Born of Fire

San Diego Geology, part 1 of 3

More so than most folks, we Californians are confronted by forces shaping the land around us. Years of drought followed by torrential rains erode hillslopes and rearrange streambeds. Earthquakes can visibly offset roads and channels or uplift mountains, reminding us more often than we might like that the planet we inhabit is not stagnant and fixed, but dynamic and changing. The earth's crust is fragmented into sections, or plates, that shift and migrate. Their titanic movements powered by thermal energy generated deep within the earth, they grind together, slowly deforming the landscape, and occasionally startling us when the build-up of pressure is released suddenly as an earthquake.

Beneath these plates, the temperature is so high that rock becomes molten. Since molten rock is less dense than solidified rock, it presses upward, tending to rise into the overlying strata wherever it can. As the crustal plates move against one another, exerting a force sufficient to uplift or depress the land on either side, the molten material moves upward into openings in the deformed and fractured rock. This molten material may rise all the way to the earth's surface as lava from a volcano, or it may become trapped below the surface by overlying rock. Either way, the molten material cools and solidifies, forming the type of rock called igneous.

The large, rounded boulders, ranging in color from pinkish-beige to tan or pale gray that are so common in the foothills and mountains of inland San Diego County, are granite, an igneous rock formed below the earth's surface, later uplifted and gradually exposed as the overlying strata eroded away. Flecks of black mica often give it a salt-and-pepper appearance at close range. Next time you pass a granite outcrop, take a moment to notice its coarse, granular texture - the result of having cooled slowly in a large mass deep below the surface, allowing plenty of time for good-sized, interlocking crystals to form.

In contrast, obsidian, another igneous rock, found only in areas of past volcanic activity, has a glassy, non-crystalline texture. It is formed from molten material which was forced to the earth's surface as lava and which cooled very quickly on contact with the air, before crystals could begin to form.

Because obsidian fractures with sharp edges and can be flaked with a great deal of precision, it was much used in the past by peoples throughout the world, including native Americans, for fashioning arrowheads and knives. Modern medicine has recently "rediscovered" this mineral resource, and obsidian-chip scalpels are now used for delicate operations such as eye surgery because, unlike steel, which tears as it cuts, obsidian makes a very clean incision that heals readily.

The Indian peoples of Southern California also found granite a very useful material. Granite's interlocking crystalline structure makes it tough stuff - slow to wear and very shatter resistant - and the women took good advantage of these properties as they pounded acorns, seeds, and other food items in granite mortars with pestles of the same material. Certain very special granite boulders, called "ringing rocks" were also important to Indian peoples of this area. The ringing rocks look like ordinary granite, but when struck with another rock, they resonate, making a clear, ringing sound, somewhat like a heavy bell. Not much is known about these ringing rocks except that they were apparently sounded to accompany singing during the girls' puberty ceremony. One has recently been discovered on Palomar Mountain.

Another igneous rock of particular importance for San Diego County is pegmatite. Narrow bands of pegmatite, known as pegmatite dykes, traverse the hillslopes near Pala and Mesa Grande. As the large, igneous mass which forms the backbone of these mountains cooled, vapors containing lithium, sodium, and other elements drifted up from the main body of molten material into fractures in the overlying rock, becoming concentrated there. The gaseous material trapped in the fractures cooled more slowly than the molten mass beneath, allowing very large crystals to form. Some of the crystals in these pegmatite dykes are gemstones - topaz, beryl, rose quartz, and tourmaline. The Pala gem district is especially noted for its pink tourmaline, and the pegmatite dykes there have been extensively mined. Specimens of Pala tourmaline are on display in the mineral room of the museum at the Pala Mission.

From time immemorial, gem crystals have fascinated people with their colors, clarity, and rhythmically refracted planes of light. In cultures throughout the world, mystical properties and healing powers have been attributed to varieties of quartz and a number of other crystals. The Luiseño used quartz and tourmaline from the Pala area in their boys' initiation ceremony, and it is believed that these gem crystals were also important in the exercise of certain shamanistic offices.

One of the oldest uses of gemstones and jewelry is as grave goods - items of value interred with the dead. This practice is known from cultures in many areas, including the Mesa Grande area, where native peoples placed local tourmaline crystals in the graves of their dead. What is the significance of this widespread custom? Is it simply that the beauty of a gemstone represents the love and esteem of the living for the departed? Or is there perhaps another reason, suggested by the derivation of our word "gem" from the Latin "gemma" which means "bud?" Anyone who has marveled to see a protruding crystal growth that appears to be sprouting from its matrix of surrounding rock, or wondered at the core of light seemingly enclosed within the facetted form, might well liken it to a rose bud or a willow bud, enclosing the incipient bloom or growth, and believe that these gemstones, placed back into the earth, express a vision and a promise of new life which is forever unfolding from the old.

Robin Hewitt, 1990