1. Figure 5.1: Often, rising magmas reach the surface and erupts as lava or pyroclastics. A volcano is a hill or mountain that forms from the accumulation of material that erupts at the surface.

2. Volcanic eruptions are often devastating and have destroyed many lives and property throughout recorded history.

1. There are several types of volcanic deposits, each producing a characteristic lava flow or accumulation. The size and shape of the resulting volcanic mountain depends on the type of volcanic material erupted.

2. Basaltic lava solidifies into a black rock called basalt (Figs 5.3 & 5.4) with a high content of mafic minerals. Basaltic lava is a high temperature melt (1000 - 1200 ^oC) that has a relatively low silica content (48 - 50 wt% SiO₂). Basaltic lava is therefore very fluid and can flow downhill at high velocities (up to 100 km/hr) for great distances.

3. Rhyolitic lava is light in color since they contain mostly felsic minerals. Rhyolitic lava erupts at lower temperatures (800-1000 ^oC) and contains more silica (> 65 wt. % SiO₂) than basaltic lava. Rhyolitic lava is therefore more sluggish, moves more slowly and builds thicker lava flows than basalt.

4. Andesitic lava has a silica content intermediate between basalt and rhyolite (55 - 65 wt. % SiO₂) and therefore has flow characteristics intermediate between these two lava types.

5. Volcanic glass forms when lava is quenched too rapidly for crystals to form. Obsidian is a dense glass that forms from non-gaseous lava whereas pumice is riddled with tiny holes and cavities due to quenching from gaseous lava (Fig. 4.3).

1. Pyroclastic Deposits: Unlike lava, pyroclastic deposits consist of an accumulation of tiny rock fragments, glasses and mineral grains (Figs 5.8, 5.23 & 5.24). Pyroclastic deposits are formed from magmas that contain relatively abundant water and gases. These wet, gaseous magmas erupt explosively (Figs 5.6 & 5.9). The lava and any overlying solid rock is shattered into fragments of various sizes, shapes and textures.

2. One type of eruption is called a pyroclastic flow that consists of a huge cloud of hot ash, dust, and gasses that cascade downhill at speeds up to 200 km per hour. Pyroclastic flows are devastating and are known to have caused many deaths and to bury entire towns (Fig. 5.9).

3. Pyroclastic rocks are classified according to size. Unconsolidated material is collectively called tephra. Loose fragments and grains (tephra) less than 2 mm in diameter are referred to as volcanic ash while the rocks they form are called volcanic or ash-flow tuffs (Figs 5.23 & 5.24). Rocks formed from larger fragments are called volcanic breccia (Fig. 5.8)

1. Central eruptions create a volcanic mountain shaped like a cone (Fig. 5.1). These eruptions discharge lava or pyroclastic materials from a central vent which consists of a feeder channel leading from the magma chamber to an open vent at the surface.

2. Shield Volcanoes are constructed from successive flows of basaltic lava that flows easily and spreads widely. Shield volcanoes therefore have broad, gentle slopes and are the classic type of volcanoes in Hawaii (Fig. 5.10).

3. Volcanic Domes (Fig. 5.11) are constructed from felsic lava and therefore have much steeper sides since the Si-rich lava is more viscous and flows more sluggishly than basaltic flows.

4. Cinder Cones (Fig. 5.12) are built from pyroclastic debris that accumulates around the vent of an explosive volcano. The sides are composed of layers of pyroclastic deposits.

5. Composite or Stratovolcanoes (Fig. 5.14) erupt both lava and pyroclastic deposits. The slopes of stratovolcanoes are therefore composed of lava flows alternating with layers of pyroclastic deposits. Stratovolcanoes have steeper slopes than shield volcanoes and are common along convergent plate boundaries (Fig. 5.30).

6. After a violent eruption, the empty magma chamber directly below the vent may no longer be able to support its roof. The overlying rock therefore collapses, leaving behind a large bowl-shaped depression called a caldera (Fig. 5.16). The resulting caldera is many times the size of the original crater and can range from a few kilometers up to 50 km in diameter.

7. Not all eruptions occur from central vents. Sometimes, lava flows out along large fractures in the crust to form long, linear fire-fountains referred to as fissure eruptions (Fig. 5.20).

8. Phreatic explosions occur when hot, gaseous magmas come in contact with groundwater or seawater. The vast amount of superheated steam that is generated causes a phreatic, or steam explosion (Fig. 5.18).

9. Sometimes, gas-charged magmas melt their way upward to the surface and erupt gases, pieces of shattered lava, chunks of the vent walls, and fragments brought up from deeper in the crust and upper mantle. The volcanic vent and feeder channel becomes filled with breccia to produce a structure known as a diatreme. These diatremes, or volcanic pipes, can be exposed as tall structures on the surface after the surrounding softer material has been eroded away. Shiprock in New Mexico is a good example of an exposed volcanic pipe or neck (Fig. 5.19).

10. Volcanoes are often associated with emissions of gas fumes and steam through small vents that are called fumaroles (Fig. 5.26). These steaming fumaroles are often located near emissions of hot, boiling water in the form of hot springs and geysers (Fig. 5.27).

11. Fumaroles, hot springs and geysers are formed when underground water (groundwater) comes in close proximity to magma reservoirs (Fig. 12.24). The water is heated to boiling temperatures and the increased pressure forces the water to the surface through fractures or vents in the crust.