Volcanoes, Rocks and Minerals

The main kinds:

• Shield volcano: built of thin, runny basaltic lava flows piled up over a long time. Broad with gentle slopes. NCERT calls shields the largest volcanoes on Earth; the Hawaiian volcanoes are the classic examples.
• Acid lava cone: viscous, silica-rich lava moves only a short way, so the cone is high with steep slopes.
• Composite cone or stratovolcano: layers of lava and ash, one on top of another. These are the highest volcanic cones. Fuji, Mayon and Vesuvius are examples.
• Cinder cone: a small cone of loose ash and cinders.
• The Volcanic Explosivity Index (VEI) grades how strong an eruption is.
• Weathered lava gives deep, fertile soils, so volcanic slopes are often farmed intensively.

The settings:

• Arc volcanoes stand above subduction zones, at convergent boundaries where one plate sinks under another. Most are steep stratovolcanoes: Fuji (Japan), Mayon (Philippines), and Ojos del Salado in the Chilean Andes, the highest volcano on Earth.
• The Pacific Ring of Fire circles the Pacific: the Andes, Rockies, Alaska, Japan, the Philippines and Indonesia.
• At divergent boundaries magma wells up between plates. Iceland sits on the Mid-Atlantic Ridge.
• Hot spots feed volcanoes in the middle of a plate, as in Hawaii.
• The Himalayas are fold mountains from a continent-continent collision, so they have no extrusive volcanoes.

The three families:

• Igneous rocks form from cooling magma or lava. Slow cooling deep down gives coarse grains (intrusive, such as granite). Fast cooling at the surface gives fine grains (extrusive, such as basalt). Most of their minerals are silicates.
• Sedimentary rocks form from layers of sediment pressed and cemented together (lithification). Mechanically formed ones are made of carried grains: sandstone, conglomerate, shale, loess. Chemically formed ones are precipitated from water: geyserite, halite, rock salt. Organically formed ones come from living things: coal, chalk.
• Metamorphic rocks are older rocks changed by heat or pressure: limestone to marble, sandstone to quartzite, shale to slate, granite to gneiss.
• Ortho-metamorphic rocks come from igneous rocks; para-metamorphic rocks come from sedimentary rocks.
• Heat from an intruding magma gives contact (thermal) metamorphism. Pressure and shearing from moving rock give dynamic metamorphism.

The main ones (NCERT):

• Feldspar: silicon, oxygen, sodium, potassium, calcium and aluminium. About half the crust. Used in ceramics and glass.
• Quartz: silica. Hard, white or colourless. A main part of sand and granite.
• Pyroxene: calcium, aluminium, magnesium, iron and silica. Green or black. About 10% of the crust; common in meteorites.
• Amphibole: aluminium, calcium, silica, iron and magnesium. Green or black; used in asbestos.
• Mica: potassium, aluminium, magnesium, iron and silica. Splits into thin sheets; used in electrical goods.
• Olivine: magnesium, iron and silica. Usually a greenish crystal, often found in basaltic rocks; used in jewellery.