Explosivity of an Eruption -
Hawaii Shield Volcanoes -
Magma can be erupted as lava or tephra.
The explosivity of an eruption depends of two factors:
In general, explosivity increases as the
Viscosity of magma depends on the
The viscosity of a magma increases as the
Magmas with higher silica content are more viscous.
Magmas that erupt at lower temperatures are more viscous than magmas that erupt at higher temperatures.
Additionally, lava flows become more viscous as they cool.
Felsic volcanoes typically have more explosive eruptions, because felsic magmas
Students should be able to use the igneous rock classification chart to answer questions about
The more viscous felsic magmas tend to inhibit the escape of gases.
The trapped gases can buildup until they are released cataclysmically.
Felsic volcanoes generally form at subduction zones.
For example, Mt. St. Helens.
After Mt. St. Helens erupted, viscous felsic lava erupted to form a lava dome.
A lava dome is a round, steep-sided dome built by very viscous magma.
Generally forms from a silica-rich magma.
Such magmas typically are too viscous to flow far from the vent before cooling and crystallizing.
Domes might be made of one or more flows.
The volcanoes the form the Hawaiian Islands are mafic volcanoes.
Mafic volcanoes typically have less explosive eruptions, because mafic magmas
Gases generally escape more readily from the less viscous mafic magmas.
Lavas are classified using the names of the rocks they form:
Three types of lavas in Hawaii:
Pahoehoe can have a smooth surface or a ropy surface.
Ropy surface forms as the solidifying skin is dragged by the flowing interior.
Pahoehoe have a relatively lower viscosity and a higher temperature.
Pahoehoe flows tend to be thinner, average 1 m thick.
Pahoehoe generally flows faster than 'a'a.
Pahoehoe flows commonly advance in lobes called lava toes.
Lava toes form when the lava breaks the solidifying skin of a flow and oozes out.
'A'a tends to be much thicker than pahoehoe flows, they can be several meters
The lava is more viscous and has a lower temperature.
'A'a flows tend to flow slower than pahoehoe flows.
'A'a flows have clinker on the top and bottom, surrounding a thick, dense core.
Clinker forms as the solidifying skin is torn and rolled into balls.
The clinker falls onto the ground around the flow, and the lava flows over it.
Accretionary lava balls form on top of 'a'a flows.
Accretionary lava balls form as a mass of lava solidifies and is rolled along on top of the flow.
The lava ball increases in size as additional lava solidifies on the outside.
Both pahoehoe and 'a'a have the same chemical composition.
Nearly all lava in Hawai’i erupts as pahoehoe.
Some pahoehoe flows change to 'a'a.
Type of flow is a function of viscosity and internal shear (disturbance).
The viscosity of a lava flow increases as the flow
If the viscosity of the flow is high, a low shear can result in the formation
For example, a slow moving flow that has cooled significantly can change to ’a’a as it advances.
If the viscosity is low, a high rate of shear is required to convert pahoehoe
For example, a fluid pahoehoe flow pours over a steep slope, its speed increases and the flow changes to 'a'a.
Pahoehoe is more common closer to vents.
Pahoehoe flows can change to 'a'a flows, but not the other way around.
Flows can start as a pahoehoe flow and change to an 'a'a flow downslope.
Lava tubes form when lava drains from beneath a crust and leave empty tube.
Lava tubes are more common is pahoehoe flows, but can form in 'a'a flows.
Transport of lava in lava tubes prevents the lava from cooling rapidly, so the lava can be transported for much longer distances.
Pillow lava forms when magma flows into water.
Either submarine or subaerial flows.
The flow surface instantaneously cools to form a glassy skin.
The skin cracks and the flow extrudes a new pillow.
Three basic types of volcanoes:
Shield volcanoes are constructed of thousands upon thousands of lava flows.
Less than 1% of the volcanic material in a shield volcano is derived from explosive eruptions.
Shield volcanoes are relatively wide compared to their height.
Shield volcanoes form broad, shield-shaped domes.
The width results because low viscosity magma tends to flow far from the vent.
Shield volcanoes generally form from lower viscosity mafic lavas.
Additionally, lava commonly is transported in lava tubes and released down
Lava flows in Hawaii commonly reach the shoreline.
Examples of shield volcanoes in Hawaii: Mauna Loa, Kilauea, etc.Pyroclastic Volcanoes
Pyroclastic volcanoes are constructed primarily from tephra.
Pyroclastic volcanoes form cone shaped volcanoes, that are tall compared to
The shape results because most of the material falls close to the vent.
Examples of pyroclastic volcanoes in Hawaii: Leahi (Diamond Head), Koko Head, Punch Bowl, etc.Stratovolcanoes
Stratovolcanoes (composite volcanoes) are constructed from significant amounts
of both lava flows and pyroclasts.
Layers of pyroclastic materials is cemented with hardened lava flows to create larger, more stable volcanic structures.
Stratovolcanoes tend to form from more viscous felsic lavas
Stratovolcanoes also form cones that are tall compared to their widths, because the materials are deposited close to vent.
The pyroclasts fall close, and the viscous lava does not flow far from the vent.
Examples of stratovolcanoes: Mount Fuji and Mount St. Helens.
Mauna Loa and Kilauea are among the most active volcanoes in the world.
In recent times, they have erupted an average of 0.1 km3/yr.
Eruptions can occur anywhere on a shield volcano.
However, most eruptions occur in two regions:
The single location with the most numerous eruptions is the caldera region.
Therefore the caldera region forms the summit of a shield volcano.
The rift zones are areas of extensive fissure concentration.
Most shield volcanoes have 2 primary, offset rift zones.
Although the caldera is the single location with the most numerous eruptions,
more eruptions occur along the length of the rift zone.
Because of the eruptions along the rift zone, Hawaiian shield volcanoes commonly are elongate parallel to the rift zone.
The rift zone forms a ridge, commonly extending in two directions from the caldera.
There is a deep magma chamber that forms in the asthenosphere.
The deep magma chamber is 60 km deep.
Each individual shield volcano also has a summit magma chamber.
Summit magma chambers develop at depth approximately equal to sea level.
The caldera of a shield volcano forms over the summit magma chamber.
Calderas are collapse features that form at the summit of a shield volcano.
The nearly vertical walls of a caldera are fault scarps.
A fault scarp is the surface exposed by faulting.
Calderas form when the magma chamber empties and the summit of a volcano collapses into the summit magma chamber.
Calderas are formed by multiple collapse and refilling events.
Examples of calderas:
Pit craters are collapse features.
Form primarily in the caldera region and along the rift zone.
Pit crater walls are fault scarps.
Pit craters generally form from a single event.
Halema'uma'u Crater is a pit crater that formed in Kilauea Caldera.
Kilauea has two rift zones:
Pu'u 'O'o is the main vent in the current eruptive phase
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