Arizona Road Trip Jan-Feb 2017 Part 3
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Arizona Road Trip Jan-Feb 2017 Part 3
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Scenic Images (3)

Weaver's Needle

The Following Description From Wikimedia (27 Jan. 2017):

Weaver's Needle is a 1,000-foot-high (300 m) column of rock that forms a distinctive peak visible for many miles around. Located in the Superstition Mountains east of Phoenix, Arizona, Weaver's Needle was created when a thick layer of tuff (fused volcanic ash) was heavily eroded, creating the spire as an erosional remnant with a summit elevation of 4,555 feet (1,388 m). It is set in a desert landscape of cactus and mesquite bush, with large Saguaro cacti particularly prominent. The peak was named after mountain man Pauline Weaver (1797-1867). Weaver's Needle has played a significant role in the stories of the Lost Dutchman's Gold Mine. The Needle's shadow reportedly indicates the location of a rich vein of gold, and many treasure hunters have searched for it. The hunt for gold around Weavers Needle has been pursued by hundreds (possibly thousands) of people.

Pauline Weaver was an American mountain man, trapper, military scout, prospector, and explorer who was active in the early Southwestern United States. He was born in 1797 in White County, Tennessee and died in 1867 at Camp Verde, Arizona. A number of geographic features in Arizona are named after him.

Wearver's Needle (also spelled Weavers Needle). An erosional remnant composed of welded tuff (fused volcanic ash). This rock is prominent in the Superstitions with its complex volcanic history.

The Superstitions have a complex geologic history dating back more than 25 million years. They are composed of a variety of extrusive volcanic rocks including basalt, dacite, rhyolite, welded tuff, and porphyritic andesite (with plagioclase crystals of feldspar). The above sample contains some of the common rocks in the alluvial areas on western side of the Superstitions.

Walking through Saguaro and Palo Verde on west side of Weaver's Peak.

Desert Varnish In The Colorado Desert Of San Diego County

Desert Varnish On Boulders In The Colorado Desert.

There is compelling evidence for a microbial origin for desert varnish. According to the classic paper by Ronald I. Dorn and Theodore M. Oberlander (Science Volume 213, 1981), desert varnish is formed by colonies of microscopic bacteria living on the rock surface for thousands of years. The bacteria absorb trace amounts of manganese and iron from the atmosphere and precipitate it as a black layer of manganese oxide or reddish iron oxide on the rock surfaces. This thin layer also includes cemented clay particles that help to shield the bacteria against desiccation, extreme heat and intense solar radiation. It has been estimated that up to 10,000 years are required for a complete varnish coating to form on boulders in extreme arid desert regions. The bacteria thrive on smooth rock surfaces on sun-baked boulders in the Colorado Desert of southern California.

Black Rock-Inhabiting Fungus (RIF)

In my many visits to the west side of the Superstitions, I thought the blackened rock surface (patina) was caused by desert varnish, a bacterial origin. I revised my previous desert varnish observations with rock-inhabiting fungus (RIF). Under 500x magnification it has the characteristic 2-celled spores produced in an ascus typical of the fungal division Ascomycota (see following link). I also observed RIF along the Hieroglyphic Trail on the east side of the Superstitions. There may be true desert varnish on exposed rock surfaces in the Superstitions. Reddish desert varnish is common on sun-baked boulders in Anza-Borrego Desert State Park and Joshua Tree National Park. Reddish desert varnish is iron oxide while black desert varnish is manganese oxide. Other sources of black surfaces on rocks are crustose lichens and cyanobacteria. They can be distinguished by examination under magnification with a good hand lens, or preferably under a dissecting or compound microscope.

Chartreuse lichen (Acarospora) & orange lichen (Caloplaca). The black rock surface is not caused by lichen or desert varnish. It is rock-inhabiting fungus (RIF) in the fungal division Ascomycota. They are sometimes called "sac fungi" because the spores (ascospores) are produced in a sac-like ascus.

A black layer of rock-inhabiting fungus (RIF) on volcanic rock in the Superstitions. The Superperstitions are composed of a variety of extrusive volcanic rocks including basalt, dacite, rhyolite, welded tuff, and porphyritic andesite (with plagioclase crystals of feldspar).

Microscopic view of black layer on rock from the Superstitions. According to lichenologist Kerry Knudsen (personal communication, 2016) this microscope slide (wet mount) contains a rock-inhabiting fungus (RIF) with mature brown, one-septate spores. The spores are produced in a sac-like ascus typical of the fugus division Ascomycota. The stroma (mycelium) consists of a mass of fungal filaments composed of spherical cells. The outer layer of the stroma is black with heavily melanized cells to protect them from intense UV radiation. Inner cells are hyaline (translucent). This slide does not have the fungal cellular structure and algal cells of crustose lichens. Magnification 500x.

Varnish-coated rocks I have observed in the southwest desert region appeared smoother and better suited for petroglyphs. Extrusive volcanic rocks in the Superstitions coated with RIF have a rough surface and are more difficult to etch.

This volcanic rock is coated with a black layer of RIF. It can be etched with difficulty.

  Rock-Inhabiting Fungi (RIF) In Superstitions  
Desert Varnish: The True Extremophile
Desert Varnish & Lichen Crust

Hieroglyphic Trail in Supertition Mtns

The trail climbs a gradual rocky slope and then enters the lower end of a boulder-filled canyon, ending at a rocky section with steep volcanic walls where pools persist for much of the year. The massive boulders are coated with black rock-inhabiting fungus (RIF) and lichens. A number of the massive boulders have remarkable petroglyphs carved into the rock surface. This was an important site for the ancient Hohokam Indians who inhabited this desert region over a thousand years ago.

Trail passes through dense sahuaro stand before it enters petroglyph canyon in distance.

Large boulders below the steep canyon walls contain petroglyphs (white arrow).
See the following image for an enlargement of boulder with white arrow.

Well-preserved, elaborate petroglyphs on face of large boulder.