate any other physical properties necessary to determine the mineral's identity

In the minerals tables that accompanies this section, the minerals are grouped according to their luster and color. They are also classified on the basis of their hardness and their cleavage or fracture. If you can identify enough of a mineral's physical properties, you can identify the mineral.

Follow this link to the minerals classification table.

How are igneous rocks classified?

There are two main types of igneous rocks: (1) plutonic (intrusive) rocks, which form by solidification of molten rock deep within the earth, and (2) volcanic (extrusive) rocks, which solidify from molten rock erupted to the surface.

Volcanic rocks themselves break down into two more categories depending on how they erupt - flowing on the ground or exploding into the air. Lava flow rocks are also known as effusive volcanic rocks. Pyroclastic rocks, which originate from explosive volcanism, are made of tephra.

Igneous rocks are classified on the basis of their igneous texture and their igneous composition.

Igneous rock textures

The texture of an igneous rock results from the cooling, crystallization, and solidification history of the magma that formed it. Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive and, if it was extrusive, whether it was formed in a lava flow or from a pyroclastic (i.e. explosive) eruption.

Texture in this context is not whether the rock feels rough or smooth to the touch. Igneous texture terms have definitions that apply only in the context of igneous rocks. Igneous textures are based on such factors as sizes of crystals, presence of glass, and presence of vesicles (bubble holes) in the rock.

Volcanic rock textures

Let us start with textures associated with rocks formed by lava flows. Magmas that erupt as lava onto the earth's surface cool and solidify rapidly. 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 may grow to a large enough size to be easily seen before the magma 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 lava has bubbles of gas escaping from it as it solidifies, it will end up with "frozen bubble holes" in it. These "frozen bubble holes" are called vesicles, and the texture of a rock containing them is said to be vesicular.

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 felsic 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 structure). 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. However, instead of originating from lava that flowed on the earth's surface, tephra is volcanic material that was hurled through the air during a volcanic eruption.

A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. The term commonly used is tuffaceous. 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. 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. The bigger chunks of material in a volcanic breccia are more than 1 cm (5/8 inch) across, and sometimes are much bigger.

Plutonic rock textures

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. This hydrothermal fluid gets into cracks and voids in the earth's crust, and as it cools it may grow very large minerals from the dissolved chemical elements. 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.

Igneous rock compositions

The most common igneous compositions can be summarized in three words: mafic, intermediate , and felsic.

A mafic composition is high in iron and magnesium and low in silica. Basalt has a mafic composition.

A felsic composition is high in silica (SiO2) and low in iron (Fe) and magnesium (Mg). Granite has a felsic composition.

Intermediate rocks or magmas contain silica, iron, and magnesium in amounts that are between felsic and mafic. Andesite is a type of rock with an intermediate composition.

Composition and color

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. If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic.

This color rule works most of the time but there are a couple of complications that you need to keep in mind. When igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. Geologists working in the field carry a rock hammer, so they can break off the weathered, outer parts of rocks to see the "fresh," unweathered rock inside.

The color rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly.

If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. For example:

Origins of igneous rocks

Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed. For example, a coarse-grained, felsic igneous rock is not only a granite, it is an intrusive igneous rock that formed from slow cooling and crystallization of a body of magma within the earth's crust.

How to identify igneous rocks

You identify igneous rocks by determining their texture and their composition. Most igneous rocks consist of a small number of minerals with excellent crystal forms, grown together into a completely interlocking pattern with no spaces around any of the minerals. This is most easily seen in igneous rocks with a phaneritic texture, such as granite with its well-formed shiny black, white, and pink minerals. The same holds true for igneous rocks with aphanitic and porphyritic textures, although those rocks require a closer look to see the interlocking crystal forms. Another quality to look for in an igneous rock is the random orientation of the minerals. Minerals in most igneous rocks have no preferred orientation, they don't tend to grow parallel to each other as the minerals do in a metamorphic rock.

The arrangement of the igneous rock classification table guides you to first determine the igneous texture of the rock you are examining, then determine the igneous composition. Remember that igneous composition is estimated on the basis of color: light = felsic composition, medium = intermediate composition, and dark = mafic composition.

Follow this link to the igneous rock classification table.

How are sedimentary rocks classified?

There are two main groups of sedimentary rocks, clastic and chemical. Each type of sedimentary rock is formed when sediment turns into rock. Clastic sediments are solid pieces of weathered and eroded rocks or minerals, for example sand on a beach. Chemical sediments are sediments that precipitate from solution, for example salt crystals that grow at the bottom of an evaporating body of water.

Clastic sedimentary rocks are classified based on their grain size. Chemical sedimentary rocks are classified based on their mineral content.

Sedimentary rock textures

Sedimentary rocks are made of lithified sediment. The texture of a particular sedimentary rock results from how the sediment was formed, where it was deposited, and how it became rock.

Sedimentary rock textures are different from igneous rock textures. Sedimentary texture names only apply to sedimentary rocks.

Clastic textures

Clastic sedimentary textures are described in terms of how big the sediment grains are, how round they are, and how well they are sorted.

Grain size: The diameter or width of a clastic sediment grain determines its grain size. The words boulder, cobble, pebble, sand, silt, and clay refer to specific ranges of grain size. Sand grains, for example, are between 1/16 mm and 2 mm in diameter.

Rounding: Clastic sediment grains can be round, angular, or in-between (subangular or subrounded). Breccia is a clastic sedimentary rock that by definition consists largely of angular grains of pebble size or larger. Conglomerate, another sedimentary rock, consists largely of rounded grains of pebble size or larger.

Sorting: The extent to which all the grains are the same size is known as sorting. If all the grains are the same size, they are well sorted. Some sandstones are well sorted, and some are not. Most conglomerates are poorly sorted, and consist of a mixture of grain sizes ranging from sand to pebble.

Chemical textures

Chemical sedimentary rocks, such as limestone, have specialized texture classification schemes of their own. However, in this course, we will not cover the textures of chemical sedimentary rocks because they do not apply to as many common rocks in the Pacific Northwest as the clastic sedimentary textures do.

Sedimentary rock compositions

The composition of a sedimentary rock is defined on the basis of the minerals that compose the rock.

Some sandstones are made almost entirely of quartz. Sandstone made entirely of quartz is called arenite.

Other sandstones have lots of feldspar mixed with quartz (and may contain smaller amounts of other minerals as well). Such sandstones are called arkose.

Other sandstones are a mixture of feldspar, quartz, clay, and small fragments of volcanic rock. Such sandstones are technically known as lithic wackes, although geologists often call them by their old name, graywackes, and they are sometimes informally described as "dirty sandstones."

The composition of a chemical sedimentary rock is usually implied by the name of the rock. For example, all limestones consist mostly of the mineral calcite. Coal is made of carbon from dead plant material. Rock salt is made of salt minerals such as halite. Gypsum rock is made of the mineral gypsum. Chalcedony is made of microcrystalline quartz, quartz grains so tiny that they cannot be distinguished even with a standard optical microscope.

How to identify sedimentary rocks

The most common indicator of a sedimentary rock is the presence of bedding. The presence of fossils also indicates that a rock is sedimentary.

Clastic sedimentary rocks contain clastic grains, so a rock that looks like it consists of gravel, sand, or mud is probably a clastic sedimentary rock. Clastic sedimentary rocks are classified on the basis of grain size. For example, a clastic sedimentary rock must be made of sand-sized grains to qualify as sandstone, regardless of what minerals the grains are made of.

Each type of chemical sedimentary rock has its own characteristics and these must be learned one-by-one. Coal, for example, consists of soft, black carbon. Limestone consists of the mineral calcite, which is too soft to scratch glass and fizzes in weak acid.

Follow this link to the sedimentary rock classification table.

How are metamorphic rocks classified?

Metamorphic rocks form from pre-existing rocks that are metamorphosed into new rocks due to changes in temperature, pressure, and chemistry. The pre-existing rocks are called parent rocks or protoliths. As rock is metamorphosed, new minerals form from the pre-existing minerals.

There is an exceptional type of metamorphic rock that undergoes partial melting during metamorphism. Such rock is called migmatite, which means a mixture of igneous and metamorphic together in one rock. For all the other types of metamorphic rock discussed here, the rocks remain in the solid state during metamorphism, without any melting taking place.

There are two main groups of metamorphic rocks, regional metamorphic rocks and contact metamorphic rocks. Regional metamorphic rocks form in zones where large volumes of the crust are subjected to heat and tectonic stress. Contact metamorphic rocks form next to igneous intrusions as a result of the heat coming from the magma.

There are two steps to classifying metamorphic rocks. The first step is to identify the rock on the basis of texture and foliation (or lack of foliation). The second step is to specify the minerals that are most visible or most noteworthy.

Foliated metamorphic rocks (regional metamorphism)

Foliated rocks originate during regional metamorphism. There are several specific types of foliation:

  1. Slaty cleavage. Slaty cleavage is a rock cleavage, not a mineral cleavage, and refers to the tendency of a very fine-grained rock to split into perfectly flat layers, with the minerals in the rock too small to see without a microscope.
  2. Phyllitic foliation. Phyllitic foliations do not split apart into perfectly flat surfaces; instead, the foliation surfaces are slightly wrinkled. Phyllitic foliation surfaces have a shiny luster from the presence of mica in the rock, even though the individual mineral crystals are too small to be discerned with the naked eye.
  3. Schistose foliation. Schistose foliation comes from mica minerals or other flat minerals that have grown large enough to be seen with the naked eye and are oriented parallel to each other. Micas are flat minerals that cleave apart into flexible sheets; examples include muscovite and biotite.
  4. Gneissic foliation. Gneissic foliation refers to layers that consist of different minerals, such as layers of black biotite and amphibole alternating with layers of clear quartz and white feldspar. This gives the rock a striped appearance. Minerals in a rock with gneissic foliation are generally large enough for the crystals to be seen with the naked eye.

Foliated metamorphic rocks are named primarily on the basis of their foliation; hence, the four common types of regional metamorphic rock are slate, phyllite, schist, and gneiss.

This sequence-slate, phyllite, schist, and gneiss-is also a sequence of increasing metamorphic grade. Higher metamorphic grade rock originates at higher temperature. Gneiss, for example, forms at much higher temperature than slate, and at somewhat higher temperature than schist.

In naming a foliated metamorphic rock, the prominent minerals should also be mentioned. For instance, if a rock has schistose foliation because of the mineral biotite, and the foliation surfaces are dotted here and there with red garnets, the rock should be called garnet biotite schist.

Unfoliated metamorphic rocks (contact and regional metamorphism)

Hornfels: Fine-grained, unfoliated metamorphic rocks that result from contact metamorphism are called hornfels.

Quartzite: Metamorphic rocks made entirely or almost entirely of quartz are called quartzite. Even if a quartzite forms during regional metamorphism, it may not be foliated. Quartzite can be distinguished from a quartz sandstone by the way the quartz grains have recrystallized and filled in around each other, eliminating the empty spaces between mineral grains that are present in clastic sedimentary rocks. The recrystallization makes quartzite a denser, tougher rock that breaks right through the quartz grains, rather than around them.

Marble: A rock made of calcite or dolomite recrystallizes into marble when it is metamorphosed. The marble may or may not be foliated (layered).

How to identify metamorphic rocks

The minerals in a metamorphic rock are mostly well-formed, intergrown crystals. Minerals would not occur in sedimentary rocks in such uneroded, un-rounded, intergrown forms. In addition, many metamorphic rocks have grown distinctive minerals that are not stable at the surface of the earth.

Gneiss is a metamorphic rock that may look like granite. However, the minerals in a gneiss are at least partly lined up and oriented parallel to each other, unlike the random orientation of minerals in granite.

The common metamorphic rocks that have no foliation, quartzite and marble, are made of virtually pure quartz grains or virtually pure calcite, respectively.

The calcite crystals in marble have grown into more well-formed, pure crystals than the calcite in a limestone. Any fossils that were in a limestone have probably been destroyed by the recrystallization of the limestone into marble.

In using the metamorphic rock classification tables that accompany this section you will see that they are grouped according to whether the rock is foliated or not. Foliated metamorphic rocks are distinguished on the basis of the size of their mineral crystals and the exact type of foliation they have, as described in the section on foliation above. In the sequence of metamorphism from low-grade to high-grade metamorphism, the rocks proceed through the sequence slate-phyllite-schist-gneiss. Proceeding through this sequence the mineral crystals start too fine-grained to see in slate and become increasingly large and easy to see in schist and gneiss, and the foliation that starts out flat and smooth in slate becomes increasingly rough and uneven in schist and gneiss.

Follow this link to the metamorphic rock classification table.

Web links

For pictures of the common minerals, go the U.S. Geological Survey Web page: http://geomaps.wr.usgs.gov/parks/rxmin/mineral.html

For pictures of common rock types, go to the U.S. Geological Survey Web page: http://geomaps.wr.usgs.gov/parks/rxmin/rock.htmll

Glossary terms that appear on this page: igneous; plutonic; volcanic; lava; pyroclastic; tephra; aphanitic; porphyritic; vesicular; frothy; pumice; glassy; obsidian; tuffaceous; tuff; brecciated; phaneritic; hydrothermal; pegmatitic; pegmatite; lithify; arenite; arkose; graywacke; bedding; intrusion; foliation; slate; phyllite; schist; gneiss


Geology of the Pacific Northwest
Basics--Rocks and Minerals
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 7/16/13