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).
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.
Don't Take it for Granite
Granite is a rock composed of all sorts of beautiful mineral crystals like quartz, potassium and plagioclase feldspar, mica, and hornblende. What's granite? This is where misunderstandings start to appear, hence the unfortunate statement one often hears that the mountains of the Sierra Nevada and Half Dome in Yosemite are all made of "granite." What?
Granite is one member of a group of rocks called granitics. There's basically three main members: granite, granodiorite, and tonalite. Most of the Sierra Nevada is granodiorite, as is Half Dome. Occasionally, a geology text or a YouTube geology buff will mention that granite is just one member of the family of granitics, but then they go off describing a rock as granite that's actually tonalite. It's enough to drive a natural historian who is trying to learn geology, crazy.
To add to the confusion, most of those "granite" countertops in home kitchens are not granite at all, but something else (often a metamorphic rock like gneiss - we discuss that below). Communities often identify themselves with granite, like Granite Hills in San Diego County where granite is nowhere to be found (the place is surrounded by tonalite). In fact, classic granite, with its characteristic pink colors (due to the mineral orthoclase feldspar) is not particularly common in California.
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!
The 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.
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.
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): cooled deep underground after magma from the mantle rises up into the crust. Entire rock is composed of visible crystals (gabbro).
2. Volcanic (bottom): 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 (basalt). However, some volcanic rocks like rhyolite will have quite a few smaller mineral crystals scattered throughout 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 when small or large particles are cemented together (conglomerate).
2. Biochemical derived from hard portions of living things like the shells from shellfish and diatoms, visible or not (limestone).
3. Chemical from minerals precipitating out of water (travertine).
4. Organic (coal).
Metamorphic
Two basic kinds of metamorphic rocks - rocks that have changed due to heat, pressure, or both:
1. Foliated (top) 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) where crystalization occurs without foliation typically due to a lack of pressure during formation (quartzite - usually from sandstone, has a frosty look when chipped).
The Igneous Rock Chart
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.
Think of yourself as a geological cook. You have a bunch of ingredients (minerals) and two forms of cooking: stovetop (volcanic) and oven (plutonic) - see definitions above. Look at the ingredients listed below.
1 /2 cup of orthoclase feldspar
1/4 cup of quartz
2 tablespoons of plagioclase feldspar
1 1/2 teaspoons of muscovite mica and biotite mica
2 teaspoons of hornblende
Imagine mixing all these minerals being able to dissolve in a sauce pan after heating. Pour the mixture into a bowl and cool quickly by placing it in the refrigerator. Look at the Mineral Recipe Chart (i.e., Igneous Rock Chart) below 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 millions of years, three to six miles below the earth's surface). Directly below each rock pair are the approximate amounts of minerals found within them. For example, gabbro can be composed of nearly 70% of the mineral augite (on the far left is the percentage scale), 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 (mafic). Since silica-rich rocks are lighter than the more mafic rocks, they rise to the surface of the crust. Hence, continents are mostly silica-rich rocks. 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 (more on that later). Basalt covers much of Washington state, nearly two miles thick in some places.
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. Another place to find the ghost of peridotite-past is 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.
OK, now that you have the basics, use the Mineral Recipe Chart, and discover what kind of rock you cooked. What kind of rock would it be if you had cooked it in the oven, turned off the heat, and let the mixture cool slowly within the oven?
The answers can be found at the bottom of this page**
* Granitics include in the following order: granite, granodiorite, and tonalite.
Back to our friend, granite, you'll notice it is not on the chart. It is actually part of in the first plutonic rock listed on the left, second row (plutonic): Granitics. Within the granitic family are the three main rocks mentioned earlier: granite (with lots of orthoclase feldspar as per the chart), granodiorite, then tonalite - each with decreasing amounts of orthoclase feldspar and increasing amounts of plagioclase feldspar.
Granitics are the plutonic equivalent of rhyolite - made of the same material as rhyolite, but slowly cooled over millions 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 rocks 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 cooled it quickly by placing it in the refrigerator: Rhyolite. The little, white specs in this cobble are quartz and plagioclase feldspar crystals.
If you placed your ingredients in a baking pan, then put it in the oven, heated it for 30 minutes at 450 degrees F, turned off the heat and let mixture cool slowly in the oven: Granite (not granodiorite or tonalite because you added too much orthoclase feldspar).