“The Rocks and Minerals of Joshua Tree National Park”
UCR Geosciences
(Elective Course for UCR’s Field Ecology Certificate)
Mr. Bruce W. Bridenbecker
64451 Brae Burn Avenue
Desert Hot Springs, CA 92240
“M. S. Geosciences, is a Professor of Earth and Physical Sciences at Copper
Mountain College. Mr. Bridenbecker graduated from Northern Arizona University
with a B.S. in Geophysics and upon graduation, worked as an exploration
geophysicist for Cities Service Company. He later received a M. A. in
environmental education. He also did research on the impact of the Glen Canyon
Dam on the Colorado River in the Grand Canyon and co-authored Mojave Desert
Issues, a curriculum guide used as part of the Parks as Classrooms program,”
according to the Desert Institute.
CLASS OUTLINE AND NOTES
I. Rock Cycle
a. See picture
II. Concepts and Terminology
a. Petrologist, one who studies the science (i.e., the logic—critical thinking and investigation—search for evidence) of rocks
b. Crust, the outer shell of planet (includes thick continental, mainly granite, and thin oceanic crust, largely basalt)
i. 98% of crust is composed of the following elements:
1. Oxygen, O (47%)
2. Silicon, Si (28%)
3. Aluminum, Al (8%)
4. Iron, Fe (5%)
5. Calcium, Ca (3.5%)
6. Sodium, Na (3%)
7. Potassium, P (2.5%)
8. Magnesium, Mg (2%)
9. Plus Hydrogen (H) and Titanium (Ti) is almost 100%
ii. On the surface of the lithosphere, cryptobiotic crust (aka Biological Soil Crust, cyanobacteria, blue-green algae)
c. mantle, above the core, mobile; believed to be largely olivine
d. outer core, liquid; inner core, solid due to pressure; hot center of iron and nickel
e. lithosphere or geosphere, solid outer part of planet: 5-50 kilometers (3-30 miles) deep
f. Igneous rock, fire-formed (such as granite or basalt)
i. Felsic (feldspar, silica), lighter elements and lower specific gravities (rhyolite)
ii. Mafic (magnesium, iron), darker colors and higher specific gravities (basalt)
iii. Intermediate (granite + basalt, andesites), part of process
iv.
Ultramafic (magnesium, iron-rich), parent rock
g. Magma, molten rock inside the crust
h. Lava, magma (molten rock) which erupts outside the crust
i. Pluton, stock, or batholith (consists of many plutons), intrusive
j. Intrusive (I) versus extrusive (E) cooling influences size of crystals which form at differing temperatures:
i. At 1200 degrees, intrusive basalt (olivine and pyroxene) and its E counterpart, gabbro (plagioclase feldspar)
ii. At 800-1200 degrees, intrusive andesite (amphibole and plagioclase feldspar) and its extrusive counterpart, diorite (quartz)
iii. At 600 degrees, intrusive granite and its extrusive counterpart, rhyolite (K-spar and quartz)
k. Sedimentary rock, earth-formed (may contain fossils; dense, compressed layers)
l. Minerals are defined as:
i. Naturally occurring versus man-made
ii. Inorganic/non-living as opposed to living
iii. Solid opposed to liquid
iv. Crystal-like
v. Unique chemical composition
vi. Definite crystal
vii. Definition includes ice (points i-vi) but excludes amber (point ii)
m. Examples of Minerals
i. Lead
ii. Copper
iii. Pyrite (aka fool’s gold)
iv. Quartz
v. Halite (aka salt; rock salt includes rock/sand)
vi. Garnet
vii. Magnetite
viii. Hematite (from Vulcan, Iron Hat mines)
n. Most common minerals (100%):
i. Feldspar, 51% (Plagioclase, 39% and Alkali, 12%)
ii. Quartz, 12%
iii. Pyroxenes, 11%
iv. Micas, 5%
v. Clays, 5%
vi. Amphiboles, 5%
vii. Silicates, 3%
viii. Non-silicates, 8%
o.
Cleavage versus Fracture
i. Former breaks along smooth surfaces/lines
ii. Latter breaks along jagged surfaces/lines
1. Note conchoidal (fan-like fractures)
iii. May need to look closely with a magnifier; along a scale of:
1. perfect,
2. imperfect,
3. good,
4. distinct,
5. indistinct, and
6. poor
iv. Difficult concept; may take years; may best be learned in field
p. Specific Gravity (relative to water)
i. Heft
q. Mohs Scale of Hardness (scratchability)
i. Talc
ii. Gypsum (fingernail)
iii. Calcite
iv. Fluorite
v. Apatite (or a knife; 5.5, glass)
vi. Potassium (Steel file, 6.5)
vii. Quartz (so hard that’s all that’s left in sand dunes)
viii. Topaz
ix. Corundum
x. Diamond
r. Keying Out Ten Common Minerals (using Table 1.2 of **)
i. Quartz, non-metallic, light; scratches glass; no cleavage—conchoidal fracture; vitreous luster; colorless/white streak
ii. Amphibole, non-metallic, dark; hard (modest scratch), cleavage (small crystals); dark gray/black streak
1. Actinolite, greenish/blackish
2. Horneblende,
iii. Potassium feldspar, non-metallic, light, hard; cleavage; pink
iv. Calcite, non-metallic, light; soft; cleavage’ effervesces in 10% HCl; rhombohedral crystal (Limestone; S-22, Badlands, Shelton (sic) mine)
v. Plagioclase feldspar, non-metallic; dark; scratches; cleavage
vi. Hematite, metallic, soft/hard?; reddish brown; silvery black
vii. Magnetite, metallic; scratches; gray black
viii. Chalcopyrite, metallic; soft (nail); dark gray; brass yellow
ix. Galena, metallic, soft, dark gray; silvery gray
x. Pyroxene, (augite), non-metallic, hard; cleavage; streak white gray
xi.
Mica,
non-metallic; soft; cleavage
1. Biotite, dark due to iron/magnesium
2. Muscovite, potassium;
a. used in Russia for windows and in US for wood burning stoves
b. aka isinglass
s. Rock, usually more than one mineral
t.
Chemical
composition (plus crystal
structure)
i.
Calcite,
CaCO3
ii.
Dolomite,
(Ca, Mg) CO3
iii.
Limestone, CaCO3
iv.
Travertine, CaCO3 (stalactite)
v.
Marble, CaCO3
u.
Color
i. Blue = copper; azurite is always blue
ii. Green = malachite is always green
iii. Orange = iron
iv. Pink = iron (quartz with rutile is called rose quartz)
v. Purple = amethyst (i.e., quartz with iron)
vi. Red = iron; realgar is red
vii. Yellow = sulfur
v. Metamorphic, process; Pre-Cambrian
w. Enclaves or xenoliths or foreign rocks (especially in granite)
III.
Field Work
a. Saturday, 15 March 2008 (Rattlesnake Canyon at 34 05.104N, 116 08.434W, 3060’), East of Indian Cove Campground
i. Jurassic period (Mesozoic era)
ii. Granite (lighter color) versus country rock (i.e., native to area)
iii. Joints (see photo) versus faults
1. Lift joint (such as Half Dome) (see photos)
a. Parallel to surface of earth
2. Conjugate joints (two joints at right angle)
a. Offset of joint revels stress
b. Look for greatest angle for direction of pressure
iv. Dikes (see photos)
1. A “mass of … rock that cuts across other rocks” (“Glossary,” NAS First Field Guide, p. 146
2. Three different types:
a. Aplite, fine grained, white intrusive rock (of alkali feldspar and quartz, granite)
b. Megmatites, large crystals
c. Quartz or quartz-sulfide, milky white (gold or silver may be present as well)
v. Desert varnish (patina)
1. Biogenic process of patina (manganese and iron oxide) or kind of rind
2. Bacterial colonies
3. Exist on clay borne silt or Aeolian dust
4. Lipid structures, with fine particles break down, under fine moisture/dew (role of hydrologic soil properties or soil geochemistry)
vi. Rusting of Ferric rocks (see photos)
vii. Accidental inclusion/zenolith/enclave
b. Sunday, 16 March 2008 (Skull Rock, White Tank, Geology Tour Road)
i. Skull Rock
1. Tafoni, cavernous weathering, breaking down (versus erosion, actual movement of component material)
2. Exfoliation, removal of the “outer casing of jointed rock” (Trent and Hazlett, 2002)
ii. So-called “Reindeer” petroglyph (pecked into the rock) at 33 55.614N, 116 04.500W, 3467’
iii.
Squaw
Tank Dam Rock Shelter at 33°55.846, 116°04.592 at 3610 feet
iv. Gold ore, defined as mined for a profit
1. On fault side in association with quartz dikes
2. Economics change over time
3. Milling of tailings
4. Concrete lined vats for cyanide
IV.
Sources
a. Harris, Tuttle and Tuttle, Geology of National Parks, 6th ed. (2004)
b. Eggers, Mining History and Geology of Joshua Tree (2004)
c. AGI/NAST, Laboratory Manual in Physical Geology, 7th ed. (2006)
d. Trent and Hazlett, Joshua Tree National Park Geology (2002)
V.
World Wide Web Resources
a. “Cryptobiotic Crusts” at http://www.mineralarts.com/artwork/cryptos.html (accessed 17 March 2008)
b. “Joshua Tree National Park—Geologic Formations (USPS) at http://www.nps.gov/jotr/naturescience/geologicformations.htm (accessed 17 March 2008)
c. “Earth’s Internal Structure” at http://geology.com/nsta/earth-internal-structure.shtml (accessed 17 March 2008)
d. “What is a Mineral?” at http://webmineral.com/Mineral_Definition.shtml (accessed 17 March 2008)
e. “Dating Tin Cans—IMAC User’s Guide” at http://www.anthro.utah.edu/IMACs/471-TinCans.pdf (accessed 17 March 2008)
f. “Tafoni, Cavernous Weathering” at http://geology.about.com/library/bl/images/bltafoni.htm (accessed 17 March 2008)
g. “Mohs Scale of Hardness” at http://www.amfed.org/t_mohs.htm (accessed 17 March 2008)
h. “Eight Common Metamorphic Rocks—including slate, schist, gneiss, marble, quartzite, and serpentine” at http://skywalker.cochise.edu/wellerr/GLG101/GLG101-metamorphic-rocks.htm (accessed 17 March 2008)
i. “Rock Piles in Joshua Tree” at http://digital-desert.com/joshua-tree-national-park/rockpiles.html (accessed 18 March 2008)
j. “Joshua Tree NP: Rock Piles” at http://www.nps.gov/archive/jotr/nature/features/geology/rockpiles/rocks.html (accessed 18 March 2008)
k. “Igneous, Sedimentary, and Metamorphic Rocks: How do Rocks Undergo Change” at http://www.classzone.com/books/earth_science/terc/content/investigations/es0602/es0602page01.cfm (accessed 18 March 2008).
l. “Minerals” at http://www.fossilwalks.com/dartmoortors1/minerals.htm (accessed 18 March 2008). WARNING: Typos and factual errors about quartz, but still useful.
m. “Introduction to the Cyanobacteria: Architects of Earth‘s Atmosphere” at http://www.ucmp.berkeley.edu/bacteria/cyanointro.html (accessed 8 April 2008)
VI.
Tools for the Geologist
a. Ward’s Rock and Mineral Test Kit (12-0300; 07037)
b. Deluxe Hardness (2, 4, 7, and 8) Picks (Probes) by Mineralab
R;tw