Igneous and Metamorphic Rock Forming Minerals Department of Geology Mr. Victor Tibane 1 SGM 210_2013
Intrusive and Effusive Rocks Effusive rocks: rapid cooling small crystalls or glas Lava & ash Magmatic intrusion (pluton) Intrusive rocks: slow cooling large crystalls Magma chamber Press & Siever, 1995
Volcanic vent Volcanic edifices Ash & lapilli deposits Vent fill (volcanic neck) with radial dikes Feeder dike volcanic conduit Sill - lateral dike parallel bedding Press & Siever, 1995
Volcanic Geosystem Pipe Side vent Central vent Lava flows...accumulating on the surface to form a volcano. Lavas erupt through a central vent and side vents,... Magma chamber...rises through the lithosphere to form a crustal magma chamber. Lithosphere Magma, which originates in the asthenosphere...
Ship Rock, New Mexico Diatreme
Diatreme (Ship Rock)
About Volcanoes Volcanoes are windows through which we can see the interior of the Earth. Volcanoes help us understand the plate tectonic process and mantle convection. Volcanoes are also connected to Earth s atmosphere and hydrosphere.
Volcanism: Volcanic products: Lava: Magma extruded at the surface. Eruption of magma can be of explosive or non-explosive nature The eruption kind is dependent on the chemistry of the magma Hawaii Äthna
Eruptions: Explosive: strato (composite) volcanoes Effusive: shield volcanoes and fissures Phreato-magmatic: Explosions in contact to external water (ground-, lake-, ocean-, melt water) Magmatic: Explosion and fragmention of lava through juvenile volatiles
Three types of volcanic eruptions are differentiated accordingly to their historical places of eruption: Hawaiian: as a rule, low viscosity basaltic magmas, up to 500 m high lava fountains and/or extensive Lava flows. Strombolian: SiO 2 -richer and thus higher viscosity, basaltic magmas, rich in volatiles with large gas accumulations in the roof of the magma chamber, such accumulations can explode violently at sudden pressure release. Magma fragments are ejected in ballistic trajectories out of the volcanic vent and are piled up to a scoria cone around the volcanoes. Eruption column can rich 10 km height, and lapilli und ash are accumulated from these columns and clouds to pyroclastic deposits. Plinian (Vesuvian): SiO 2 -rich, differentiated and volatile rich magmas with up to 65 km high eruptive columns and laterally extensive tephra-sheaths.
Bimodality of magmas in the Earth crust
Properties of Magma Magmas are silicatic melt solutions molecular-disperse or dissociated At conditions of... Association or Polymerisation equilibrium
Rheology (flow behaviour) The rheological behaviour of a melt (magma) (or of a solid rock under very high pressure) describes the flow properties. The flow properties of a magma depend on its viscosity Viscosity: The viscosity of a magma depends on temperature, chemical composition and on volatile content of the magmas. Low viscosity magmas can flow with an average of 16 km/h.
Viscosity: Generally: The lower the temperature the higher the viscosity The more (SiO 4 ) 4- -anions the higher the viscosity (polymerisation degree) The higher the gas content in the melt the higher the viscosity Rhyolite e Viscosity (Poise) e Tholeiitic Basalt Alcaline basalt e Temperature ( C) Therefore, the highest viscosity occurs in the felsic differentiates (temperature (SiO 4 ) 4- and high gas content!). At volcanic eruptions these are the most dangerous magmas, with the highest hazard potential.
Building stones of polymers The more (SiO 4 ) 4- -anions the higher the viscosity (degree of polymerisation) SiO 4 -Tetrahedron
Recognition of laminar flow in crystalline rocks by alignment of components
Lava flows: non explosive magma eruptions, basaltic, low viscosity, low gas content. Texture of the lava flows: Pahoehoe lava Block lava (aa-lava) subaquatic Pillow Lava Pahoehoe lava, Iceland Block lava, Iceland
Block-Lava (LaPlama, Canary Island) Aa-Lava
Aa-Lava flow, Iceland
Aa-Lava flow, Iceland
Hawaii Pahoehoe lava
La Palma, Canary Islands Pahoehoe-Lava
Turbulent lava outflow from a lava tube, basaltic lava, Hawaii
Pahoehoe lava
Low viscosity, fast flowing basaltic lava, Hawaii
The lava flows of Hawaii have a lateral extension of 100 s of square kilometres and reach often flow velocities of 10-20km/h
Pillow lavas on the sea floor
Lavas Deposits Summary Types of lava basaltic lavas (1000 1200ºC) aa pahoehoe pillow lavas andesitic lavas (<1000ºC) rhyolitic lavas (600 800ºC)
Prism - lava: Lembobo basalts
Pyroclastic deposits fragmentation of lava in eruptions Arenal, Costa Rica: pyroclastic eruption
Vesicles in basalt - gas in lava large small
Pyroclastics: explosive transport - glowing hot rock fragments at different sizes, the eruption column carries hot gases and ashes into the stratosphere (above 45 65 km height). Tephra: accumulation of pyroclastics, classified by it s diameter (sediment) Volcanic Ash (<2mm) Lapilli (<64mm) Volcanic Bombs (>64mm) Tuff: hardened, solidified volcanic ash and lapilli content: solidified lava fragments and xenoliths, country rocks Pumice (bims): glassy lava fragments with gas-filled pores and bubbles
Tuffs Nomenclature of Pyroclastics Grain size: > 64 mm (2 6 ) Bombs 64 2 mm (2 6 2 1 ) Lapilli < 2 mm (2 1 ) Ash Tuffites Mixture of fine-coarse grained epi- and pyroclastics Grains size of tuffites classifies the rock as in other non volcanic sedimentary deposits (e.g. volcanic sandstone)
Pumice Lapilli tuff Ash-tuff Bomb
Volcanic ejected bomb
Volcanic tuff
Volcanic breccia
Thought questions for this chapter Give a few examples of what geologists have learned about Earth s interior by studying volcanoes and volcanic rocks. Which rocks give more information on the Earth s interior, basaltic or rhyolitic? In which way are extrusive and pyroclastic rocks different? Why is basaltic volcanism much more common then rhyolitic volcanism?