1. Volcanoes and Other Igneous Activity

2. Types of Volcanoes ► Three basic types  Shield Volcano  Cinder Cone  Composite (Stratovolcano)

3. Shield Volcanoes ► Characteristics  Broad, slightly domed-shaped  Composed primarily of basaltic lava  Generally cover large areas  Produced by mild eruptions of large volumes of lava

4. Examples Mauna Loa

5. Cinder Cones ► Characteristics  Built from ejected lava (mainly cinder-sized) fragments  Steep slope angle (Generally not greater than 30 degrees)  Rather small size  Frequently occur in groups

6. Example Sunset Crater

7. Composite Volcano ► Characteristics  Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)  Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)  Composed of interbedded lava flows and layers of pyroclastic debris ► Most violent type of activity (e.g., Mt. Vesuvius) ► Often produce a nueé ardente ► Fiery pyroclastic flow made of hot gases infused with ash and other debris  Move down the slopes of a volcano at speeds up to 200 km per hour ► May produce a lahar, which is a volcanic mudflow

8. Examples ► Mt. Fuji

9. Why so Different? ► Viscosity of Magma  Viscosity is a measure of a fluids resistance to flow ► Water has a very low viscosity ► Honey has a very high viscosity

10. Viscosity of Magma ► Factors affecting viscosity  Temperature - Hotter magmas are less viscous  Composition - Silica (SiO2) content ► Higher silica content = higher viscosity ► Lower silica content = lower viscosity or more fluid-like (Basaltic vs. Felsic magmas)  Dissolved Gases ► One to six percent of a magma by weight ► Mostly water and CO2  Gas content affects magma mobility  Gases expand within a magma as it nears the Earth’s surface due to decreasing pressure

11. So What??? ► Viscosity will determine not only how the lava behaves on the surface but also how violent the eruption is  *  *Fluid basaltic lavas generally produce quiet eruptions  *  *Highly viscous lavas (rhyolite or andesite) produce more explosive eruptions

12. Lava Flows ► Basaltic lavas are the most fluid and probably the type we are all most familiar ► Types of basaltic flows  Pahoehoe lava (resembles a twisted or ropey texture)  Aa lava (rough, jagged blocky texture)

13. Examples Pahoehoe

14. Examples ► Aa

15. What comes out…. ► Types of pyroclastic debris:  - fine, glassy fragments  Ash and dust - fine, glassy fragments  - porous rock from “frothy” lava  Pumice - porous rock from “frothy” lava  - walnut-sized material  Lapilli - walnut-sized material  - pea-sized material  Cinders - pea-sized material ► Particles larger than lapilli  - hardened or cooled lava  Blocks - hardened or cooled lava  - ejected as hot lava  Bombs - ejected as hot lava Bomb

16. Other Volcanic Landforms: ► Fissure eruptions and lava plateaus  Fluid basaltic lava extruded from crustal fractures called fissures  e.g., Columbia River Plateau ► Lava Domes  Bulbous mass of congealed lava  Most are associated with explosive eruptions of gas-rich magma ► Volcanic pipes and necks  Pipes are short conduits that connect a magma chamber to the surface  Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone

17. Plutonic Activity Most magma is emplaced at depth in the Earth ► An underground igneous body, once cooled and solidified, is called a pluton ► Classified by shape and orientation:  Shape: ► Tabular (sheetlike) ► Massive  Orientation with respect to the host (surrounding) rock ► Discordant – cuts across sedimentary rock units ► Concordant – parallel to sedimentary rock units

18. Types of intrusive igneous features ► Dike – a tabular, discordant pluton ► Sill – a tabular, concordant pluton (e.g., Palisades Sill in New York) ► Laccolith-Similar to a sill  Lens or mushroom-shaped mass  Arches overlying strata upward ► Batholith- Largest intrusive body  Surface exposure of 100+ square kilometers (smaller bodies are termed stocks)  Frequently form the cores of mountains

19. Emplacement of magma ► Magma at depth is much less dense than the surrounding rock ► Increased temperature and pressure causes solid rock to deform plastically ► The more buoyant magma pushes aside the host rock and forcibly rises in the Earth as it deforms the “plastic” host rock ► At shallower depths, the host rock is cooler and exhibits brittle deformation ► Movement of magma here is accomplished by fractures in the host rock and stoping

20. Global distribution of igneous activity ► Not random!  Most volcanoes are located within or near ocean basins ► Basaltic rocks are common in both oceanic and continental settings, whereas granitic rocks are rarely found in the oceans

21. Global distribution of igneous activity ► Spreading centers  The greatest volume of volcanic rock is produced along the oceanic ridge system  Mechanism of spreading ► Lithosphere pulls apart ► Less pressure on underlying rocks results in partial melting of mantle ► Large quantities of basaltic magma are produced

22. Global distribution of igneous activity ► Subduction zones  Occur in conjunction with deep oceanic trenches ► Descending plate partially melts ► Magma slowly moves upward ► Rising magma can form either an island arc if in the ocean or a volcanic arc if on a continental margin  Associated with the Pacific Ocean Basin ► Region around the margin is known as the “Ring of Fire”  Most of the world’s explosive volcanoes are found here

23. Global distribution of igneous activity ► Intraplate volcanism  Activity within a tectonic plate  Associated with plumes of heat in the mantle ► Form localized volcanic regions in the overriding plate called a hot spot  Produces basaltic magma sources in oceanic crust (e.g., Hawaii and Iceland)  Produces granitic magma sources in continental crust (e.g., Yellowstone Park)

24. Yeah and…. ► Volcanoes and the Climate  Explosive eruptions emit huge quantities of gases and fine- grained debris into the atmosphere which filter out and reflect a portion of the incoming solar radiation  Examples of volcanism affecting climate ► Mount Tambora, Indonesia – 1815  The year without a summer ► Krakatau, Indonesia – 1883 ► Mount Pinatubo, Philippines – 1991