MAGMA and IGNEOUS ROCKS

•Extrusive and Intrusive Rocks

•Different magmas produce different rocks

•Different magmas make different eruptions

•Different magmas make different volcanoes

Extrusive and Intrusive Rocks

• Igneous rocks begin as magma. Intrusive igneous rocks, like granite, form when magma cools inside the Earth. They make large grains.

• Extrusive igneous rocks, like the basalt lava flow in this photo, form at the Earth's surface. They make fine grained rocks.

Different Magmas Make Different Rocks

Intrusive rocks cooled slowly underground making large grains.Extrusive rocks cooled rapidly above ground making small grains.

All Magmas Are Not the Same

Hawaiian Basalt Mt St. Helens Dacite

Different Magmas Affect the Rocks

• Basalts are characterized by a relatively low abundance of silica and high abundances of iron and magnesium.

• In contrast, most volcanic rocks from stratovolcanoes along continental margins are andesite or dacite. Andesite or dacite are characterized by a relatively high abundance of silica and low abundances of iron and magnesium.

All Magmas Are Not the Same

Gentle eruption of lava from a fissure, Mauna Loa, Hawaii.

Basalts are characterized

by a relatively low

abundance of silica

and high abundances of

iron and magnesium (dark

minerals).

Different Magmas affect the Rocks

• Most volcanic rocks from stratovolcanoes along continental margins are andesite or dacite. Andesite or dacite are characterized by a relatively high abundance of silica (light colored minerals) and low abundances of iron and magnesium (dark colored).Violent eruption of ash from a

central vent, Mount St. Helens

Different Magmas affect the eruptions

– The dramatic differences in eruptions of Kilauea and Mount St. Helens are a result of their compositions. The different abundances of elements in a magma, especially silica, exert the primary control on the type of eruption, either non-explosive or explosive. The viscosity (resistance to flow) of a magma is greatly influenced by its silica content

– Magmas (and lavas) which are low in silica, like basalt, tend to be very fluid. Magmas (and lavas) which are high in silica, like andesite and dacite, tend to be very sticky. Because Hawaiian magma is fluid, gas dissolved in the magma can escape prior to the eruption, resulting in the gentle effusion of lava onto the surface.

– In contrast, gas is trapped inside of more viscous andesitic or dacitic magmas. The gas cannot escape until the magma enters the throat of the volcano. Then, due to the reduction in pressure, the gas bubbles nucleate and grow. The outward pressure exerted by the bubbles is greater than the strength of the magma/lava. The lava is fragmented to produce ash that is ejected violently at high velocity.

Different Magmas affect volcano shape

• Lava composition also influences the ultimate shape of a volcano. Fluid Hawaiian lava flows travel great distances from their vent, producing broad, shield shaped volcanoes.

• Sticky lava flows from continental volcanoes travel only a short distance from their vent. The lava flows accumulate near the vent, producing a steep-sided stratovolcano.