Part IV: Sand Grain Natural History Glossary of Explanations and Definitions
E1:
Our Sand Grain Structure. There are many different types and chemistries of minerals that can make sand, but quartz is one of the most common in California. The strong covalent bonds between silicon and oxygen in this mineral help it to survive weathering processes that can break weaker minerals down more quickly. This diagram shows the chemical structure of the sand grain we are following. Source: Slideplayer.com
E2:
Sources of Sand. Most California mountain ranges have granitic batholith cores with chemistries that trend toward the left of this diagram. These rocks weather into the light-colored, high-silica sands typical of our beaches. In contrast, many Hawaiian and other volcanic landscapes dominated by basalt will shed darker, mafic rocks that weather into their black sand beaches. Source: Opengeology.org.
Generally, combining to form lighter to darker minerals (felsic to mafic), elements grade from aluminum to potassium, sodium, calcium, iron, and magnesium. They combine to form minerals (again, generally grading from lighter to darker and heavier) such as feldspars, mica, amphibole, pyroxene, and olivine. These minerals then combine to form (again, roughly grading from lighter and to darker) the intrusive igneous rocks called granite, diorite, gabbro, and peridotite. Since they all cooled slowly deep below the surface, these rocks exhibit larger crystals, as compared to extrusive igneous rocks.
E2a:
Orthoclase Feldspar: Potassium (K) and aluminum (Al) bond with the dominant silicon and oxygen atoms to form this feldspar. These minerals that commonly weather out of granitic rocks often join quartz, adding more color to our river and beach sands. Source: Opengeology.org.
E2b:
Structure of Mica. Light-colored granitic rocks in California may also be smattered with flat sheets of mica crystals containing more complex chemical properties. Notice here that elements such as potassium, aluminum, magnesium, and iron join the silicon and oxygen to produce this shiny, flaky mineral that stands out from the lighter felsic content (higher in silica) of the larger granitic rock mass in our story. Muscovite forms without the iron and magnesium, so its glassy shine is lighter than biotite mica, which contains iron and magnesium. Intrusive igneous rocks with even more iron and magnesium (grading toward diorites and gabbros) are less common in California. Source: OpenGeology.org
E2c:
Identifying Mica. “Micas are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite mica (lower left) has Fe (iron) and Mg (magnesium) cations. Muscovite mica (lower right) has Al (aluminum) and K (potassium) elements instead of iron and magnesium. The muscovite mica shows how thin layers can split away in a sheet silicate.” Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Top left- modified after Steven Earle (2015) CC BY 4.0. Top right- modified after Klein & Hurlbut (1993). Photos by R. Weller/ Cochise College. Source: Physical Geology, First University of Saskatchewan Edition.
E3: Granitic rocks stand in contrast to darker, heavier intrusive igneous rocks that contain more iron and magnesium, elements which combine to make minerals such as pyroxene and olivine. They also contrast with the extrusive igneous (volcanic) rocks that cooled faster near or at the surface and have smaller crystals. For instance, rhyolite is the chemical extrusive equivalent of our granites, but with small crystals. Such extrusive (volcanic) rocks may also grade darker into andesites and basalts. We can find excellent examples of all these intrusive and extrusive igneous rocks (and every other rock category and type) in a state as diverse as California. (We featured some of these volcanic rocks in a previous web page story about great volcanoes of Northern California.) So, it is not surprising that the sand grains weathering from these rock formations are just as diverse. As another example, we examined some of the rock formations containing the gold that attracted California’s epic Gold Rush in a previous story on this web page, Geologic History in Sierra Nevada Gold Country.
E4: Physical or mechanical weathering includes small expansions and contractions during alternating periods of hot and cold. Frost action pries open some of the rocks when water occasionally wedges and freezes into cracks. Thin sheets are exfoliated in the lower-pressure surface environments. Plant roots pry into joints and cracks and wedge them further. Pressure exerted from salt crystals (salt crystal growth) and other minerals that were precipitated can expand rock cavities. These processes would produce the clasts that could be eroded, transported, and deposited, then lithified into clastic sedimentary rocks.
Roots that Pry. Several different weathering processes are attacking this weakened granitic outcrop. You can see roots growing into cracks and joints, expanding and wedging wider openings. Dislodged pieces and course sand grains accumulate below, waiting for an erosional agent (likely running water) to carry them away.
Chemical and Physical Weathering Attacks. Dominant light-colored quartz and felspars are joined with speckles of darker minerals, such as mica and amphibole, in these highly weathered granitic outcrops that are common to California mountain ranges.
E5: Chemical weathering processes also attacked the exposed rock layers. When oxygen from the air and water attached to elements in the rocks, this oxidation produced weaker, stained, rusty surfaces and the flaky mineral hematite (Fe2O3). Such oxidation produces dark streaks where lingering water may accelerate weathering processes and it is responsible for blushing exposed rocks into colorful red formations. Hydration (technically, a physical weathering process, when water is added to the mineral, causing swelling and stress), hydrolysis (when water chemically combines with minerals to break them down), weak carbonic acids, organic acids from decaying plant materials, and other irritants gradually ate away at the rocks and dissolved their weaker minerals. When these chemicals are liberated from the rocks and dissolved into water, they may eventually be precipitated out again or form as evaporates, accumulating in layers of nonclastic chemical sedimentary rocks. These include limestone (calcium carbonate, or calcite), dolostone (calcium-magnesium dolomite), gypsum (calcium sulfate with water), chert (silica), and rock salt (sodium chloride or halite). As you might expect, you will also find excellent samples of all these nonclastic sedimentary rocks scattered around the state. Continue to the final photo essay to see some colorful illustrations of how they are part of the rock cycle.
Weaker Exceptions at Boyden Cave. These “limestone” cave formations in Sierra Nevada’s Kings Canyon contrast with California’s more common granitic rocks. This is actually marble that was metamorphosed from limestones, which were deposited before the Mesozoic magma chambers intruded. The stalactites, stalagmites, and columns are evidence that the calcium carbonates are easily weathered and sculpted, compared to the relatively resistant granitic rocks in the surrounding mountains.
A Different Type of Bedrock. These marble outcrops look tough on the surface, but they are not likely to make it as far or survive as long as the more resistant granitic outcrops that dominate the Sierra Nevada and other California mountain ranges. Weathering processes, accelerated by the relatively high solubility of calcite in water, are likely to destroy them long before they reach the coast. They may become dissolved chemicals in the water as it transports other rocks and sand grains downstream.
E6: A wealth of other relatively older metamorphic rocks (changed by heat and pressure or other agents) are exposed and can be found weathering throughout California. You will see examples where sandstone or chert has been changed to quartzite, limestone to marble, and shale to slate. Higher levels of metamorphism may produce schist and gneiss. Extensive outcrops take us far back to the intense heat and pressure that was generated around subduction zones that molded many California rocks especially before and during the Paleozoic and Mesozoic Eras.
Contrasting Rock Outcrops Produce Different Sediment. The darker, rusty, oxidized, metasedimentary rocks are sometimes referred to as pendants in and east of California’s Sierra Nevada. These older rocks (many date back to the Paleozoic Era) contrast with the granitic outcrops in the background, as they have different chemistries, have been changed by heat and pressure, are less common on western slopes facing the coast, and may be more vulnerable to weathering in the long run. Which of these colors do you see dominating California beaches?
E7: You may have wondered what happened to all the salts and other minerals that had been chemically weathered out of inland rock formations and dissolved into streams and rivers running downhill. Some of those chemicals were trapped within stagnant water in deep inland basins with no outlet to the ocean. As the water evaporated, high concentrations of salts and other mineral evaporates accumulated on valley bottoms, leaving us with sparkling white desert playas. A shorter story may follow that addresses these rocks and landforms, or you might search back on our web page to find our separate story about Searles Valley.
Salts Accumulate in Inland Basins. On most of California’s western slopes, streams and rivers carry salts and other minerals, dissolved from rock outcrops, to the sea. But streams that flow east are often trapped in inland basins, where the water evaporates, leaving dissolved salts behind. This is certainly the case for Owens Lake, which once nearly covered this part of Owens Valley from one side to the other. It shrunk in response to warmer and drier climates that ended the Pleistocene more than 11,000 years ago. But its salty water disappeared when the L.A. Department of Water and Power tapped Owens River water and sent it south in the L.A. Aqueduct. Today, L.A.D.W.P. spends a fortune each year trying to keep fine, toxic dust in the dry lake from polluting the air around the Owens Valley. Much of that dust and its chemicals originated from weathered rocks and was dissolved and carried by running water and then concentrated into Owens Lake over thousands of years.
But the bulk of California’s surface water flows toward the sea, carrying those weathered and dissolved salts and other chemicals with it. Runoff from the continents continues to add these salts and minerals and that is why the ocean is so salty. More dissolved chemicals are always being added to the oceans by this runoff, while ocean water gradually evaporates. But a rough equilibrium has been achieved: as accumulating salt and other mineral concentrations increasingly saturate ocean water, layers of these chemicals are precipitated on the ocean floor, where nonclastic chemical and organic sedimentary rocks continue to form. Much of the salt in the water you swim in when you visit California beaches originated on the continent and may have been weathered from the very rock formations we have examined in this story.
Sunset in the Fog on the Mendocino Coast. Salts and other minerals dissolved from rock formations are carried in solution by streams and rivers off the continent and then concentrated into the sea until they are precipitated on the ocean floor. This section of the cool, foggy Mendocino Coast is an exposed, high-energy beach, where powerful waves scour and erode the sand down the coast or deposit it out in deeper, quieter water. Sea cliffs, resistant sea stacks, and large rocks on the beach are subject to pounding and occasional brutal wave action.
