Rhyolite in the context of Glass

Dacite (/ˈdeɪsaɪt/) is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. It is composed predominantly of plagioclase feldspar and quartz.

Dacite is relatively common, occurring in many tectonic settings. It is associated with andesite and rhyolite as part of the subalkaline tholeiitic and calc-alkaline magma series.

A stratovolcano, also known as a composite volcano, is a typically conical volcano built up by many alternating layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and explosive eruptions. Some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and solidifies before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high to intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less viscous mafic magma. Extensive felsic lava flows are uncommon, but can travel as far as 8 kilometres (5 miles).

The term composite volcano is used because strata are usually mixed and uneven instead of neat layers. They are among the most common types of volcanoes; more than 700 stratovolcanoes have erupted lava during the Holocene Epoch (the last 11,700 years), and many older, now extinct, stratovolcanoes erupted lava as far back as Archean times. Stratovolcanoes are typically found in subduction zones but they also occur in other geological settings. Two examples of stratovolcanoes famous for catastrophic eruptions are Krakatoa in Indonesia (which erupted in 1883 claiming 36,000 lives) and Mount Vesuvius in Italy (which erupted in 79 A.D killing an estimated 2,000 people). In modern times, Mount St. Helens (1980) in Washington State, US, and Mount Pinatubo (1991) in the Philippines have erupted catastrophically, but with fewer deaths.

In volcanology, a lava dome is a circular, mound-shaped protrusion resulting from the slow extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of eruptions on Earth form lava domes. The geochemistry of lava domes can vary from basalt (e.g. Semeru, 1946) to rhyolite (e.g. Chaiten, 2010) although the majority are of intermediate composition (such as Santiaguito, dacite-andesite, present day). The characteristic dome shape is attributed to high viscosity that prevents the lava from flowing very far. This high viscosity can be obtained in two ways: by high levels of silica in the magma, or by degassing of fluid magma. Since viscous basaltic and andesitic domes weather fast and easily break apart by further input of fluid lava, most of the preserved domes have high silica content and consist of rhyolite or dacite.

Existence of lava domes has been suggested for some domed structures on the Moon, Venus, and Mars, e.g. the Martian surface in the western part of Arcadia Planitia and within Terra Sirenum.

Andesite (/ˈændəzaɪt/) is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

Andesite is the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in island arcs. The average composition of the continental crust is andesitic. Along with basalts, andesites are a component of the Martian crust.

The tholeiitic magma series (/ˌθoʊleɪˈɪtɪk/) is one of two main magma series in subalkaline igneous rocks, the other being the calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the evolution of a mafic magma into a more evolved, silica rich end member. Rock types of the tholeiitic magma series include tholeiitic basalt, ferro-basalt, tholeiitic basaltic andesite, tholeiitic andesite, dacite and rhyolite. The variety of basalt in the series was originally called tholeiite but the International Union of Geological Sciences recommends that tholeiitic basalt be used in preference to that term.

Tholeiitic rock types tend to be more enriched in iron and less enriched in magnesium and aluminium than calc-alkaline rock types. They are thought to form in a less oxidized environment than calc-alkaline rocks. Tholeiitic basalt is formed at mid-ocean ridges and makes up much of the oceanic crust. Almost all the basalt found on the Moon is tholeiitic basalt.

The calc-alkaline magma series is one of two main subdivisions of the subalkaline magma series, the other subalkaline magma series being the tholeiitic series. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it fractionally crystallizes to become a felsic magma, which is low in magnesium and iron and produces rhyolite or granite. Calc-alkaline rocks are rich in alkaline earths (magnesia and calcium oxide) and alkali metals and make up a major part of the crust of the continents.

The diverse rock types in the calc-alkaline series include volcanic types such as basalt, andesite, dacite, rhyolite, and also their coarser-grained intrusive equivalents (gabbro, diorite, granodiorite, and granite). They do not include silica-undersaturated, alkalic, or peralkaline rocks.

Chaitén is a volcanic caldera 3 kilometres (2 mi) in diameter, 17 kilometres (11 mi) west of the elongated ice-capped Michinmahuida volcano and 10 kilometres (6 mi) northeast of the town of Chaitén, near the Gulf of Corcovado in southern Chile. The most recent eruptive phase of the volcano erupted on 2008. Originally, radiocarbon dating of older tephra from the volcano suggested that its last previous eruption was in 7420 BC ± 75 years. However, recent studies have found that the volcano is more active than thought. According to the Global Volcanism Program, its last eruption was in 2011.

The caldera rim reaches 1,122 metres (3,681 ft) above sea level. Before the current eruption, it was mostly filled by a rhyolite obsidian lava dome that reached a height of 962 metres (3,156 ft), partly devoid of vegetation. Two small lakes occupied the caldera floor on the west and north sides of the lava dome.

Porphyry (/ˈpɔːrfəri/ POR-fə-ree) is any of various granites or igneous rocks with coarse-grained crystals such as feldspar or quartz dispersed in a fine-grained silicate-rich, generally aphanitic matrix or groundmass. In its non-geologic, traditional use, the term porphyry usually refers to the purple-red form of this stone, valued for its appearance, but other colours of decorative porphyry are also used such as "green", "black" and "grey".

The term porphyry is from the Ancient Greek πορφύρα (porphyra), meaning "purple". Purple was the colour of royalty, and the Roman "imperial porphyry" was a deep purple igneous rock with large crystals of plagioclase. Some authors claimed the rock was the hardest known in antiquity. Thus porphyry was prized for monuments and building projects in Imperial Rome and thereafter.

In archaeology, ground stone is a category of stone tool formed by the grinding of a coarse-grained tool stone, either purposely or incidentally. Ground stone tools are usually made of basalt, rhyolite, granite, or other cryptocrystalline and igneous stones whose coarse structure makes them ideal for grinding other materials, including plants and other stones.

Organic and inorganic materials are processed on ground stones into edible products. They are sometimes the only artefacts preserved on archaeological sites and are found worldwide.

In archaeology, a tool stone is a type of stone that is used to manufacture stone tools,or tools that use stone as raw material.

Generally speaking, tools that require a sharp edge are made using cryptocrystalline materials that fracture in an easily controlled conchoidal manner.Cryptocrystalline tool stones include flint and chert, which are fine-grained sedimentary materials; rhyolite and felsite, which are igneous flowstones; and obsidian, a form of natural glass created by igneous processes. These materials fracture in a predictable fashion, and are easily resharpened. For more information on this subject, see lithic reduction.

Obsidian (/əbˈsɪdi.ən, ɒb-/ əb-SID-ee-ən ob-) is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Produced from felsic lava, obsidian is rich in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, giving them a high viscosity. The high viscosity inhibits the diffusion of atoms through the lava, which inhibits the first step (nucleation) in the formation of mineral crystals. Together with rapid cooling, this results in a natural glass forming from the lava.

Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.

Gabbro (/ˈɡæbroʊ/ GAB-roh) is a phaneritic (coarse-grained), mafic (magnesium- and iron-rich), intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro has the same chemical composition and mineralogy as rapid-cooling, fine-grained basalt. Much of the Earth's oceanic crust is made of gabbro, formed at mid-ocean ridges. Gabbro is also found as plutons associated with continental volcanism. Due to its variant nature, the term gabbro may be applied loosely to a wide range of intrusive rocks, many of which are merely "gabbroic". By rough analogy, gabbro is to basalt as granite is to rhyolite.

The Massif de l'Esterel (Occitan Provençal: Esterèu; English: Esterel Massif) is a Mediterranean coastal mountain range in the departments of Var and Alpes-Maritimes on the French Riviera. Neighbouring cities are Mandelieu-la-Napoule and Cannes on the east, as well as Saint-Raphaël and Fréjus on the west.

The soil and rocks of the range are of volcanic origin, composed mainly of rhyolite, which gives the hills a red color. The terrain is rugged, with deep ravines and oak forests. The highest point of the massif is Mont Vinaigre (618 metres or 2,028 feet).

The Big Raven Plateau is an intermontane plateau in Cassiar Land District of northwestern British Columbia, Canada. It lies on the Tahltan Highland and is surrounded by several valleys, including those of Mess Creek, Kakiddi Creek, Chakima Creek, Walkout Creek and the Klastline River. The plateau is drained by many small streams that flow into these neighbouring valleys and, unlike the neighbouring valleys, it is relatively barren of vegetation. Stream erosion has resulted in the creation of canyons with intervening ridges on the eastern and western sides of the Big Raven Plateau. The plateau is in Mount Edziza Provincial Park, which is one of the largest provincial parks in British Columbia. Access to the Big Raven Plateau is mainly by aircraft or by a network of horse trails from surrounding roads.

The Big Raven Plateau is volcanic in origin, consisting mostly of basaltic lava flows of the Mount Edziza volcanic complex that were erupted in the last 7.5 million years. These lava flows are interbedded with rhyolite and trachyte which are in the form of rock fragments produced and ejected by explosive eruptions. The dominant feature on the Big Raven Plateau is Mount Edziza, an ice-covered stratovolcano reaching an elevation of 2,786 metres (9,140 feet). Its composition is more diverse, consisting of alkali basalt, hawaiite, trachybasalt, tristanite, mugearite, benmoreite, trachyte and rhyolite which are in the form of lava flows, lava domes and breccia. The Big Raven Plateau is subdivided into at least 10 geological formations, each being the product of a distinct period of volcanic activity.

The Mono–Inyo Craters are a volcanic chain of craters, domes and lava flows in Mono County, Eastern California. The chain stretches 25 miles (40 km) from the northwest shore of Mono Lake to the south of Mammoth Mountain. The Mono Lake Volcanic Field forms the northernmost part of the chain and consists of two volcanic islands in the lake and one cinder cone volcano on its northwest shore. Most of the Mono Craters, which make up the bulk of the northern part of the Mono–Inyo chain, are phreatic (steam explosion) volcanoes that have since been either plugged or over-topped by rhyolite domes and lava flows. The Inyo volcanic chain form much of the southern part of the chain and consist of phreatic explosion pits, and rhyolitic lava flows and domes. The southernmost part of the chain consists of fumaroles and explosion pits on Mammoth Mountain and a set of cinder cones south of the mountain; the latter are called the Red Cones.

Eruptions along the narrow fissure system under the chain began in the west moat of Long Valley Caldera 400,000 to 60,000 years ago. Mammoth Mountain was formed during this period. Multiple eruptions from 40,000 to 600 years ago created the Mono Craters and eruptions 5,000 to 500 years ago formed the Inyo volcanic chain. Lava flows 5,000 years ago built the Red Cones, and explosion pits on Mammoth Mountain were excavated in the last 1,000 years. Uplift of Paoha Island in Mono Lake about 250 years ago is the most recent activity. These eruptions most likely originated from small magma bodies rather than from a single, large magma chamber like the one that produced the massive Long Valley Caldera eruption 760,000 years ago. During the past 3,000 years, eruptions have occurred every 250 to 700 years. In 1980, a series of earthquakes and uplift within and south of Long Valley Caldera indicated renewed activity in the area.

Ignimbrite is a type of volcanic rock, consisting of a typically welded tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrounding atmosphere. New Zealand geologist Patrick Marshall (1869–1950) coined the term ignimbrite from the Latin igni- [fire] and imbri- [rain].

Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice and/or scoria lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be fairly loose and unconsolidated, or a poor to strongly lithified (solidified) rock called lapilli tuff. Near the volcanic source, ignimbrites often contain thick accumulations of lithic blocks, and distally, many show meter-thick accumulations of rounded cobbles of pumice. Ignimbrites may be white, grey, pink, beige, brown, or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic. Darker-coloured ignimbrites may be densely welded volcanic glass or, less commonly, mafic in composition.

Opal is a hydrated amorphous form of silica (SiO₂·nH₂O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to the amorphous (chemical) physical structure, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.

The name opal is believed to be derived from the Sanskrit word upala (उपल), which means 'jewel', and later the Greek derivative opállios (ὀπάλλιος).