VOLCANO IN BEND EarthCache

Basic types of volcano (see here for more info):

1. Composite or Stratovolcanos: Three Sisters (which can be seen some 23miles West of here) is an good example (see Gallery & illustration below). Typically large steep-sided, symmetrical cones built up of alternating layers (strata) of lava flows, ash, cinders, blocks, and bombs resulting from successive eruptions. They may rise up to 8,000 feet above their bases. Typical examples include Mt Fuji (Japan), Mt Mayon (Philippines) and Mts Vesuvius & Stromboli (Italy)

2. Cinder Cones: (see graphic left) the simplest type, built from particles and blobs of congealed lava ejected from a single vent. As the gas-charged lava is blown violently into the air, it breaks into small fragments that solidify and fall as cinders around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than a 1,000 feet from their base. Bend is one of four cities in the United States to have a volcano within its boundaries. Portland is the only other city in Oregon with a volcano within its city limits, with Mount Tabor. Jackson Volcano in Jackson, Mississippi and Diamond Head in Honolulu, Hawaii being the others.

3. Shield Volcanos: consist of thousands of highly fluid basaltic lava flows which pour out in all directions from a central summit vent, or group of vents, spreading widely over a large area and then cooling and slowly building up (by accretion) into thin gently dipping sheets which form a broad, gently sloping cone of flat, domical shape (ie. like a shield). Lavas also often erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest (but not highest!) volcanoes in the world are shield volcanoes. Newberry volcano east of the Cascade Range, South of Pilot Butte, is one of the largest volcanoes on the US mainland. It covers about 1600 km2 and contains a large caldera, Newberry Crater, that was created several 100,000 years ago by a series of devastating eruptions. The volcano last erupted about 1300 years ago, blasting a plug of viscous lava and creating the well-known "Big Obsidian Flow." Background: The low-angle basaltic to basaltic-andesite shield volcano is dotted with more than 400 cinder cones, and most of its edifice is the result of voluminous lava flows. However, Newberry volcano has also produced major explosive silicic eruptions, which formed the 6 x 8 km wide summit caldera containing two caldera lakes. The earliest eruptive products (<0.73 million years ago) (Ma) consist of a sequence of ash-flow and airfall tuffs. Caldera collapse is thought to be associated with 2 major ash flows emplaced about 0.5 and 0.3-0.5 Ma, respectively. These eruptions were preceded by the emplacement of numerous basaltic cones and vents and silicic lava domes and flows, many of which are aligned NNW and NNE parallel to regional fault zones. A rhyolitic magma chamber has been present throughout the past 10,000 years. 6 major eruptive episodes within the past 10,000 years until about 1300 years ago have included both the eruption of basaltic lava flows from flank vents and the explosive ejection of rhyolitic pumice and pyroclastic flows and the extrusion of obsidian flows within the caldera.

4. Lava (Volcanic) Domes: (right) formed by relatively small, bulbous masses of lava too viscous to flow far. On extrusion, the lava piles over and around its vent and a dome grows mainly by internal expansion. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs or spines over the volcanic vent, others form short, steep-sided lava flows known as 'coulees'. They typically occur within the craters or on the flanks of large composite volcanoes.

Types of Lava

Volcanoes may also be classified by the composition of ejected material (lava) as this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions, depending on silica content as follows:

1. Felsic: (also known as dacites or rhyolites) which contains a high percentage (>63%) of silica.These are usually highly viscous and are erupted as domes or short, stubby flows. Such lavas tend to form stratovolcanoes or lava domes and trap volatiles (gases) causing magma to erupt catastrophically.

2. Andesitic: which contains 52–63% silica. Such volcanoes generally only occur above subduction zones, like Mt Merapi (Indonesia), and are typically formed at convergent boundaries/margins of tectonic plates.

3. Mafic (basaltic): which contains 45-52% silica - so called as it has higher percentages of magnesium (Mg) and iron (Fe). These lavas are usually much less viscous and are hotter than felsic/rhyolitic lavas, depending on the eruption temperature, and occur in a wide range of settings: at mid-ocean ridges, where two oceanic plates are pulling apart, basaltic lava erupts as pillows to fill the gap; in shield volcanoes like the Hawaaian Islands and Mt Suswa) on both oceanic and continental crust; and as continental flood basalts.

4. Ultramafic: which contains <=45% silica. These lava flows (aka askomatiites) are very rare and very few have been erupted on Earth since the Proterozoic period when the planet's heat flow was higher. They are the hottest and most fluid lavas.

Tephra and Tuff

Tephra is fragmental material ejected in an eruption regardless of composition, fragment size or emplacement mechanism. Airborne fragments are also called pyroclasms. Once clasts (rock fragments or grains resulting from the breakdown of larger rocks) have fallen to the ground they remain as tephra unless hot enough to fuse together into pyroclastic rock or tuff (see photo) which usually consists mainly of volcanic ash with larger embedded particles like cinders. There are many types of tuff depending on its composition and the type of lava involved in its formation. (see here for more information on this)

Obsidian

Obsidian is a naturally occurring volcanic glass ( extrusive igneous silicate tock) produced when felsic lava cools rapidly with minimum crystal growth. It is often found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces the formation of a lava with a high viscosity and degree of polymerization. The resulting prevention of atomic diffusion through this lava explains the lack of growth of pure quartz crystals. With the rapid cooling any impurities are 'frozen in'. Common impurities include iron oxides which give the rock a very dark, often black colour. Obsidian (like glass) is hard and brittle and therefore fractures with very sharp edges, which have been used in the past for cutting and piercing tools (eg. stone tools used by early man) and has been used experimentally as surgical scalpel blades! (see here for more information on this fascinating rock).

(Cache description material adapted from information found on various Wikipedia pages and USGS web page. For graphics sources see Gallery images notes)