Ch. 7.2 Volcanic Ch. 7.2 Volcanic EruptionsEruptions

Two general types of lava.Two general types of lava. Mafic lavaMafic lava—dark when hardened; —dark when hardened;

rich in iron and magnesium; usually rich in iron and magnesium; usually thin and flowing; commonly forms thin and flowing; commonly forms ocean floor.ocean floor.

Felsic lavaFelsic lava—rich in silica…less iron —rich in silica…less iron and magnesium; forms continental and magnesium; forms continental crust; lighter colored when crust; lighter colored when hardened.hardened.

Kinds of EruptionsKinds of Eruptions

Lava flow eruptionsLava flow eruptions—usually quiet; —usually quiet; produce red-hot rivers of flowing produce red-hot rivers of flowing lava. Thin mafic lava. Gases escape lava. Thin mafic lava. Gases escape easily.easily.

Explosive eruptionsExplosive eruptions—thicker felsic —thicker felsic lava with lots of trapped gases, lava with lots of trapped gases, produce ash, and various sizes of produce ash, and various sizes of molten and solid particles shooting molten and solid particles shooting into the air. into the air.

Volcanic Rock FragmentsVolcanic Rock Fragments

Pyroclastic materialPyroclastic material results from results from exploded felsic lava. The type of exploded felsic lava. The type of pyroclastic material depends on particle pyroclastic material depends on particle size.size.

Volcanic ashVolcanic ash—particles less than 2mm in —particles less than 2mm in diameter.diameter.

Volcanic dustVolcanic dust—particles less than .25mm.—particles less than .25mm. LapilliLapilli—larger particles less than 64mm.—larger particles less than 64mm.

Volcanic bombsVolcanic bombs—large red-hot lava —large red-hot lava blobs thrown out of the volcano.blobs thrown out of the volcano.

Volcanic blocksVolcanic blocks—solid rock blasted —solid rock blasted away from the volcano. May be as away from the volcano. May be as big as houses.big as houses.

Volcanic FeaturesVolcanic Features

Volcanic coneVolcanic cone—the buildup of lava —the buildup of lava and pyroclastic materials around the and pyroclastic materials around the vent. Three main types…vent. Three main types…

Shield VolcanoesShield Volcanoes

Broad at the base with gently sloping Broad at the base with gently sloping sides.sides.

Result from eruptions of mafic lava.Result from eruptions of mafic lava.

Cinder ConesCinder Cones

Only a few hundred meters high at Only a few hundred meters high at most; very steep sides.most; very steep sides.

Result from explosive eruptions of Result from explosive eruptions of solid fragments.solid fragments.

Stratovolcanoes (Composite Stratovolcanoes (Composite Volcanoes)Volcanoes)

Formed when a volcano experiences Formed when a volcano experiences both quiet lava flows and explosive both quiet lava flows and explosive eruptions.eruptions.

Alternating layers of different Alternating layers of different materials build up into very high materials build up into very high volcanic mountains.volcanic mountains.

CraterCrater—the funnel-shaped pit at the —the funnel-shaped pit at the top of a volcanic vent.top of a volcanic vent.

CalderaCaldera—large basin-shaped —large basin-shaped depression formed when a volcanic depression formed when a volcanic cone collapses because the magma cone collapses because the magma chamber below empties (Ex: chamber below empties (Ex: Yellowstone Park).Yellowstone Park).

Predicting EruptionsPredicting Eruptions

Small earthquakes often increase in Small earthquakes often increase in frequency and strength just before frequency and strength just before eruptions.eruptions.

There may be slight bulging of the There may be slight bulging of the volcano’s surface prior to an volcano’s surface prior to an eruption.eruption.

There may be changes in gases There may be changes in gases coming out of the volcano.coming out of the volcano.

Past history is important.Past history is important.