Geo. 101 1st Edition Lecture 10Outline of Last Lecture I. Igneous RocksII. Igneous Rock TexturesOutline of Current Lecture I. Volcanisma. ViscosityII. Eruption: Why?Current LectureWhy does Volcanism Occur?- Molten rock is less dense than solid rock- Rises buoyantly to the surface.- Why does some molten rock make it to the surface (extrusive) and some does not (intrusive)?- Many factors – temperature of magma, volume of magma, path magma follows and viscosity.~~~Viscosity:- A magma/lava’s resistance to flow.o Compositiono Temperature Felsic  Lower temperature = VERY high viscosity = sticky, slow moving Intermediate  Intermediate temperature = High viscosity Mafic  Higher temperature = Low viscosity = quick movingWhy do volcanoes erupt?- Think of a bottle of beer or a can of pop:o Water & CO2 are dissolved in magmao Gases exsolve as pressure is reduced from the magma – get eruption- Volcanic Eruptions:o Effusive: Gentle eruptions Lava dominatesThese 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. Magma/lava generally low in silica (Mafic) Low viscosity Release gases Occurs commonly at divergent boundaries and hot spotso Pyroclastic: Violent eruptions Mostly pyroclastic Lava high in silica (intermediate to felsic) High viscosity Gas is trapped Occur commonly at convergent boundaries and hot spots- Lava and gases erupt from volcanoes in two locations:o Central Vent: (Ex: Mt. St. Helens, WA)o From a Fissure (Crack): (Ex: Kilauea,HI)- Landforms produced by different styles of volcanic eruptions:o Effusive: (Fissure Eruptions) Flood Basalts:- Fissure eruption- Mafic lavas- Generally flat layers- Cover 100-1000km²- Ex: Columbia River Basaltso Central Eruption: Shield Volcanoes:- Vent eruptions- May also have fissure eruptions- Primarily mafic lava- Large area (100s km across)- Broad and gently sloping- Convex Up profile (shape)- Ex: Mauna Loa, HI; Newberry Volcano, OR Cinder Cone (Vent eruptions) Spatter Cone (Vent eruptions)o Pyroclastic: (Central Eruption) Composite Volcano (stratovolcano)- Vent eruptions- Steep slopes- Composed of alternation pyroclastic layers and felsic to intermediate lava flows- Concave up profile- Up to 10s km across- Ex: Cascade Volcanoes (Mt. Rainier, Mt. St. Helens, etc.); Volcanoes of the Japan Ocean Island Arc; Andes Volcanoes Calderas (think Calderon)- Central Vent- Extremely large explosive eruption cause the collapse of the summit into a partially empty magma chamber- Primarily felsic pyroclastic rocks- 10s km across- EX: Crater Lake, OR and Yellowstone Nat.