Page 447 - Geosystems An Introduction to Physical Geography 4th Canadian Edition
P. 447

Chapter 13 Tectonics, Earthquakes, and Volcanism 411
   Continental rift valley
Seafloor Island-arc volcanoes spreading
Transform fault
Asthenosphere
▲Figure 13.22 tectonic settings of volcanic activity. Magma rises and lava erupts from rifts, through crust above subduction zones, and where thermal plumes at hot spots break through the crust. [Adapted from U.S. geological Survey, The Dynamic Planet (Washington, DC: government Printing Office, 1989).]
Animation
Forming Types of Volcanoes
Kliuchevskoi, Siberia) or oceanic plate–oceanic plate
convergence (the Philippines; Japan)
• Along sea-floor spreading centres on the ocean floor
(Iceland, on the Mid-Atlantic Ridge; off the coast of British Columbia, Oregon, and Washington) and along areas of rifting on continental plates (the rift zone in East Africa)
• At hot spots, where individual plumes of magma rise to the crust (Hawai‘i; Yellowstone National Park; Nazko region in central B.C.)
Volcanic Materials
A volcano is the structure in the Earth’s crust containing an opening at the end of a central vent or pipe through which magma rises from the asthenosphere and upper mantle. Magma rises and collects in a magma chamber deep below the volcano until conditions are right for an eruption. This subsurface magma emits
As discussed in Chapter 12, solidified magma forms igneous rock. When magma emerges at the surface, it is lava. The chemistry of lava determines its behaviour (whether it is thin and liquid or is thick and forms a plug).
Geologists classify lava as felsic, intermediate, or mafic, depending on its chemical composition (review Table 12.2). Mafic, or basaltic, lava has two principal forms, both known by Hawaiian names (Figure 13.23). The composition of both these forms of lava is the same; the texture difference results from the manner in which the lava flows while it cools. Rough and jagged basalt with sharp edges is aa; it forms as a thick skin over the surface of a slowing lava flow, cracking and breaking as it cools and solidifies. Shiny and smooth basalt that resem- bles coiled, twisted rope is pahoehoe; it forms as a thin
Chain of
volcanic mountains
Fracture zone
Oceanic Mid-ocean rift plate
Upwelling zone Asthenosphere
Extinct seamount
 Hot-spot volcano Lithosphere
Thermal plume
   tremendous heat; in some areas, it boils groundwater, as seen in the thermal springs and geysers of Yellowstone Na- tional Park and at other locations with surface expressions of geothermal energy.
Various materials pass through the central vent to the surface to build volcanic landforms, including lava (magma that has cooled to form rock), gases, and pyroclastics—pulverized rock and clastic materials of various sizes ejected violently during an erup- tion (also called tephra). These mate- rials may emerge explosively, or they may emerge effusively (flowing gen- tly) from the vent (eruption types are discussed ahead).
(a) Aa is a rough, sharp-edged lava said to get its name from the sounds people make if they attempt to walk on it.
(b) Pahoehoe forms ropy cords in twisted folds.
▲Figure 13.23 two types of basaltic lava—Hawaiian examples, close up. [Bobbé Christopherson.]
 Continental plate
Flood basalts
Lithosphere
S
u
b
d
u
c
t
i
o
n
z
o
n
e
Subduction zone



















































   445   446   447   448   449