1. General Definitions: Acid: a substance which when added to water produces hydrogen ions [H+]. Base: a substance which when added to water produces hydroxide ions [OH-]. 2. Properties: husain project this the way of talking acids are very bad Acids: react with zinc, magnesium, or aluminum and form hydrogen (H2(g)) react with compounds containing CO32- and form carbon dioxide and water turn litmus red taste sour (lemons contain citric acid, for example) DO NOT TASTE ACIDS IN THE LABORATORY!! Bases: feel soapy or slippery turn litmus blue they react with most cations to precipitate hydroxides taste bitter (ever get soap in your mouth?) DO NOT TASTE BASES IN THE LABORATORY!! 3. Water dissociation: H2O(l) → H+(aq) + OH-(aq) equilibrium constant, KW = [H+][OH-] / [H2O] Note: water is not involved in the equilibrium expression because it is a pure liquid, also, the amount of water not dissociated is so large compared to that dissociated that we consider it a constant Value for Kw = [H+][OH-] = 1.0 x 10-14 Note: The reverse reaction, H+(aq) + OH-(aq) → H2O(l) is not equal to 1 x 10-14 [H+] for pure water = 1 x 10-7 [OH-] for pure water = 1 x 10-7 Definitions of acidic, basic, and neutral solutions based on [H+] acidic: if [H+] is greater than 1 x 10-7 M basic: if [H+] is less than1 x 10-7 M neutral: if [H+] if equal to 1 x 10-7 M Example 1: What is the [H+] of a sample of lake water with [OH-] of 4.0 x 10-9 M? Is the lake acidic, basic, or neutral? Solution: [H+] = 1 x 10-14 / 4 x 10-9 = 2.5 x 10-6 M Therefore the lake is slightly acidic Remember: the smaller the negative exponent, the larger the number is. Therefore: acid solutions should have exponents of [H+] from 0 to -6. basic solutions will have exponents of [H+] from -8 on. Example 2: What is the [H+] of human saliva if its [OH-] is 4 x 10-8 M? Is human saliva acidic, basic, or neutral? Solution: [H+] = 1.0 x 10-14 / 4 x 10-8 = 2.5 x 10-7 M The saliva is pretty neutral. 4. pH relationship between [H+] and pH pH = -log10[H+] Definition of acidic, basic, and neutral solutions based on pH acidic: if pH is less than 7 basic: if pH is greater than 7 neutral: if pH is equal to 7 The [H+] can be calculated from the pH by taking the antilog of the negative pH Example 3: calculate the [OH-] of a solution of baking soda with a pH of 8.5. Solution: First calculate the [H+] if pH is 8.5, then the antilog of -8.5 is 3.2 x 10-9. Thus the [H+] is 3.2 x 10-9 M Next calculate the [OH-] 1.0 x 10-14 / 3.2 x 10-9 = 3.1 x 10-6 M Example 4: Calculate the pH of a solution of household ammonia whose [OH-] is 7.93 x 10-3 M. Solution: This time you first calculate the [H+] from the [OH-] 7.93 x 10-3 M OH- = 1.26 x 10-12 M H+ Then find the pH -log[1.26 x 10-12] = 11.9 Now you try a few by yourself. You can then check your answers using the Java applet that follows, but remember, you wont learn how to do them if you dont try by yourself first. Practice #1. What is the pH of a solution of NaOH that has a [OH-] of 3.5 x 10-3 M? Practice #2. The H+ of vinegar that has a pH of 3.2 is what? Practice #3. What is the pH of a 0.001 M HCl solution? How can pH be determined experimentally? By using pH paper or a pH meter 5. Strength of Acids and Bases: Acids 1. Strong Acids: completely dissociate in water, forming H+ and an anion. example: HN03 dissociates completely in water to form H+ and N031-. The reaction is HNO3(aq) → H+(aq) + N031-(aq) A 0.01 M solution of nitric acid contains 0.01 M of H+ and 0.01 M N03- ions and almost no HN03 molecules. The pH of the solution would be 2.0. There are only 6 strong acids: You must learn them. The remainder of the acids therefore are considered weak acids. HCl H2SO4 HNO3 HClO4 HBr HI Note: when a strong acid dissociates only one H+ ion is removed. H2S04 dissociates giving H+ and HS04- ions. H2SO4 → H+ + HSO41- A 0.01 M solution of sulfuric acid would contain 0.01 M H+ and 0.01 M HSO41- (bisulfate or hydrogen sulfate ion). 2. Weak acids: a weak acid only partially dissociates in water to give H+ and the anion for example, HF dissociates in water to give H+ and F-. It is a weak acid. with a dissociation equation that is HF(aq) ↔ H+(aq) + F-(aq) Note the use of the double arrow with the weak acid. That is because an equilibrium exists between the dissociated ions and the undissociated molecule. In the case of a strong acid dissociating, only one arrow ( → ) is required since the reaction goes virtually to completion. An equilibrium expression can be written for this system: Ka = [ H+][F-] / [HF] Which are the weak acids? Anything that dissociates in water to produce H+ and is not one of the 6 strong acids. Molecules containing an ionizable proton. (If the formula starts with H then it is a prime candidate for being an acid.) Also: organic acids have at least one carboxyl group, -COOH, with the H being ionizable. Anions that contain an ionizable proton. ( HSO41- → H+ + SO42- ) Cations: (transition metal cations and heavy metal cations with high charge) also NH4+ dissociates into NH3 + H+ Bases 1. Strong Bases: They dissociate 100% into the cation and OH- (hydroxide ion). example: NaOH(aq) → Na+(aq) + OH-(aq) a. 0.010 M NaOH solution will contain 0.010 M OH- ions (as well as 0.010 M Na+ ions) and have a pH of 12. Which are the strong bases? The hydroxides of Groups I and II. Note: the hydroxides of Group II metals produce 2 mol of OH- ions for every mole of base that dissociates. These hydroxides are not very soluble, but what amount that does dissolve completely dissociates into ions. exampIe: Ba(OH)2(aq) → Ba2+(aq) + 2OH-(aq) a. 0.000100 M Ba(OH)2 solution will be 0.000200 M in OH- ions (as well as 0.00100 M in Ba2+ ions) and will have a pH of 10.3. 2. Weak Bases: What compounds are considered to be weak bases? Most weak bases are anions of weak acids. Weak bases do not furnish OH- ions by dissociation. They react with water to furnish the OH- ions. Note that like weak acids, this reaction is shown to be at equilibrium, unlike the dissociation of a strong base which is shown to go to completion. When a weak base reacts with water the OH- comes from the water and the remaining H+ attaches itsef to the weak base, giving a weak acid as one of the products. You may think of it as a two-step reaction similar to the hydrolysis of water by cations to give acid solutions. examples: NH3(aq) + H2O(aq) → NH4+(aq) + OH-(aq) methylamine: CH3NH2(aq) + H20(l) → CH3NH3+(aq) + OH-(aq) acetate ion: C2H3O2-(aq) + H2O(aq) → HC2H302(aq) + OH-(aq) General reaction: weak base(aq) + H2O(aq) → weak acid(aq) + OH-(aq) Since the reaction does not go to completion relatively few OH- ions are formed. Acid-Base Properties of Salt Solutions: definition of a salt: an ionic compound made of a cation and an anion, other than hydroxide. the product besides water of a neutralization reaction determining acidity or basicity of a salt solution: split the salt into cation and anion add OH- to the cation a. if you obtain a strong base. the cation is neutral b. if you get a weak base, the cation is acidic Add H+ to the anion a. if you obtain a strong acid, the anion is neutral b. if you obtain a weak acid. the anion is basic Salt solutions are neutral if both ions are neutral Salt solutions are acidic if one ion is neutral and the other is acidic Salt solutions are basic is one of the ions is basic and the other is neutral. The acidity or basicity of a salt made of one acidic ion and one basic ion cannot be determined without further information. Examples: determine if the following solutions are acidic, basic, or neutral Click on each one to find out the answer. KC2H3O2 NaHPO4 Cu(NO3)2 LiHS KClO4 NH4Cl 6. Acid-Base Reactions: Strong acid + strong base: HCl + NaOH → NaCl + H2O net ionic reaction: H+ + OH- → H2O Strong acid + weak base: example: write the net ionic equation for the reaction between hydrochloric acid, HCl, and aqueous ammonia, NH3. What is the pH of the resulting solution? Strong base + weak acid: example: write the net ionic equation for the reaction between citric acid (H3C6H507) and sodium hydroxide. What is the pH of the resulting solution? 7. Titrations 1. Nomenclature: these are terms that are used when talking about titrating one substance with another. You need to learn these definitions well enough to explain them to someone else. titration titrant indicator equivalence point end point titration cuve 2. Strong acid-strong base titration example: titration curve pH at equivalence point species present appropriate indicators 3. Strong acid-weak base titration example titration curve pH at end point species present appropriate indicators 4. Weak acid-strong base titrations example: titration curve for the titration of vinegar with NaOH pH at end point- approximately 8.5 species present- H2O and NaC2H3O2 appropiate indicator-phenolphthalein Titration of Vinegar with NaOH Note: no matterwhat type of titration you do, at the equivalence (end) point the number of moles of H+ is equivalent to the number of moles of OH-. This applies whether you have weak or strong acids and/or bases. Problems: l. Citric acid (C6H807) contains a mole of ionizable H+/mole of citric acid. A sample containing citric acid has a mass of 1.286 g. The sample is dissolved in 100.0 mL of water. The solution is titrated with 0.0150 M of NaOH. If 14.93 mL of the base are required to neutralize the acid. then what is the mass percent of citric acid in the sample? 2. A sample of solid calcium hydroxide is mixed with water at 30 oC and allowed to stand. A 100.0 mL sample of the solution is titrated with 59.4 mL of a 0.400 M solution of hydrobromic acid. a. What is the concentration of the calcium hydroxide solution? b. What is the solubility of the calcium hydroxide in water at 30 oC? Express your answer in grams of Ca(OH)2 / 100 mL water? 8. Three models of acids: l. Arrhenius Model Basis for the model--action in water acid definition: produces H in water solution base definition: produces OH1- in water solution 2. Bronsted-Lowry Model Basis for the model-- proton transfer acid definition: donates a proton ( H ) base definition: accepts a proton conjugate acid definition: the acid becomes the conjugate base after it donates the proton because it can now accept it back. conjugate base definition: the base becomes the conjugate acid after it accepts the proton because it can now donate it back. 3. Lewis Model Basis for model--electron pair transfer acid definition: accepts a pair of electrons base definition: donates a pair of electrons Send questions, comments or suggestions to Gwen Sibert, at the Roanoke Valley Governors School [email protected] Mineral acid From Wikipedia, the free encyclopedia Jump to: navigation, search Acids and bases Diagrammatic representation of the dissociation of acetic acid in aqueous solution to for acetate and hydronium ions. Acid dissociation constant Acid-base extraction Acid–base reaction Acid–base titration Dissociation constant Acidity function Buffer solutions pH Proton affinity Amphoterism Self-ionization of water Acid strength Acid types Brønsted Lewis Mineral Organic Strong Superacids Weak Base types Brønsted Lewis Organic Strong Superbases Non-nucleophilic Weak v t e A mineral acid (or inorganic acid) is an acid derived from one or more inorganic compounds, and all mineral acids form hydrogen ions and the conjugate base ions when dissolved in water. Characteristics Commonly used mineral acids are sulfuric acid, hydrochloric acid and nitric acid (They are also known as bench acids). Mineral acids range from acids of great strength (example: sulfuric acid) to very weak (boric acid). Mineral acids tend to be very soluble in water and insoluble in organic solvents. Mineral acids are used in many sectors of the chemical industry as feedstocks for the synthesis of other chemicals, both organic and inorganic. Large quantities of these acids, especially sulfuric acid, nitric acid and hydrochloric acid are manufactured for commercial use in large plants. Mineral acids are also used directly for their corrosive properties. For example, a dilute solution of hydrochloric acid is used for removing the deposits from the inside of boilers, with precautions taken to prevent the corrosion of the boiler by the acid. This process is known as descaling. Examples Hydrochloric acid HCl Nitric acid HNO3 Phosphoric acid H3PO4 Sulfuric acid H2SO4 Boric acid H3BO3 Hydrofluoric acid HF Hydrobromic acid HBr Perchloric acid HClO4 External links Mineral Acids: Reregistration Eligibility Decision Fact Sheet – U.S. Environmental Protection Agency Categories: Mineral acids Navigation menu Create account Log in Article Talk Read Edit View history Main page Contents Featured content Current events Random article Donate to Wikipedia Wikimedia Shop Interaction Help About Wikipedia Community portal Recent changes Contact page Tools Print/export Languages العربية Català Deutsch Español فارسی Français 한국어 Bahasa Indonesia Italiano עברית Nederlands ָױ±¾ױZ Polski Portuguךs Romגnד ׀ףססךטי Svenska தமிழ் ה·ֲ Tiếng Việt ײ׀־ִ Edit links This page was last modified on 18 November 2013 at 07:15. Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy. Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization. On Earth, volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are not usually created where two tectonic plates slide past one another. Volcanoes can also form where there is stretching and thinning of the Earths crust in the interiors of plates, e.g., in the East African Rift, the Wells Gray-Clearwater volcanic field and the Rio Grande Rift in North America. This type of volcanism falls under the umbrella of Plate hypothesis volcanism.[1] Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. Volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbine blades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earths lower atmosphere or troposphere; however, they also absorb heat radiated up from the Earth, thereby warming the stratosphere. Historically, so-called volcanic winters have caused catastrophic famines.The word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn originates from Vulcan, the name of a god of fire in Roman mythology.[2] The study of volcanoes is called volcanology, sometimes spelled vulcanology. Plate tectonics Map showing the divergent plate boundaries (OSR – Oceanic Spreading Ridges) and recent sub aerial volcanoes. Main article: Plate tectonics Divergent plate boundaries Main article: Divergent boundary At the mid-oceanic ridges, two tectonic plates diverge from one another. New oceanic crust is being formed by hot molten rock slowly cooling and solidifying. The crust is very thin at mid-oceanic ridges due to the pull of the tectonic plates. The release of pressure due to the thinning of the crust leads to adiabatic expansion, and the partial melting of the mantle causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, therefore most volcanic activity is submarine, forming new seafloor. Black smokers or deep sea vents are an example of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, Iceland. Convergent plate boundaries Main article: Convergent boundary Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the oceanic plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the overlying mantle wedge, creating magma. This magma tends to be very viscous due to its high silica content, so often does not reach the surface and cools at depth. When it does reach the surface, a volcano is formed. Typical examples for this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire. Hotspots Main article: Hotspot (geology) Hotspots is the name given to volcanic provinces postulated to be formed by mantle plumes. These are postulated to comprise columns of hot material that rise from the core-mantle boundary. They are suggested to be hot, causing large-volume melting, and to be fixed in space. Because the tectonic plates move across them, each volcano becomes dormant after a while and a new volcano is then formed as the plate shifts over the postulated plume. The Hawaiian Islands have been suggested to have been formed in such a manner, as well as the Snake River Plain, with the Yellowstone Caldera being the part of the North American plate currently above the hot spot. This theory is currently under criticism, however.[1] Volcanic features Lakagigar fissure vent in Iceland, source of the major world climate alteration of 1783–84. Skjaldbreiður, a shield volcano whose name means broad shield The most common perception of a volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit. This describes just one of many types of volcano, and the features of volcanoes are much more complicated. The structure and behavior of volcanoes depends on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, whereas others present landscape features such as massive plateaus. Vents that issue volcanic material (lava, which is what magma is called once it has escaped to the surface, and ash) and gases (mainly steam and magmatic gases) can be located anywhere on the landform. Many of these vents give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Hawaiis Kīlauea. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn and Neptune; and mud volcanoes, which are formations often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes, except when a mud volcano is actually a vent of an igneous volcano. Fissure vents Main article: Fissure vent Volcanic fissure vents are flat, linear cracks through which lava emerges. Shield volcanoes Main article: Shield volcano Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically. Since low-viscosity magma is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland, as well. Lava domes Main article: Lava dome Lava domes are built by slow eruptions of highly viscous lavas. They are sometimes formed within the crater of a previous volcanic eruption (as in Mount Saint Helens), but can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but their lavas generally do not flow far from the originating vent. Cryptodomes Cryptodomes are formed when viscous lava forces its way up and causes a bulge. The 1980 eruption of Mount St. Helens was an example. Lava was under great pressure and forced a bulge in the mountain, which was unstable and slid down the north side. Volcanic cones (cinder cones) Main articles: volcanic cone and Cinder cone Volcanic cones or cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 meters high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico, Caja del Rio is a volcanic field of over 60 cinder cones. Based on satellite images it was suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and Moon.[3][4][5] Stratovolcanoes (composite volcanoes) Cross-section through a stratovolcano (vertical scale is exaggerated): 1. Large magma chamber 2. Bedrock 3. Conduit (pipe) 4. Base 5. Sill 6. Dike 7. Layers of ash emitted by the volcano 8. Flank 9. Layers of lava emitted by the volcano 10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud Main article: Stratovolcano Stratovolcanoes or composite volcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes, created from several structures during different kinds of eruptions. Strato/composite volcanoes are made of cinders, ash and lava. Cinders and ash pile on top of each other, lava flows on top of the ash, where it cools and hardens, and then the process begins again. Classic examples include Mt. Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy. Throughout recorded history, ash produced by the explosive eruption of stratovolcanoes has posed the greatest hazard to civilizations as compared to other types of volcanoes. Shield volcanos have smaller pressure buildup from the underlying lava flow as compared to stratovolcanoes. Fissure vents and monogenetic volcanic fields (volcanic cones) have less powerful eruptions, as they are many times under extension. Stratovolcanoes have been a greater historical threat because they are steeper than shield volcanos, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars.[6] Supervolcanoes Main article: Supervolcano See also: List of largest volcanic eruptions A supervolcano is a large volcano that usually has a large caldera and can potentially produce devastation on an enormous, sometimes continental, scale. Such eruptions would be able to cause severe cooling of global temperatures for many years afterwards because of the huge volumes of sulfur and ash erupted. They are the most dangerous type of volcano. Examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States), Lake Taupo in New Zealand, Lake Toba in Sumatra, Indonesia and Ngorogoro Crater in Tanzania, Krakatoa near Java and Sumatra, Indonesia. Supervolcanoes are hard to identify centuries later, given the enormous areas they cover. Large igneous provinces are also considered supervolcanoes because of the vast amount of basalt lava erupted, but are non-explosive. Submarine volcanoes Main article: Submarine volcano Submarine volcanoes are common features on the ocean floor. Some are active and, in shallow water, disclose their presence by blasting steam and rocky debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them prevents the explosive release of steam and gases, although they can be detected by hydrophones and discoloration of water because of volcanic gases. Pumice rafts may also appear. Even large submarine eruptions may not disturb the ocean surface. Because of the rapid cooling effect of water as compared to air, and increased buoyancy, submarine volcanoes often form rather steep pillars over their volcanic vents as compared to above-surface volcanoes. They may become so large that they break the ocean surface as new islands. Pillow lava is a common eruptive product of submarine volcanoes. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on dissolved minerals. Subglacial volcanoes Main article: Subglacial volcano Subglacial volcanoes develop underneath icecaps. They are made up of flat lava which flows at the top of extensive pillow lavas and palagonite. When the icecap melts, the lavas on the top collapse, leaving a flat-topped mountain. These volcanoes are also called table mountains, tuyas or (uncommonly) mobergs. Very good examples of this type of volcano can be seen in Iceland, however, there are also tuyas in British Columbia. The origin of the term comes from Tuya Butte, which is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analyzed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near the boundary with the Yukon Territory. Mud volcanoes Main article: Mud volcano Mud volcanoes or mud domes are formations created by geo-excreted liquids and gases, although there are several processes which may cause such activity. The largest structures are 10 kilometers in diameter and reach 700 meters high. Erupted material Pāhoehoe lava flow on Hawaii. The picture shows overflows of a main lava channel. The Stromboli stratovolcano off the coast of Sicily has erupted continuously for thousands of years, giving rise to the term strombolian eruption. Lava composition Another way of classifying volcanoes is by the composition of material erupted (lava), since this affects the shape of the volcano. Lava can be broadly classified into 4 different compositions (Cas & Wright, 1987): If the erupted magma contains a high percentage (>63%) of silica, the lava is called felsic. Felsic lavas (dacites or rhyolites) tend to be highly viscous (not very fluid) and are erupted as domes or short, stubby flows. Viscous lavas tend to form stratovolcanoes or lava domes. Lassen Peak in California is an example of a volcano formed from felsic lava and is actually a large lava dome. Because siliceous magmas are so viscous, they tend to trap volatiles (gases) that are present, which cause the magma to erupt catastrophically, eventually forming stratovolcanoes. Pyroclastic flows (ignimbrites) are highly hazardous products of such volcanoes, since they are composed of molten volcanic ash too heavy to go up into the atmosphere, so they hug the volcanos slopes and travel far from their vents during large eruptions. Temperatures as high as 1,200 °C are known to occur in pyroclastic flows, which will incinerate everything flammable in their path and thick layers of hot pyroclastic flow deposits can be laid down, often up to many meters thick. Alaskas Valley of Ten Thousand Smokes, formed by the eruption of Novarupta near Katmai in 1912, is an example of a thick pyroclastic flow or ignimbrite deposit. Volcanic ash that is light enough to be erupted high into the Earths atmosphere may travel many kilometres before it falls back to ground as a tuff. If the erupted magma contains 52–63% silica, the lava is of intermediate composition. These andesitic volcanoes generally only occur above subduction zones (e.g. Mount Merapi in Indonesia). Andesitic lava is typically formed at convergent boundary margins of tectonic plates, by several processes: Hydration melting of peridotite and fractional crystallization File:Sarychev Peak eruption on 12 June 2009, oblique satellite view.ogv Sarychev Peak eruption, Matua Island, oblique satellite view Melting of subducted slab containing sediments[citation needed]]] Magma mixing between felsic rhyolitic and mafic basaltic magmas in an intermediate reservoir prior to emplacement or lava flow. If the erupted magma contains 45% silica, the lava is called mafic (because it contains higher percentages of magnesium (Mg) and iron (Fe)) or basaltic. These lavas are usually much less viscous than rhyolitic lavas, depending on their eruption temperature; they also tend to be hotter than felsic lavas. Mafic lavas 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; Shield volcanoes (e.g. the Hawaiian Islands, including Mauna Loa and Kilauea), on both oceanic and continental crust; As continental flood basalts. Some erupted magmas contain
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