Introduction: Volcanic eruptions are perhaps nature's most exciting display of Earth processes. For centuries, humankind has been both fascinated and intrigued by the forces of nature which result in volcanic eruptions. We know today that certain types of volcanoes are characteristic of certain types of plate tectonic settings. Therefore the occurrence of volcanoes and volcanic eruptions must be studies in the context of plate tectonics. While studying this topic, be sure to understand why volcanoes have different forms and shapes, and why volcanoes erupt different types of materials. Be sure to know definitions of the most common eruptive products and the most common volcano types. Importantly, be sure to know the given examples of each type of volcano.
I. Materials Produced During an Eruption
Lava Flow - A 'stream' of molten rock (Fig 9.4, 9.6)
'aa' lava flow - jagged, rubbly, broken surface (Fig 9.6a)
'pahoehoe lava flow - smooth, ropy surface (Fig 9.6b)
columnar joints (Fig 9.28) - form as lava flows cool and develop shrinkage fractures that produce elongate, pillar-like columns.
Gasses - Dissolved in magma (similar to the dissolved gasses in carbonated beverages). Most abundant gasses: H2O, CO2, SO2, N2, Cl2, H2, Ar. In the photograph below, volcanic gasses which escaped from a near-surface body of magma are being vented into the atmosphere through cracks in the Earth's surface called fumaroles. Explosive volcanic eruptions are triggered by the build-up of gas pressures inside the magma chamber. When the outward pressure exerted by gasses within the magma chamber exceeds the pressure of overlying rock, an explosive eruption is triggered.
Pyroclastic Materials (Fig 9.8) - Pulvarized rock and magma produced by explosive eruption. (volcanic ash, pumice, lapilli, cinder, blocks, bombs).
volcanic ash - microscopic volcanic glass (Fig 9.8)
pumice - vesicular, glassy rock formed by rapid solidification of frothy lava
lapilli - pyroclastic material visible to the naked eye
cinder - pyroclastic material 2 to 64 mm in diameter
block - pyroclastic material > 64 mm in diameter, erupted as hardened lava (Fig 9.8)
bomb - pyroclastic material > 64 mm in diameter, erupted as molten lava (Fig 9.8)
II Types of Eruptions
Hawaiian - lava fountains and low viscosity lava flows (Fig 9.4)
Strombalian - blocks and bombs (Fig 9.9)
Vulcanian - mostly gas, ash, cinder, bombs (Fig 9.6)
Plinian - large quantities of volcanic ash and pyroclastic flows (Fig 9.19)
III. Types of Volcanoes
Listed in order of increasing magma viscosity:
1. Basaltic Plateau (Flood Basalt Plateau) (Fig 9.23, 9.24)- Example = Columbia River Plateau - Forms as a result of eruption of very low viscosity basaltic lavas from fissures (Fig 9.23) (long cracks in Earth's surface from which lavas are erupted). Columbia River basalts were erupted about 17 my ago, covering an area of approximately 200,000 km2 with lava flows averaging 1 km in thickness.
2. Shield Volcano (Fig 9.13, 9.14) - Forms as a result of eruption of low viscosity basaltic lavas. Broad, gently sloping flanks. Earth's highest volcanoes are shield volcanoes. Most (not all) form on the ocean floor, forming volcanic islands or submerged volcanic peaks (seamounts). Example = Hawaii (Mauna Loa, Kilauea) , Iceland, Galapagos Islands
3. Cinder Cone (Fig 9.16) - - Forms as a result of eruption and build-up of mostly loose, cinder-sized pyroclastic material from a gas-rich basaltic magma. Sometimes erupt basaltic lava flows. Typically no more than 300 m in elevation. The photograph below shows a cinder cone in Utah which is being mined for 'lava rock' to be used in gas grills, aquariums, etc. Example = Paricutin, Mexico (Fig 9.12)
4. Composite Volcano (Stratovolcano) (Fig 9.11, 9.18) - Forms as a result of alternating eruptions of pyroclastic material and lava flows. Results in alternating layers of lava and volcanic ash. The lava flows protect underlying ash deposits from erosion. Due to their relatively high viscosity, the flows cannot flow great distances from their source. Consequently, composite volcanoes are typically high and have very steep slopes. Due to high magma viscosity, composite volcanoes are characterized by highly explosive eruptions, producing a greater volume of pyroclastic material than lava flows. Examples: St. Helens (WA), Rainier (WA), Hood (OR), Popocatepetl (Mexico)
Materials and Eruptive Processes Common to Composite Volcanoes:
Crater - (Fig 9.16) Steep-walled depression at or near volcano summit from which lava and pyroclastic materials are ejected.
Dome (Spine) - An accumulation of viscous lava directly over a volcano's vent. Acts as a surface plug to the underlying magma plumbing system.
Ash Fall - Ash erupted into atmosphere is carried away from volcano before falling to earth. Volcanic ash particles are typically tricuspate in shape, representing regions between magmatic gas bubbles which were filled with magma prior to eruption. Rocks which form by the accumulation of valcanic ash are called tuff. If the glass particles are very hot when deposited, they may fuse together to form a welded tuff.
Ash Flow (Pyroclastic Flow) (Fig 9.19)- Hot, dense mixture of rock fragments, gas, and ash rapidly flows down flanks of volcano (speeds up to 125 mph). Temperatures inside ash flow ~ 1000 0C. Typically form when a vertical eruption cloud begins to collapse, causing massive amounts of pyroclastic debris to rush down the volcano's flanks. Pyroclastic flows have been documented to move up to 100 km from their source. It is unlikely that any human will survive a pyroclastic flow. If not burned to death, humans are usually asphixiated by the lack of oxygen or suffocate after lungs fill up with volcanic ash particles. The hurricane force winds typically destroy all man-made structures. The 1902 eruption of Mt. Pelee on the Caribbean island of Martinique produced a pyroclastic flow that killed all but a few of the town of St. Pierre's 28,000 residents. One of the survivors was a prisoner who was being kept in an underground prison cell.
Lahar - (Fig 9.21) Volcanic mudflow. Produced when large amounts of water mix with volcanic ash forming a thick mud with the density of concrete. The lahars move down the volcano flanks, filling low-lying areas with thick accumulations of mud. Lahars may be triggered by excessive rainfall or melting of snow during warm weather, or by melting of glaciers by the heat associated with an eruption. Lahars pose the greatest threat to people living on and around stratovolcanoes because they may form without warning. Furthermore, the formation of lahars may occur when a volcano is dormant (not erupting).
In 1995, asmall eruption at Nevado del Ruiz volcano (Andes Range, Columbia, South America) melted ice and snow resulting in the formation of lahars which traveled down river valleys, killing 25,000 people. In the United States, Mt. Rainier volcano (Cascade Range, Washington) is considered a potential site of deadly lahars. Approximately 100,000 people live around the volcano in towns built on ancient lahar deposits. It is feared that future lahars at Mt. Rainier will follow the similar paths through heavily populated areas, resulting in widespread damage and casualties.
IV. Other Volcanic Landforms
Volcanic Pipe (Volcanic Neck) - (Fig 9.25) A conduit which connects the underlying magma chamber to volcano's vent. Represents the 'plumbing system' through which magma flows from the magma chamber to the Earth's surface. Sometimes is exposed at the surface after erosion of the overlying volcano.
Caldera (Fig 9.22) - Large crater which forms when a volcano collapses into partially empty magma chamber. The formation of calderas is sometimes caused by a highly explosive eruption which removes the overlying volcano and empties the underlying magma chamber. The weight of the overlying rock then causes collapse into the underlying void, resulting in a large depression on the Earth's surface.
Lava Tubes - Underground conduits through which lava
(usually basalt) flows. After the conduit is drained of lava, a tube-like
V. Intrusive Landforms (Fig 9.26)
Intrusive igneous landforms result from the cooling and crystallization of magmas beneath the surface, followed by erosion of overlying rock so that the intrusive landform is exposed at Earth's surface. The classification of intrusive landforms is based upon landform size, shape, and relationship to the surrounding 'country' rock. The study of intrusive landforms is important in that rocks contained within them provide important information about internal earth igneous processes which, of course, cannot be directly observed.
Discordant intrusive bodies - The margins of the intrusive landform cut across layering in the surrounding country rock.
1. Pluton - General term describing an intrusive landform.
2. Batholith (Fig 9.29) - A pluton with > 100 km2 exposed at the surface. Typically, are composed of multiple smaller intrusive bodies containing a variety of igneous rock types.
3. Stock - A pluton with < 100 km2 exposed at the surface.
4. Dike - Sheet-like intrusive body.
Concordant intrusive bodies - The margins of the intrusive landform are parallel to layering in the surrounding country rock.
1. Sill (Fig 9.27) - Tabular or sheet-like intrusive body formed when magma is injected aling sedimentary bedding surfaces. Usually form from low viscosity magma.
2. Laccolith - Similar to a sill, but magma collects as a lens-shaped mass that arches the overlying strata upward. Magma viscosity is slightly higher than that for a sill.
Study Questions - Volcanoes and Intrusive Landforms