Page 451 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 13 Tectonics, Earthquakes, and Volcanism 415
Ash
Ash fall
Vent Lava Dome
Sill
Aerial bombs
Crater Nue´e ardente Buried dome
Pyroclastic flow Fissure flows
Flank eruption
Conduit
Magma chamber
A typical composite volcano with its cone-shaped form in an explosive eruption.
▲Figure 13.29 a composite volcano.
intensity for 9 hours and then blasted new material inter- mittently for days (Figure 13.31).
The slumping that occurred along the north face of the mountain produced the greatest landslide witnessed in recorded history; about 2.75 km3 of rock, ice, and trapped air, all fluidized with steam, surged at speeds approaching
250 km·h−1. Landslide materials travelled for 21 km into the valley, blanketing the forest, covering a lake, and fill- ing the rivers below.
As destructive as such eruptions are, they also are constructive, for this is the way in which a volcano even- tually builds its height. Before the eruption, Mount St. Helens was 2950 m tall; the eruption blew away 418 m. Today, Mount St. Helens is building a lava dome within its crater. The thick lava rapidly and repeatedly plugs and breaks in a series of lesser dome eruptions that may con- tinue for several decades. The buildup of the lava dome is now more than 300 m high, so a new mountain is being born from the eruption of the old.
Mount Pinatubo The 1991 eruption of Mount Pinatubo in the Philippines was the second largest of that century (the largest was Novarupta on the Katmai Peninsula in Alaska in 1912), and the largest to affect a densely populated area. The eruption produced 15 to 20 million tonnes of ash and 12 km3 of magma, ash, and pyroclastics—about 12 times the volume of material produced by Mount St. Helens. The loss of this vast amount of material caused the summit of the volcano to collapse, forming a caldera 2.5 km in diameter. The eruption killed 800 people and devastated many surrounding villages. However, accurate prediction of this event saved many lives, as approximately 60000 evacuated their homes prior to the eruption.
Although volcanoes are local events, they have global effects. As discussed in Chapters 1 and 6, the Mount Pina- tubo eruption affected Earth’s climate, releasing an aerosol
(b) After the eruption, the scorched earth and tree blow-down area covered some 38 950 hectares.
(a) Mount St. Helens prior to the 1980 eruption.
▲Figure 13.30 Mount St. Helens, before and after the 1980 eruption. [(a) Pat and Tom Lesson/Science Source. (b) Krafft-Explorer/ Science Source.]
Georeport 13.4 Slow Slip Events across Kı¯lauea’s South Flank
The entire south flank of the Kı¯lauea volcano is in a long-term motion toward the ocean, along a low-angle fault at a rate of about 7 cm per year. gPS units, tiltmeters, and small earthquakes mark the slow movement. In February 2010, in one
36-hour period, a slow slip event of 3 cm occurred, accompanied by a flurry of small earthquakes. A sizeable collapse or failure of this new basaltic landscape into the sea is possible, but the eventual outcome of these slow slip events is uncertain. In 2011, lava flows to the ocean changed course as new eruptions began west of Pu’u O’o.