Transcript What is sand?

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.
W. H. Berger, Ph.D.
Geosciences Research Division
Scripps Institution of Oceanography
University of California, San Diego
La Jolla, California, Ca 92093
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Editorial assistance:
Patricia Anderson and Debra Brice
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.
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.
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.
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, sending the corer to the sea floor
within a pipe from the ship 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.
The most pleasant place to study sand is on the beach.
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?
Beaches: San Diego, Monterey.
We also want to know how
animals live with sand.
•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.
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
beach.
However, we do have
lots of pocket beaches.
Highway 101 between Monterey and Moro Bay
Here we are in earthquake
country: uplift of the shores and
coastal mountain building makes
pictoresque cliffs. No beach here.
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).
Dana Point
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.
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.
On the lower beach, we
find sand ripples, made
by currents.
Solana Beach
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.)
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.
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.
This piece of granodiorite has quartz and
feldspar and hornblende
in it. Quartz and feldspar
are light-colored, and
hornblende is dark. The
result is a salt-andpepper look.
quartz vein
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.
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.
Hills are bare or covered by brush
in the lower elevations, by pine
forests farther up.
Clevenger Canyon, San Diego. Avocado groves.
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 wholerock 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.
Vein-filling quartz is commonly milky.
White Mtns. Ca.
Brazil
When quartz grows into open spaces
(water-filled cracks deep in the Earth), it
can make beautiful crystals.
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.
Transport is by floods.
The “normal” river flow
is puny and cannot
transport much.
Gridley 1962. Berkeley Collections.
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.
NASA
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.
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
The mountain roots are
much the same in San Diego
and in the southern Sierras.
Sierras, near Bishop
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.
Anderson
Brice
Berger
For samples, contact [email protected]