GLG 101

Chapter 4

Volcanism

Volcanism: processes whereby magma (molten rock) and its gases are extruded onto the Earth's surface. During Mount St. Helens explosive eruption (Figure 4.1, p85), magma solidified rapidly blasted explosively by gases into air, producing pyroclasts (rock fragments).

Volcanic gases: most of gas released during volcanic eruptions water vapor, condenses as steam. Other gases, carbon dioxide, sulfur dioxide, hydrogen sulfide (rotten egg smell), hydrochloric acid, given off lesser amounts with steam.

Lava flows:

Pahoehoe lava – characterized by ropy surface (Figure 4.16), formed by quick cooling (slide 341).

Aa lava – characterized by rough, jagged angular surface (Figure 4.17), forms basalt cool enough partially solidified and moves slowly (slide 342).

Pillow structure - basalt erupts underwater in ocean forms pillow structure. Pillow basalts commonly form along crests mid-oceanic ridges.

Pyroclastic material:

Ash (very fine, <2.0 mm) (slide 347)

Lapilli (fine, 2 to 64 mm) (slide 350)

Bomb, block (>64 mm) (slide 351)

Volcanoes:

Crater: more or less circular central depression over vent at summit of cone, <1km in diameter (Figure 4.14).

Caldera: volcanic depression much larger than crater, having diameter of at least 1 kilometer. Created volcano’s summit blown off exploding gases, or as case Crater Lake, when volcano collapses into partially emptied magma chamber.

Three major types volcanoes (shield, cinder cone, and composite).

Shield volcanoes: large gently sloping volcanoes. During eruptions, lava spreads widely and thinly due to low viscosity. Because lava flows from central vent, without building up much near vent, slopes usually between 2 degrees and 10 degrees from horizontal, producing volcano in shape of flattened dome or shield (Figure 4.15). Mafic or basaltic composition (slides 354 – 358).

Cinder cones: volcano consisting mostly pyroclastic material ejected from central vent (Figure 4.20). Commonly have slopes of approx 30 degrees. Most of ejected material lands near vent during eruption, building up cone to peak. Commonly found on flanks and in calderas of Hawaii’s shield volcanoes. Few exceed height of 500 meters (slides 359 – 361).

Composite volcanoes (stratovolcanoes): composed of pyroclastics and lava flows (intermediate). Slopes not as steep cinder cones, but steeper than shield volcanoes. Young composite volcanoes, predominantly composed andesite, aligned along circum-pacific belt (slides 362-370).

Volcanic domes: steep-sided, domed or spine shaped masses of volcanic rock formed from viscous lava solidifies in or immediately above volcanic vent. Example: Volcanic dome grew within crater of Mount St. Helens after eruption in May 1980 (Figure 4.25).

Distribution of volcanoes:

Nearly all larger and better known volcanoes of world are composite volcanoes. Tend to align along two major zones or belts, Circum-pacific belt or “Ring of Fire” (Figure 4.22) and the Mediterranean belt.