A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks and occasionally organic materials that are not minerals are also used for jewelry and are therefore often considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity and notoriety are other characteristics that lend value to gemstones.
Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It lies 52 miles (83 km) northeast of Portland, Oregon, and 98 miles (158 km) south of Seattle. Mount St. Helens takes its English name from that of the British diplomat Lord St Helens, a friend of explorer George Vancouver who surveyed the area in the late 18th century. The volcano is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire.
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous. Tuff composed of sandy volcanic material can be referred to as volcanic sandstone.
Rhyolite is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral assemblage is predominantly quartz, sanidine, and plagioclase. It is the extrusive equivalent to granite.
A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high-to-intermediate levels of silica, with lesser amounts of less-viscous mafic magma. Extensive felsic lava flows are uncommon, but have travelled as far as 15 km (9 mi).
Pumice, called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular volcanic rock that differs from pumice in having larger vesicles, thicker vesicle walls, and being dark colored and denser.
Volcanic glass is the amorphous (uncrystallized) product of rapidly cooling magma. Like all types of glass, it is a state of matter intermediate between the closely packed, highly ordered array of a crystal and the highly disordered array of liquid. Volcanic glass may refer to the interstitial material, or matrix, in an aphanitic (fine-grained) volcanic rock, or to any of several types of vitreous igneous rocks.
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli is Latin for "little stones".
Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts. It is typically dark in color, and basaltic or andesitic in composition. Scoria is relatively low in density as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity greater than 1 and sinks in water.
On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of phreatic blasts occurred from the summit and escalated until a major explosive eruption took place on May 18, 1980, at 8:32 AM. The eruption, which had a Volcanic Explosivity Index of 5, was the most significant to occur in the contiguous United States since the much smaller 1915 eruption of Lassen Peak in California. It has often been declared the most disastrous volcanic eruption in U.S. history.
The 2004–2008 volcanic activity of Mount St. Helens in Washington, United States has been documented as a continuous eruption in the form of gradual extrusion of magma. Starting in October 2004 and ceasing in January 2008, a new lava dome was built up. The new dome did not rise above the rim of the crater created by the 1980 eruption of Mount St. Helens.
Volcanic lightning is an electrical discharge caused by a volcanic eruption rather than from an ordinary thunderstorm. Volcanic lightning arises from colliding, fragmenting particles of volcanic ash, which generate static electricity within the volcanic plume, leading to the name dirty thunderstorm. Moist convection and ice formation also drive the eruption plume dynamics and can trigger volcanic lightning. Unlike ordinary thunderstorms, volcanic lightning can also occur before any ice crystals have formed in the ash cloud.
Volcanic eruptions can have various impacts on maritime transportation. When a volcano erupts, large amounts of noxious gases, steam, rock, and ash are released into the atmosphere; fine ash can be transported thousands of miles from the volcano, while high concentrations of coarse particles fall out of the air near the volcano. The high concentrations of hazardous toxic gases are localized in the immediate vicinity of the volcano.
A volcanic hazard is the probability a volcanic eruption or related geophysical event will occur in a given geographic area and within a specified window of time. The risk that can be associated with a volcanic hazard depends on the proximity and vulnerability of an asset or a population of people near to where a volcanic event might occur.
The Mazama Ash is an extensive, geologically recent deposit of volcanic ash that is present throughout much of northern North America. The ash was ejected from Mount Mazama, a volcano in south-central Oregon, during its climactic eruption about 7640 ± 20 years ago when Crater Lake was formed by caldera collapse. The ash spread primarily to the north and east due to the prevailing winds, and remnants of the ash have been identified as far northeast as the Greenland ice sheet.
Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gases shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away.
