Geo Primer
PHOTO: The mountain above is made of ancient ocean sediments that have been changed (metamorphized) by extreme heat and pressure deep underground, creating beautiful patterns. The auburn foliation is composed of early Carboniferous Period (about 350-million-yrs-old) hornfels (rocks that have been metamorphized by contact with heat), overtopped by a blue-gray marble-like deposit from the late Carboniferous/early Permian Periods (323 to 280-million-yrs-old). Location: Near Convict Lake, eastern Sierra Nevada.
Falling in Love with Geology Starts with Getting the Names Right
Like chaparral, geology is underappreciated and often misunderstood. Let's fix that.
During our Chaparral Naturalist program, we have found that it helps to establish a basic foundation about geology by discussing something everyone is familiar with, rocks. We pick them up, collect them, throw them, see them everywhere. Then, with a few creative analogies, we expand the understanding of rocks by examining their basic ingredients (minerals) and how they are made. The following process assumes that your only exposure to geology was back in grade school or that you have taken a geology course at some point, but have let the knowledge slip away. Regardless of your level of understanding, this adventure will increase your enjoyment of being out in Nature, and consequently, improve your mental and physical health.
First, let's get something straight. Rocks are not always rocks. If you throw a chunk of quartz through a window, you're not throwing a rock. You are throwing a mineral. A chunk of quartz is actually a pure crystal composed of one kind of molecule, silicon dioxide SiO2. Now, if you really want to throw a rock through a window, you'll need to pick up a chunk of something that's composed of several minerals, something like granite - that's a rock. Another way to look at it is that a rock is like a book, its pages are minerals.
Don't Take it for Granite
Granite is a plutonic rock (formed from magma deep underground) that is composed of all sorts of beautiful mineral crystals like quartz, orthoclase and plagioclase feldspar, mica, and hornblende.
Unfortunately, the word granite is thrown around to identify all sorts of beautiful rocks that are not granite at all. For example, the mountains of the Sierra Nevada, including Half Dome in Yosemite, are said to be made of "granite." They're not. Communities often identify themselves with granite, like Granite Hills in San Diego County, where granite is actually nowhere to be found. And those "granite" countertops people have in their kitchens? Most of them are something else entirely, like a beautiful metamorphic rock like gneiss with swirling patterns or diorite with large, reflective crystals. In fact, despite what you may read or hear from seemingly authorative sources, true granite, with 2/3rds of its feldspar mineral component being orthoclase (causing its classic pink coloration), is not something you'll likely find in the wilds of California.
So what is granite?
Granite is actually just one member of a group of plutonic rocks called granitics. There's basically three main members of the granitic family: granite, granodiorite, and tonalite. More than 60% of the Sierra Nevada is tonalite. Half Dome is composed of granodiorite (the second most common granitic in the Sierra Nevada, followed by non-granitic monozonite - see the plutonic rock pyramid below). Granite Hills in San Diego County is surrounded by tonalite, not granite. The misconceptions about granite spread by kitchen counter manufacturers are too many to mention.
Adding to the confusion, geology texts and online geology presenters will sometimes initially mention that granite is just one member of the granitic family, but then will go on later to describe a rock as granite when it's not. It's enough to drive a natural historian who is trying to learn geology, crazy.
Does all this matter? Sure. Calling a bunch of things granite masks the wonderful geological diversity that surrounds us. It's the details that makes life interesting. And the correct names help us reveal those details.
So, let's dig in!
Three memebers of granitic family from left to right. Granite (with a lot of orthoclase feldspar - commonly pink), granodiorite (usually light in color - more plagioclase feldspar), and tonalite (usually darker).
The basic minerals found in granite, from upper left, clockwise. Orthoclase feldspar (often pink), quartz (somewhat clear), plagioclase feldspar (usually off white), hornblende (black), biotite mica (shiny black sheets), and muscovite mica (shiny silver sheets).
The Mother of All Rocks
The fascinating thing about tonalite, granodiorite, and granite is that they are igneous rocks, rocks that have been formed from molten material (magma) miles underground and are essentially the mother of all rocks. Why? Once igneous rocks form, they can erode to forms layers of sediment, which can eventually lead to the formation of the second kind of rock, sedimentary. Sedimentary and igneous rocks can be heated and/or squeezed by high pressure deep underground and change into the third kind of rock, metamorphic. Remember, all rocks are made of combinations of minerals. We discuss more about sedimentary and metamorphics rocks further below.
Rock ID Warning! If you think you'll be able to identify a rock or mineral you find by looking at a guide, the color, or the physical arrangement of crystals you'll often become frustrated because there are huge variations in the appearance of most rocks and minerals. Sometimes it's impossible without chemical analysis and/or by examining thin slices of the material and counting the visible crystals. But this is one of the fun parts of becoming a rock lover - solving the identification mystery of what you've found. The secret is to take on the challenge, lots of experience, and talking with geologists who know their rocks.
The Igneous Rock Chart
* Granitics include in the following order (L to R): granite, granodiorite, and tonalite mostly based on the % of orthoclase (K - potassium) feldspar within.
Since igneous rocks are the mother of all rocks, it is best to examine them first. We'll start by using an analogy to understand the igneous rock chart (above).
Think of yourself as a geological cook. You have a bunch of ingredients (minerals) and two forms of cooking: stovetop (i.e., volcanic - formed at the earth's surface) and oven (i.e., plutonic - formed deep underground) - see further definitions below. Look at the ingredients listed below.
3/4 cup of orthoclase feldspar1/4 cup of quartz2 tablespoons of plagioclase feldspar1 1/2 teaspoons of muscovite mica and biotite mica2 teaspoons of hornblende
Imagine all these minerals dissolving together in a sauce pan after heating them on the stove. Pour the mixture into a bowl and set aside to cool. Examine the Mineral Recipe Chart above to figure out the rock you have made. But first, a brief explanation.
At the top of the chart are two rows of the basic igneous rocks found on earth. The top row are volcanic rocks (magma that reaches the surface). The second row are plutonic rocks (magma that cools over thousands of years, three to six miles below the earth's surface). Directly below each rock pair are the approximate amounts of minerals found within each. For example, gabbro can be composed of nearly 70% of the mineral augite (the percentage scale is on the far left), 10% olivine, and 20% plagioclase feldspar. The bottom row identifies other variables: color/weight, amount of silica and iron/magnesium, and temperature during formation.
Silica is a major element in rocks, but the % decreases as it is replaced by the heavier elements, iron and magnesium. These heavier rocks are classified as mafic. Since silica-rich (siliceous) rocks are lighter than the more mafic rocks, they rise to the surface of the crust. Hence, continents are mostly silica-rich. This is why basalt, a heavier rock, makes up the floor of the ocean (or what's under all the ooze that has accumulated). Basalt can also invade continents due to the antics of tectonic plates. For example, basalt covers much of Washington state, nearly two miles thick in some places, and in the Owen's Valley area in California's eastern Sierra Nevada. The last two igneous rock in the last column on the far right are somewhat rare to find: Peridotite and Komatiite. Peridotite is only found in places where it has been pushed to the surface through the collision of tectonic plates. It usually has a rich greenish coloration due to the large concentration of the mineral olivine. However, you've probably seen it in modified forms. Serpentine, California's official state rock, found mostly in the coastal ranges in central and northern California and in the western foothills of the Sierra Nevada, is actually just peridotite plus water. The peridotite was "hydrolyzed" when it was part of the mashing of tectonic plates. Consequently, serpentine is a metamorphic rock. Another place to find the ghost of peridotite-past is at the green sand beach of Hawaii.
Komatiite is rarely included in typical igneous rock charts because it is so rare. It's a volcanic rock of the ancient Archean Eon, 4 to 2.5 billion years ago. It hasn't emerged on the surface for a very long time. Komatiite is named for its type location in South Africa, Komati.
OK, now that you have the basics, use the Mineral Recipe Chart to discover what kind of rock you cooked. Then consider what kind of rock it would be if you had cooked it in the oven, turned off the heat, and let the mixture cool slowly over several thousand years within the oven?
The answers can be found below**
Think of yourself as a geological cook. You have a bunch of ingredients (minerals) and two forms of cooking: stovetop (i.e., volcanic - formed at the earth's surface) and oven (i.e., plutonic - formed deep underground) - see further definitions below. Look at the ingredients listed below.
3/4 cup of orthoclase feldspar1/4 cup of quartz2 tablespoons of plagioclase feldspar1 1/2 teaspoons of muscovite mica and biotite mica2 teaspoons of hornblende
Imagine all these minerals dissolving together in a sauce pan after heating them on the stove. Pour the mixture into a bowl and set aside to cool. Examine the Mineral Recipe Chart above to figure out the rock you have made. But first, a brief explanation.
At the top of the chart are two rows of the basic igneous rocks found on earth. The top row are volcanic rocks (magma that reaches the surface). The second row are plutonic rocks (magma that cools over thousands of years, three to six miles below the earth's surface). Directly below each rock pair are the approximate amounts of minerals found within each. For example, gabbro can be composed of nearly 70% of the mineral augite (the percentage scale is on the far left), 10% olivine, and 20% plagioclase feldspar. The bottom row identifies other variables: color/weight, amount of silica and iron/magnesium, and temperature during formation.
Silica is a major element in rocks, but the % decreases as it is replaced by the heavier elements, iron and magnesium. These heavier rocks are classified as mafic. Since silica-rich (siliceous) rocks are lighter than the more mafic rocks, they rise to the surface of the crust. Hence, continents are mostly silica-rich. This is why basalt, a heavier rock, makes up the floor of the ocean (or what's under all the ooze that has accumulated). Basalt can also invade continents due to the antics of tectonic plates. For example, basalt covers much of Washington state, nearly two miles thick in some places, and in the Owen's Valley area in California's eastern Sierra Nevada. The last two igneous rock in the last column on the far right are somewhat rare to find: Peridotite and Komatiite. Peridotite is only found in places where it has been pushed to the surface through the collision of tectonic plates. It usually has a rich greenish coloration due to the large concentration of the mineral olivine. However, you've probably seen it in modified forms. Serpentine, California's official state rock, found mostly in the coastal ranges in central and northern California and in the western foothills of the Sierra Nevada, is actually just peridotite plus water. The peridotite was "hydrolyzed" when it was part of the mashing of tectonic plates. Consequently, serpentine is a metamorphic rock. Another place to find the ghost of peridotite-past is at the green sand beach of Hawaii.
Komatiite is rarely included in typical igneous rock charts because it is so rare. It's a volcanic rock of the ancient Archean Eon, 4 to 2.5 billion years ago. It hasn't emerged on the surface for a very long time. Komatiite is named for its type location in South Africa, Komati.
OK, now that you have the basics, use the Mineral Recipe Chart to discover what kind of rock you cooked. Then consider what kind of rock it would be if you had cooked it in the oven, turned off the heat, and let the mixture cool slowly over several thousand years within the oven?
The answers can be found below**
Back to our friend granite, you'll notice it is not on the chart above. As mentioned earlier, granite is only one of several kinds of rocks within the granitic family (the first plutonic "rock" listed on the left, second row).
There are three main rocks within granitic family, with granite just being one of them with granodiorite and tonalite being the other two - all determined by their mineral composition.
The most significant difference between the rocks with the granitic family is their ratio of the minerals orthoclase and plagioclase feldspar. Granite has 35% or more orthoclase. When the orthoclase % increases significantly, the mineral's characteristic pink color often appears. The classic pink granite prized by many with its high percentage of orthoclase feldspar is uncommon in California. The closest deposits of classic granite to the southern part of the state are near Yuma, Arizona.
Granitics are the plutonic equivalent of the volcanic rock rhyolite. Granitics and rhyolite are made of the same mineral composition, but granitics are cooled slowly over thousands of years, miles underground. And because of the slower cooling process, crystals have time to form. Consequently, granitics can contain some especially beautiful crystals, some of which can become quite large. Sometimes, mineral-rich magma or water can squeeze into openings and cracks in granitic rock, forming one of the most interesting mineral accumulations you can find, pegmatite.
There are three main rocks within granitic family, with granite just being one of them with granodiorite and tonalite being the other two - all determined by their mineral composition.
The most significant difference between the rocks with the granitic family is their ratio of the minerals orthoclase and plagioclase feldspar. Granite has 35% or more orthoclase. When the orthoclase % increases significantly, the mineral's characteristic pink color often appears. The classic pink granite prized by many with its high percentage of orthoclase feldspar is uncommon in California. The closest deposits of classic granite to the southern part of the state are near Yuma, Arizona.
Granitics are the plutonic equivalent of the volcanic rock rhyolite. Granitics and rhyolite are made of the same mineral composition, but granitics are cooled slowly over thousands of years, miles underground. And because of the slower cooling process, crystals have time to form. Consequently, granitics can contain some especially beautiful crystals, some of which can become quite large. Sometimes, mineral-rich magma or water can squeeze into openings and cracks in granitic rock, forming one of the most interesting mineral accumulations you can find, pegmatite.
**Answer. What Rock Did You Cook?
If you mixed your ingredients in a sauce pan, heated to boiling, put the mixture into a bowl, and let it cool quickly, you made rhyolite. The little, white specs in this cobble are quartz and plagioclase feldspar crystals. So, volcanic rocks can some, however small, crystal structures.
If you placed your ingredients in a baking pan, put it in the oven until melted, then turned off the heat and let mixture cool slowly, you made granite. Its granite rather than granodiorite or tonalite because you added a large amount of orthoclase (about 75% of the ingredients).
The Wonderful Granitic Details
Now that you have been exposed to the granitic basics, let's enjoy the details. Look at the middle section of the plutonic rock triangle diagram and you'll see our three friends: granite, granodiorite, and tonalite. Don't get overwhelmed by all the rest. It's actually quite simple.
The diagram shows the percentage of three minerals found in different plutonic rocks: Q (top of triangle) for quartz, P for plagioclase feldspar. The percentage of A for orthoclase feldspar (defined in the diagram as alkali feldspar) would be reversed from what is shown in the diagram. For example, if you have a rock that is 55% quartz (moving up the diagram) and 95% plagioclase (moving left to right), you would have tonalite. The % of orthoclase it would be 5%.
There is an excellent description about how to use the diagram here. You can see there are actually three kinds of granite: light monzogranite (which composes most of the rocks in Joshua Tree National Park), and true granite with 65% orthoclase feldspar - syneogranite and alkali feldspar granite (with at least 90% orthoclase). All granitics have between 20 - 60% quartz.
The Three Basic Rock Types
Igneous
There are two basic types of the igneous rocks - the mother of all rocks - formed by the solidification of molten rock:
1. Plutonic (top): Gabbro - cooled deep underground after magma from the mantle rises up into the crust. Entire rock is composed of visible crystals.
2. Volcanic (bottom): Basalt - rises up to the surface and cools there (lava) or cools near the surface (e.g., throat of a volcano). Few, if any observable crystals. However, some volcanic rocks like rhyolite have quite a few smaller mineral crystals scattered in a plain matrix.
Sedimentary
Four basic kinds of sedimentary rocks - formed by particles (clasts of various sizes) cementing together or minerals precipitating out of water (clockwise from upper left):
1. Clastic: Conglomerate - when small or large particles are cemented together.
2. Biochemical: Limestone - derived from hard portions of living things like the shells from shellfish and diatoms, visible or not.
3. Chemical: Travertine - from minerals precipitating out of water.
4. Organic: Coal.
Metamorphic
Two basic kinds of metamorphic rocks - rocks that have changed due to heat, pressure, or both:
1. Foliated (top): Slate - with different colored "layers" from the concentration of different mineral crystals (as in the Julian schist above) or with sheets that can be separated (like slate).
2. Non-foliated (bottom): Quartzite - where crystalization occurs without foliation typically due to a lack of pressure during formation. Quartzite has a frosty look when chipped and is usually created from sandstone.
MORE TO COME
SEDIMENTARY ROCK DETAILS METAMORPHIC ROCK DETAILS