Presentation on theme: "W. H. Berger, Ph.D. Geosciences Research Division Scripps Institution of Oceanography ….. University of California, San Diego ….. La Jolla, California,"— Presentation transcript:
W. H. Berger, Ph.D. Geosciences Research Division Scripps Institution of Oceanography ….. University of California, San Diego ….. La Jolla, California, Ca 92093 Editorial assistance: Patricia Anderson and Debra Brice GOT SAND? A project of the Frances L. Parker Program for Outreach and Education in the Earth Sciences, directed by Frances Parker (1906-2002) Scripps paleontologist; world expert in Foraminifera and ocean history. Parker helped initiate the program, in 2001. Sand Stories No. 1
What is sand? Sand is what feels gritty between your toes when you walk barefoot. If it gets in your food, it damages your teeth. Geologists use the word “sand” to describe solid particles of a certain size, say, between 1/16 mm and 2 mm in diameter. To study fine sand, at the lower end of the range, one needs a microscope. For coarse sand, a hand lens provides sufficient magnification.
In common English usage, sand has many meanings. It is a symbol for very large numbers. In the Bible we find sand in the context of “numerous as the sand that lieth upon the sea shore” or “abundant as the sand of the sea”, and this usage was adopted into everyday English. It is a symbol for instability and uncertainty, as in the advice not to build one’s house on sand. It is a symbol of the passing of time as in reference to the sand-glass or hour-glass (“our sands are almost run”). It is a symbol of the inhospitable desert (“barren and pathless sands”). It is a symbol of fruitless labor (like “ploughing the sands”). In slang, it refers to courage (“she’s got sand”). This may be related to “grit”, which refers to stamina. But what is sand really?
There are basically two types of sand: sand made of minerals sand made of shell The upper left photo shows a rare type of mineral sand from Hawaii: olivine, with mineral grains from volcanic rock. To the right: sand made from shell, also from Hawaii. Lower left: deep-sea shell sand.
The Hawaiian Islands make a string with an active and an inactive end. The sands at the active end, naturally, are mostly of volcanic origin. At the inactive end, they are mostly made of shell and coral fragments, nicely washed and rounded by the wave action. Univ. of Hawaii Because the Hawaiian Islands are in the trade wind zone, where big waves are made, there is always plenty of surf energy to move the sand on the beach.
A coconut has settled on this volcanic sand, on the Big Island, but it is unlikely to survive here. The white shell sand (below) is delivered by the reef offshore. Also, one can see sand moving out to sea in a sand river on the right of the image.
Deep-sea sand occurs where the currents are strong and wash the sediment. Normally, the sand particles (the empty shells of single-celled organisms called foraminifers) must be washed free on a sieve in the lab, to study them. Foraminifers make up the most widespread sand type on the planet. Deep-sea sediments are recovered at sea with a device called a “corer.” It is a steel tube with a heavy weight on top. In the last several decades, most deep-sea sediment has been raised by deep ocean drilling, within a pipe from the ship sending the corer to the sea floor to the bottom. ODP
The most familiar type of sand, to most people, is the mineral sand making up the beaches on the West Coast and much of the East Coast. Also, we find such sand in the desert. The sand in the desert (left, Anza) looks raw and unsorted. It has may different types of minerals and even some rock fragments. The sand from the beach (right, La Jolla), in contrast, is well sorted and has much more quartz than anything else. It has traveled quite a ways from the mountains to the sea and is being worked over by wave action.
Beaches: San Diego, Monterey. What we want to know is this: What is it made of? How did it get there? What stories does it tell about the evolution of the landscape? The most pleasant place to study sand is on the beach.
This flatfish is trying to look like the sand and pebbles it lies on. This shore-bird probes the beach for sand crabs and hidden worms. London Aqu., Dana Pt. We also want to know how animals live with sand.
In California, we have lots of shoreline. So, where is the beach? Much of the coastline has cliffs and is rocky, both in northern and southern California. A narrow terrace in front of the cliffs commonly does not leave much room for a wide Here we are in earthquake country: uplift of the shores and coastal mountain building makes pictoresque cliffs. No beach here. beach. However, we do have lots of pocket beaches. Highway 101 between Monterey and Moro Bay
Pocket beaches form where the shoreline is set back, at the inner end of an embayment. Here the waves are a little less powerful than elsewhere, allowing the sand to accumulate. Highway 101 between Monterey and Moro Bay
The rocks making up the cliffs were made in a subduction zone -- where the ocean floor and the continent collide, moving in opposite directions. In fact, the continent builds out into the ocean here, incorporating material from the sea floor. How do we know that the rocks in the cliffs were made in a subduction zone? Because we find pieces of altered basaltic sea floor, like this “snake-skin” rock. (Geologists say “ophiolite” and “serpentinite”, derived from “snake” in Greek and Latin, respectively.)
In southern California, subduction debris is exposed at Dana Point (photo).
The cliffs are the main source of the beach sand here, so we can find out about the origin of the sand by noting what the cliffs are made of. The cliffs consist of a mixture of boulders and pebbles of volcanic and metamorphic rocks, and a soft muddy “matrix” which holds rocks like dough does pieces of fruit. Dana Point This material is landslide debris, which moved downhill under the sea long ago, probably as a result of major earthquakes. It has since been uplifted and is now part of the land.
San Diego is the county farthest south in California. It has a narrow strip of beach along much of the water front, on a terrace cut into its cliffs. The cliffs here are made of sandstone, mainly. Most of the beach sand comes from the mountains in the back country and is delivered by rivers and carried south by the winter waves. (But there are some pocket beaches also, with only local supply from cliffs and broken shells.) Solana Beach On the lower beach, we find sand ripples, made by currents.
The cliffs are made of sandstone (ancient lagoonal sand deposits, cemented by carbonate). At the base, they are eroded by breaking waves, which use pebbles as hammering tools.
The sand contains quartz, feldspar and dark (iron-rich) minerals, mainly hornblende. The quartz looks like glass. It is hard and very resistant to chemical attack. The feldspar looks like porcelain. It is more readily destroyed. So, the minerals on the beach come from the granitic rocks to the east, but the proportions have changed in favor of quartz.
This piece of grano- diorite has quartz and feldspar and hornblende in it. Quartz and feldspar are light-colored, and hornblende is dark. The result is a salt-and- pepper look. This type of rock is common in the high mountains of California. This rock was made deep in the Earth. On the surface, it is attacked chemically by water and the air’s oxygen. As it disintegrates (slowly), the minerals in it make sand grains. quartz vein The granitic rock in the mountains of San Diego that is the source of most of the mineral grains on the beach is called “granodiorite” by geologists.
Hills are bare or covered by brush in the lower elevations, by pine forests farther up. Clevenger Canyon, San Diego. Avocado groves. The mountains are largely made of granitic rock, although other rock types also can be found. It is not difficult to spot the rocks. The vegetation is thin, owing to lack of rain and sporadic fire.
Quartz is very common in the beach sand. It is a mineral, that is, a crystalline solid compound found in rocks. Other common minerals are: feldspar, hornblende, mica, and magnetite. The most common sand grains on California beaches are quartz grains and whole- rock fragments. Quartz is much more abundant on beaches than in rocks, because it is a hard mineral resistant to abrasion, and also to chemical attack. So, it survives the longest of all the common mineral grains. When quartz grows into open spaces (water-filled cracks deep in the Earth), it can make beautiful crystals. Brazil Vein-filling quartz is commonly milky. White Mtns. Ca.
When granitic rock is attacked chemically and mechanically, it crumbles and makes gritty debris. The surviving mineral grains make sand. The weathering products move downhill after rainstorms, carried by rushing runoff.
Chemical and mechanical attack on the rocks is aided by plant roots. Much of the mineral matter is dissolved or turned into very fine particles, even smaller than sand. The red color in this soil in the mountains of San Diego shows the presence of iron oxide. The iron comes from the dark minerals of the rocks. Its oxidation destroys the minerals and weakens the rock.
Coastal San Diego has a Mediterranean-type climate, with only 10-12 inches of precipitation per year. Commonly, there is a series of drought years interspersed with El Niño years with greatly increased flooding and erosion. Transport is by floods. The “normal” river flow is puny and cannot transport much. Gridley 1962. Berkeley Collections.
Southern California has a “semi-arid” type of climate much like the Mediterranean, with lots of sunshine. This type of climate produces lots of sediment for transport to the sea, if mountains are around where it snows in winter. This is the case in our region. Freezing water cracks the rocks. The snowmelt provides for rushing waters and transport. NASA
In the high country of San Diego, the granitic rocks are well exposed. Unequal weathering makes for strange sculptures. In many countries, legends attach to such natural landmarks, as people “see” heads and figures, or make up stories about how large boulders came to be on top of a mountain.
In fact, the granitic rocks (“diorites”) are the roots of ancient mountains that have been eroded away. Nothing has been carried up. Everything has been carried down and away. San Diego Sierras, near Bishop The mountain roots are much the same in San Diego and in the southern Sierras.
This sand, from a pocket beach in northern California, consists mainly of quartz grains derived from nearby granitic rocks. Debris from organisms (shells and organic matter) is thrown onto the beach by the waves. Note two types of molluscs and a calcareous alga. After breaking up into fragments, such debris will be added to the sand (“biogenous” grains). Mineral sands tell a story of mountain building, weathering, transport and wave action. Shells sands tell about the conditions of the sea where they are deposited. END
About the production team: The author is a geologist and oceanographer at Scripps Institution of Oceanography, UCSD. His research is in ocean and climate history. Editorial assistants Patty Anderson and Debra Brice teach Earth Science at a community college and middle school, respectively. All are enthusiastic about sand and the stories it tells. Berger AndersonBrice For samples, contact firstname.lastname@example.org