Volcanism and Its Landforms

Objectives Describe the distribution of volcanic activity and explain its relationship with plate boundaries Explain how the composition of magma influences the processes that occur during volcanic eruptions Discuss the major types of volcanic landforms, and the hazards associated with them Cite some dramatic historical examples of human interaction with volcanic environments Describe the landscapes that result from volcanism

Volcanoes The eruption of molten rock at Earth’s surface Process of creating new lithosphere Kanaga stratavolcano, Alaska. © USGS

Distribution of Volcanic Activity Volcanoes occur along plate boundaries Divergent boundaries – mid-ocean ridges Convergent boundaries – Subduction zones

Distribution of Some of the World’s Major Volcanoes Figure 7.26

Volcanoes Active Volcanoes – One that has erupted in recorded history Dormant Volcanoes – No evidence of eruption, but shows evidence of recent activity – Shows little sign of being worn down Extinct Volcanoes – Shows no sign of eruption and long-term weathering and erosion Belknap Shield Volcano, Oregon, is an extinct volcano. © USGS

Properties of Magma Viscosity – A fluid’s resistance to flow – Some magmas have higher viscosity than others because of their composition – Higher viscosity magmas typically have higher silica content and produce explosive eruptions Pyroclastics – solid fragments erupted from a volcano – Basaltic lavas have low viscosity and have effusive eruptions

Volcanic Landforms Calderas – After an eruption, the magma chamber empties and no longer supports the overlying surface – Surface rocks collapse where the magma chamber once subsisted leaving a large depression [Insert Fig caldera]

Composite Volcanoes Also called stratovolcanoes Explosive volcanoes Composed of alternating layers of lava and pyroclastics

Deadly Composite Volcanoes Lahars – Viscous mudflow of pyroclastic debris and water – May be triggered by rapid snow melt or rain Insert Fig Nevado del Ruiz

Composite Volcanoes Pyroclastic Flows – Ground-hugging avalanche – Composed of hot ash, pumice, rock fragments, and volcanic gas – May move as fast as 100 km/hr and be up to 500°C Right: Mayon pyroclastic flow, Philippines © USGS

Composite Volcanoes Predicting Risks – Understanding precursors to explosive eruptions – Frequent earthquakes – Growing bulge due to rising magma chamber – Increased gas emissions

A Nueé Ardente on Mt. St. Helens Figure 7.14

Formation of Crater Lake, Oregon Figure 7.17

Cinder Cones Single eruptive event Consists primarily of small fragments (cinders) Low silica content [Insert Fig East Africa cinder cone]

Shield Volcanoes Fluid basaltic, low- silica magma produce quiet eruptions Result in smooth, ropy lava called pahoehoe Broad, gentle-sloping flanks

A Lava Flow Figure 7.5 B

Shield Volcanoes

Profiles of Volcanic Landforms Figure 7.9

Basalt Plateaus and Plains Flood basalts represent the accumulation of many lava flows that spread over large areas [Insert Fig Columbia Plateau and Snake River Plain]

Hot Spots Intra-plate volcanism occurs over a stationary hot spot Lithospheric plate moves over the hot spot producing an active volcano As the plate moves off the hot spot, it produces a long chain of mostly extinct volcanoes

Landscapes of Volcanism Composite cones are limited to their geographic extent, but dominate the landscape

Landscapes of Volcanism

PREDICTING VOLCANIC ERUPTIONS