WHAT IS MAGMA MADE UP OF?

At divergent boundaries and hot spots, magma forms by PARTIAL MELTING (not complete melting) of the mantle Minerals with higher melting points stay in the mantle. Minerals with relatively lower melting points melt to form the magma

All terrestrial planets have silicate mantles with compositions similar to CHONDRITIC meteorites (pages 11-12) BASALTS result when chondritic silicates are partially melted Basaltic magma contains ~ 44-50% silica (SiO 2 ) (figs. 1.8 and 1.9, page 14) Relatively high iron and magnesium (also called MAFIC magma) (MORB = Mid Ocean Ridge Basalt)

“ WET” magmas contain volatiles (liquids and gasses) such as water (H 2 O) and carbon dioxide (CO 2 ) And you need to know these because… The nature of volcanic eruption depends on magma composition Would it flow or would it blow…?

VISCOSITY of magma (pages ) controls the nature of volcanic eruption Viscosity depends on (fig 4.6, page 102) 1.TEMPERATURE Hotter magma flows more easily than cooler magma 2.SILICA CONTENT Higher silica makes magma more viscous 3.GAS CONTENT More gas makes magma more fluid

Mafic magma is silica deficient Tends to have FLUID eruptions instead of explosive eruptions Common in divergent boundaries and hot spots

More viscous (less fluid) magmas cause more explosive eruptions Explosive volcanoes are most common around subduction zones

Sometimes mafic magma is produced below continental crust (e.g. East African Rift, Yellowstone, subduction zones etc.) When it rises through continental crust, the continental crust melts The rocks in continental crust are silica rich and SILICIC magma is formed (silica >65%)

Silicic magmas produce extremely explosive eruptions (Yellowstone eruption, 640,000 years ago) Common rocks produced from silicic magma are GRANITE and RHYOLITE Mixing of silicic and mafic magma, as well as melting of ocean sediments produces INTERMEDIATE magma (ANDESITE, silica ~55%) in subduction zones