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Lightning Rod of the Cascades

A cache by Vadrosaul Send Message to Owner Message this owner
Hidden : 09/07/2010
2 out of 5
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

Located several miles north of Crater Lake national park, along North Umpqua Highway 138, is a rest stop within Umpqua National Forest. This stop provides a place to stretch your legs, answer the call of nature, and to revel in the beauty of Diamond Lake, Mount Thielsen, and Mount Bailey, all visible and picture-perfect from where you park. There are some information signs highlighting the geology and volcanology of what you see.

View of Mount Bailey from coordinates (photo © Bill Dahl)

Lying directly west is Mount Bailey, the southernmost volcano in a north-south trending volcanic chain 6 miles (10 km) long that rises west of Diamond Lake. Bailey is about the same age as Diamond Peak, 27 miles (43 km) north: less than 100,000 years but greater than 11,000 years old, on the basis of glacial evidence and morphologic comparisons with dated volcanoes. Like Diamond Peak, Bailey consists of a tephra cone surrounded by basaltic-andesite lava. Bailey is slightly smaller than Diamond Peak, and minor andesite erupted from the summit cone in its late stages, whereas Diamond Peak eruptions were never more siliceous than basaltic andesite.

Mt. Thielsen snow-covered (photo © Bill Dahl)

To the southeast, Mount Thielsen (2,800 meters; 9,187 feet) is a shield volcano comprising approx. 8 cubic kilometers of basaltic andesite built atop a broad pedestal (24 cubic kilometers) of older lava. Thielsen is remarkable even at a distance for its colorfully interbedded pyroclastic rocks that dip away from the jagged spire of the central plug, often called the "lightning rod of the Cascades". The most spectacular views are on the north and east sides (accessible only by foot or horseback) where now-vanished glaciers have carved precipitous cirque walls that reveal the construction. Thielsen's age is approximately 290,000 years. Very little of Thielsen's underpinnings are exposed because ash, which erupted from vents at Crater Lake National Park 7 thousand years ago, formed a shroud 4-20 meters thick in the Thielsen area.

Diamond Lake is a shallow, post-volcanically created lake that will eventually be filled in with sediment by inflowing streams. Mt. Mazama's pyroclastic flows created during its great eruption 7 thousand years ago altered the area's drainage characteristics. They dammed a former glacial stream outflow, creating a shallow basin in which streams began to flow into, creating the lake. Diamond Lake eventually found a new outlet via a resistant ledge of lava, and the Lake creek outlet was established, and will maintain the lake level for a long time to come.

Important Geological Terms

Ash – Unconsolidated, fine-grained material ejected from the crater of a volcano during eruptions. Essentially bits of pulverized rock and glass less than 2 millimeters (0.1 in) in diameter.

Andesite - Mostly pale colored, fine-grained volcanic rock derived from moderately hot (900-1100°C) sluggish lava that contains a greater abundance of silica, but less iron & magnesium than basalt. Andesite typically contains visible crystals of light & dark colored minerals called phenocrysts.

Caldera – A large, circular basin-shaped volcanic depression created by the destruction of the upper part of the volcanic cone by an eruption of great power. Many times precipitation will fill the depression, creating a Crater Lake.

Cinder – A reddish-brown, spongy-looking volcanic rock formed when molten lava is blasted into the air during an explosive eruption. Cinder is more compact and heavier than pumice because it is derived from lava with greater iron content.

Composite cone – A volcanic cone built of alternating layers of lava and pyroclastic material over a lengthy time period.

Dike – A sheet-like body of intrusive igneous rock which rises upwards from a magma chamber and cuts discordantly through existing structures of the country rock.

High Cascades – A stretch of exceptionally tall volcanic mountains ranging from central Washington state (Mt. Rainier), through central Oregon (Mt. Hood, Three Sisters, Crater Lake), and ending in northern California (Mt. Shasta). The Cascades range is part of the Pacific Ring of Fire.

Lapilli - A pyroclastic particle of material ejected in volcanic eruptions that is larger than ash, with a size ranging from 2 – 64 millimeters (0.1 – 2.5 in) in diameter.

Pleistocene glaciation - A time period in Earth's history from approximately 2.6 million years ago to 12 thousand years ago. This stretch of time saw North America and much of Eurasia undergo repeated ice ages that severely altered surficial landscapes. Remnants of those ice ages can be seen in a multitude of alpine glaciers throughout Oregon.

Pumice – A generally pale colored, frothy volcanic rock ejected from a volcanic vent during a highly explosive gaseous eruption. It is exceptionally lightweight because it is riddled with air pockets, formed when rapidly expanding gas escapes from stiff lava enriched in silica.

Pyroclastic flow – Fast-moving currents of hot gas and rock, collectively known as tephra, which travel away from the volcano at great speeds. The flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. The word pyroclast is derived from Greek, pyro = fire, and clast = broken.

Shield volcano – A broad, dome-shaped volcanic cone with very gentle slopes and an extensive circumference owing to the fluid nature of the alkaline (basaltic) lava from which it forms.

Tephra – Encompassing term for fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism. Includes ash, lapilli, and volcanic bomb sizes.

Tuff – A variably-colored, gritty-textured volcanic rock composed of small lumps of pumice and bits of angular rock fragments embedded in a matrix of consolidated volcanic ash. The ash, pumice, and rock fragments are produced during highly explosive, gaseous eruptions.

Vesicular Basalt - A form of basalt riddled with irregular-shaped cavities termed vesicles. These voids are created when expanding volcanic gases bubble out of the congealing lava.

Volcanic bomb – A globular mass of lava ejected from a volcano in a liquid or plastic form, which solidifies in the air prior to hitting the ground. Their diameter is generally larger than 64 millimeters (2.5 in).

Volcanic neck/plug – An eroded remnant of solidified lava which formerly filled the pipe of a volcano, but which has now been exposed by denudation of the surrounding volcanic cone.

Logging the Earthcache:

In order to claim this earth-cache, please email Vadrosaul (via the profile link at the top of the page) with answers to the following questions. All the information you need is available on information signs at the Diamond view rest stop. Although it is optional, pictures of you, your group at the earthcache site, and the surrounding landscape would be appreciated:

1. What was the name of the ancient volcano that altered this area's drainage pattern? What is its current name?

2. What three river valleys did pyroclastic flows funnel down?

3. Why do you think Mt. Thielsen's basalt pinnacle attracts lightning so much?

4. Why is Mt. Bailey considered to be younger than Mt. Thielsen?

5. Find a rock around the rest stop that's the same as one of the six on display, and take a picture of your find beside the encased sample (please return the rock you found where you got it).

6. Name another volcanic plug in the United States (try to identify one not in the list below, plenty to choose from and some have earthcaches).

Plugs mentioned so far:
Mount Washington (44° 19.918'N 121° 50.299'W)
Steins Pillar (44° 24.700'N 120° 37.182'W)
Morro Rock (35° 22.155'N 120° 52.056'W)
Devil's Tower (44° 35.423'N 104° 43.193'W)
Lizard Head (37º 50.150'N 107º 57.033'W)
Superstition Mountain (33° 26.300'N 111° 26.865'W)
Tsé Bit' A'í (36º 38.382'N 108º 49.561'W)
Agathla Peak (36° 49.529'N 110° 13.553'W)
Boars Tusk (41º 57.728'N 109º 11.814'W)
Cabezon Peak (35º 35.970'N 107° 05.740'W)
Sutter Buttes (39º 13.971'N, 121º 49.509'W
Bishop Peak (35° 18.136'N, 120° 41.866'W)
Black Hill (35° 21.516'N, 120° 49.905'W)
The Bumpheads (42° 03.621'N, 121° 05.945'W)


John B. Whittow - Dictionary of Physical Geography
Dr. Johannes Koch - Volcanoes
Umpqua National Forest - Mt. Thielsen Wilderness
USGS - Mt. Bailey and vicinity
USGS - Mt. Thielsen volcano
William B. Purdom - Fulgurites from Mount Thielsen
Bill Dahl - On Porpoise's photostream

Additional Hints (No hints available.)

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