Today you will be stopping for a little rest at the Wadworth Rest Area in Nevada. This rest area is essentially in the middle of the Great Basin area. Around the building is a sandy area containing numerous, various boulders/large rocks. When I was last here I saw various interesting features and rock types, basalt, rhyolite, sandstone, some granite, and so on. These are Great Basin volcanic rocks, used here as an aesthetic design feature for the rest area, but these volcanic rocks have stories to tell.
While here you will need to walk around and observe these various rocks, looking to identify a variety of features these rocks contain.
REQUIREMENTS: Listed below are a series of 8 different features you will be looking for among the rocks in this rest area. Brief description of each feature and an example picture for each will help you one your observations. You need to find and identify the listed features at each location and describe the rock you found this feature in (size, color, shape, texture). Describe the feature you observed in that rock.
FEATURES TO OBSERVE: (You only need to find three). At each of these locations among the various rocks here you will find a different feature found in volcanic rocks.
N 39° 36.186′ W 119° 20.215′
- Silicate Crystal: Silicate minerals are the largest class of rock-forming minerals. Silicates include minerals such as quartz, feldspar, mica, and pyroxene. Crystals form when molten magma cools. (NOTE: I only saw very small ones, but they caught my eye because they sparkled)
- Describe the rock you find these crystals in. Describe the crystals (mainly color and size).
N 39° 36.180′ W 119° 20.218′
- Vesicles: rounded holes in volcanic rocks due to the presence of bubbles of a vapor phase when the rock was solidifying.
- Describe the rock you find these vesicles in. Look at the vesicles, locate the largest one you can find on the rock. How big is it?
N 39° 36.186′ W 119° 20.227′
- Layering: stratification, the layering that occurs in most sedimentary rocks and in those igneous rocks formed at the Earth's surface, as from lava flows and volcanic fragmental deposits.
- Look for a large flat rock, looking at it you should see evidence of layering (north-east-ish side). How many layers can you see? Describe the color of the layers you observe.
N 39° 36.193′ W 119° 20.179′
- Quartz/Quartz Vein: Found in all forms of rock: igneous, metamorphic and sedimentary. Quartz is physically and chemically resistant to weathering. An inclusion is a material that was trapped inside a mineral as it formed.
- Describe the rock with the quartz feature. Describe the quarts feature (color, size, etc.)
OPTIONAL: Pictures are always welcom and appreciated.
WADSWORTH
Located about 30 miles east of Reno, Wadsworth is located fairly centrally in the Great Basin region. Pioneers trailing west in the 1800s headed towards California for the Gold Rush also passed this way, and this area was considered treacherous due to long stretches of lack of water. The millions of years of volcanic activity that created this region contributes to that. As the water, rivers, streams, creeks, that may be present, due to the geographical and geological make up, at times could be all underground and inaccessible. This regions water does not flow out but rather sinks (or evaporates) leaving a dry, crusted surface even though water may be found deep beneath porous volcanic rock formation. This area is also part of a smaller region with the Great Basin called the Reno Quadrangle, and within the Quadrangle three groups of rocks/deposits are present: pre-Tertiary metasedimentary and metavolcanic rocks, Mesozoic plutonic rocks, and Cenozoic volcanic, sedimentary, and plutonic rocks and alluvial deposits. To break this down simply, this area is full of volcanic rocks dating back millions of years, and the unique build of the landscape within and surrounding the Great Basin and the Reno Quadrangle have created an environment right in geological history. The U.S. GEOLOGICAL SURVEY BULLETIN 2019 (linked below) has a much more in-depth look into the geology of this region, starting on page 8 of the report.
THE GREAT BASIN
The Great Basin covers a large swath of land including most of Nevada, about half of Utah, and smaller sections of Idaho, Wyoming, Oregon, and California. It covers roughly 200,000 square miles. It is a hydrographic region essentially ringed in by various mountains, and so the precipitation of the Great Basin region evaporates, sinks underground or flows into lakes. There are no outlets so creeks, streams, or rivers cannot drain into the Gulf of Mexico or the Pacific Ocean.
Older Volcanic Rocks: Within the Great Basin many volcanic rocks dating back to more that 40 million years can be found. Basalt, andesite-dacite, and rhyolite, are not uncommon, in fact, they can appear in abundance. Older rocks (20 to 40 million years old) thend to consist of andesites and rhyolites; the rhyolites are typically larger and more widespread. The andesites tend to be higher in silicic content. The rhyolites tend to be low in silica content as these formed when they erupted as ash-flow sheets.
Younger Volcanic Rocks: Younger volcanic rocks date back to 16 million years old, most abundantly these consist of basalt rocks. Rhyolite is the second largest abundancy of the younger volcanic rocks making up this region. Andesites and dacites are much rarer from this time period.
Volcanic Activity: The Great Basin acts basically like a large bowl where surrounding volcanic activity would flow into, being there are no outlets pasts the various mountains, the vocanic material would be trapped and cool with the Great Basin. Initially the volcanic activity center in the central areas of the Great Basin but over time moved progressively westward. The younger volcanic rocks are more abundant closer to the margins of the Great Basin region. Over time, tectonic activity formed and helped to create the configuration of the Great Basin. This tectonic activity led to a crusted layer that covered the area. The basaltic volcanism was so abundant and widespread across the Great Basin that it is associated with forming the unique tectonic break up of the Great Basin area.
SOURCES:
http://neotectonics.seismo.unr.edu/0_COURSES/Geo730-2021/Mckee71.pdf
https://www.science.smith.edu/~jbrady/petrology/rock-library/rock-features/igfeatures-list.php
https://www.geologyin.com/2023/05/cross-cutting-relationships.html
https://www.nps.gov/grba/planyourvisit/the-great-basin.htm#:~:text=The%20Great%20Basin%20includes%20most,drains%22%20in%20the%20Great%20Basin.
https://pubs.usgs.gov/bul/2019/report.pdf