Download Igneous Rocks and Intrusive Igneous Activity

Igneous Rocks
and Intrusive
Igneous Activity
Introduction ú  Igneous rocks make up large parts of the continents and all the oceanic crust -­‐ indicate plate boundaries and hot spot activity Granite ú  Mostly quartz and feldspar ú  Used for kitchen counters, facings on buildings, tombstones, etc ú  Molten rock material below the Earth's surface is called magma. ú  If the magma reaches the surface it is called lava. ú  Magma is erupted as either:   Lava flow or Pyroclastic materials. Composition of Magma ú  Silica is the primary constituent of magmas ú  Magmas are differentiated based on the relative proportions of silica, iron, and magnesium.   Felsic – silica rich, iron and magnesium poor   Intermediate – intermediate between felsic and mafic   Mafic – silica poor, iron and magnesium rich How Hot Are Magma and Lava? ú  Measured temperatures of lavas suggest that typical ú  mafic magma is somewhat hotter than 1200ºC ú  felsic magma is somewhat hotter than 900ºC. Viscosity – Resistance to Flow ú  Viscosity is controlled primarily by temperature and composition (silica and iron content).   felsic magma Mafic magma – high viscosity low viscosity MORE EXPLOSIVE !!!!!
Bowen s reaction series ú  Bowen s reaction series describes the sequence of mineral crystallization in a cooling magma. ú  There are two branches in the reaction series:   Discontinuous series. -­‐ produces ferromagnesian minerals   Continuous series -­‐ produces a variety of plagioclase feldspars How Does Magma Originate and Change? The Origin of Magma at Spreading Ridges. ú  Magmas that forms beneath spreading ridges as a result of mantle plumes are mafic. Fig 4.5, p. 92
How Does Magma Originate and Change? Subduction Zones and the Origin of Magma ú  Along subduction zones at convergent boundaries, partial melting of the subducting plate produces intermediate to felsic magmas. Fig 4.5, p. 92
How Does Magma Originate and Change? Hot Spots and the Origin of Magma. ú  Magmas can originate from hot spot activity   Rising plumes in the ocean create volcanic islands far from plate boundaries.   Flood basalts, huge thick flat-­‐lying layers of basalt, originate this way. Igneous Rock Textures ú  Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow. ú  Two broad groups based on texture are:   Volcanic (extrusive) Rapid cooling = = Aphanitic texture =mineral grains are not easily visible   Plutonic (intrusive) Slow cooling = = Phaneritic texture: mineral grains that are easily visible   Rocks with more complex cooling histories are characterized by porphyritic textures. Mineral grains with different sizes. Intrusive Igneous Bodies -­‐ Plutons ú  Plutons are bodies of igneous rock which have been intruded in country rock or have formed in place far beneath the surface. Geo-inSight 1., p. 100
v Dikes and Sills Dikes and sills are the most common sheet-­‐like igneous intrusions. ú  Dikes are discordant features (meaning they cut across layering in the country rock) ú  Sills are concordant (parallel to the rock layers). Laccoliths, Volcanic pipes and necks ú  Laccoliths are sill-­‐like bodies with inflated cores. Geo-inSight 6. and 7., p. 101
Laccoliths, Volcanic pipes and necks ú  Volcanic pipes are magma-­‐
filled, cylindrical feeder channels beneath volcanoes. ú  Pipes can become volcanic necks with deep erosion. Geo-inSight 3., p. 100
Laccoliths, Volcanic pipes and necks Dikes, sills, and laccoliths radiate from many volcanic pipes and necks. Batholiths and Stocks ú  Batholiths are plutons that have more than 100 km² in area of exposure. ú  Stocks are somewhat smaller plutonic bodies.