“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.




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