CEE 1030 1st Edition Lecture 4 Which of the following minerals are harder than diamond 1 graphite 2 calcite 3 quartz 4 gypsum 5 none of these White asbestos are blue asbestos are different forms of the same mineral false What controls the style of a volcanic eruption Effusive lava flow vs explosive eruption Magma composition volcanic rock and SiO2 content Nature of volcanic eruptions these factors control viscosity of a magma this measures a material s resistance to flow Temperature hotter magmas are less viscous more runny Types of volcanic deposits non explosive eruptions lava flows mainly basaltic and intermediate compositions explosive eruptions pyroclastic deposits mainly intermediate to rhyolitic compositions basaltic lava flows very fluid behavior can flow at rate typically 10 200 meters hour travel distances of several 100km rhyolitic lava flows very sticky pahoehoe ropy texture Aa rough jagged blocky texture main difference between pahoehoe and Aa is the viscosity Aa very rocky would hurt to walk on Aaaaa if walked on it in bare feet Basaltic volcanoes form shield volcanoes and cinder cones scoria cones shield volcanoes large broad circular shape with gentle dipping slopes 3 to 5 degrees built by successive fluid lava flows from a central vent cinder cones also known as scoria cones built from ejected gas rich lava fragments have very steep angles fissure eruptions 1783 laki eruption Iceland largest historic lava flow These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute flood basalt plateaus huge stacks of thick fissure fed basalt lavas Intermediate rhyolitic volcanoes composite cones or stratovolcanoes interbedded lava flows and pyroclastic debris potentially most dangerous volcano type complex histories Mt St Helens 1980 largest eruption in historic times in the USA summit lowered by more than 400 m over 1km 3 of ash and rock debris erupted Different types of pyroclastic material falls from eruption cloud of material explosively ejected from the vent flows are gravity controlled surface flows from collapse of eruption column or lava dome pyroclastic material rapid expansion of gas bubbles results in the obliteration and fragmentation of magma and rock Lahars volcanic mud flows Mixture of volcanic debris and water eruption triggered melting of snow and glaciers heavy rainfall saturates loose ash deposits 1991 eruption of Mt Pinatubo Philippines coincided with a major typhoon 2 nd largest eruption in the 20th century 1985 Nevado de Ruiz Columbia 23 000 people killed hydrothermal systems circulation of water around a magma chamber leaches elements from surrounding rocks can form economically important metal ores massive intrusions batholiths batholith largest intrusions cover areas or 100 km 2 made of smaller intrusions called stocks or plutons