While many of us have been impatiently awaiting the coming of the spring season, there are some lovely aspects to the last of winter. The winter storms that dumped precipitation and broke branches have also greatly affected the shoreline.
Wind drives waves, so our beaches change shape between summer winds and winter winds. Last summer’s gentler wind-born waves pushed sand onto the face of our beaches, forming a humped profile. The larger, more powerful waves driven by winter storms scour the sand off our beaches and, typically, deposit the sediments in bars just outside the surf zone, leaving a flatter beach profile. The sand is usually pushed back on our beaches with next summer’s waves. (During the higher seas and higher sea-levels produced by El Niños, however, the beach sand pulled off by winter’s waves can be moved too far off for the next summer’s waves to readily push it back.)
The bare beaches and larger waves of winter do require additional caution by beach walkers—e…
This seasonal deflating of the beach exposes the foredune and cliff face to more pounding winter surf. Sometimes aided by a load of battering driftwood, heavy winter waves erode new sand and rocks from the vulnerable cliffs and dunes onto the beach.
The sand's temporary off-shore movement often also reveals gravel beds that were beneath the blanket of last summer’s beach. The freshly exposed gravel lures rock hounds seeking treasures: chief among the quarry are agates and jasper.
Agates and jasper are types of chalcedony. Chalcedony is a high-silica rock, related to quartz, that has crystals too small to see with the unaided eye. (Such tiny crystals are called “microcrystalline” or “cryptocrystalline.”)
Agates are translucent chalcedony, often with layers or bands of different color. By some definitions, agate must have bands, but most call any microcrystalline translucent rock “agate.” (Obsidian, volcanic glass, is also nearly transparent and microcrystalline.) A translucent rock that has crystals big enough to see is more likely quartzite, not agate, although some people call quartzite “sugar agate” for the crystalline texture.
Jasper are opaque chalcedony, often with more than one color, but rarely banded. Like agate, jasper has enough silica to take a nice polish, but jasper also has enough impurities to block the light.
So how do agate and jasper form?
Chalcedonies start out as dissolved silica. While we don’t think of silica as being particularly soluble in water—water doesn’t readily eat through a glass, for example—the chalcedony variety of quartz is more soluble in hot water than regular quartz is.
Where does rock make water hot enough to do that? Deep under the earth or in a volcano. Subsurface water superheated by magma or lava can dissolve the silicas in the rock. The water then transports the silica-rich water to an opening—a hole or a crack. Sometimes the hot water has so much silica that the fluid becomes a gel. The super-saturated silicas then precipitate out of the water/gel to fill the opening. The gel hardens over time.
In agate, pulses of fairly pure silica-rich water or gel leave layers; in jasper, the silica-rich water or gel mixes with fine sediments, such as volcanic ash, coloring the jasper and making it opaque. Agate and jasper both come in many colors, depending on the impurities in the silica-rich water/gel. Our agates are usually nearly-clear white or yellow/amber; our jaspers are usually brownish red or dark green. Other colors are possible, depending on the impurities in the silica-rich water/gel.
The silica-rich water/gel can fill any opening, even filling the inside of an empty snail shell to make a spiral agate. Geodes, those cool rock spheres of layered agate with crystals inside, are essentially incomplete agates.
Have you ever found a beach pebble that looked like it was half jasper and half agate? “Jaspagate” is the common term for such a combination.
Once solidified, chalcedony is hard enough to take a nice polish, making its varieties favored semi-precious gemstones. In addition to agate and jasper, prized chalcedony also includes carnelian, onyx, and bloodstone.
While you’re waiting for the garden to warm up, a walk along the end-of-winter beach can be very rewarding.