Igneous rocks Geology 103

Igneous rocks’ origin Ignis (Latin for fire) – these rocks were crystallized from a molten state. They are not formed by sediment accumulation.

Igneous rock classification The names of different igneous rocks depends first on if they have a distinctive texture, and second on composition and grain size.

Grain size allows sorting of igneous rocks Terminology: Volcanic rocks = extrusive = fine-grained (cannot see mineral grains within rocks) Plutonic rocks = intrusive = coarse-grained (easily seen mineral grains)

Why the different grain sizes? Grain size is dependent on the rate of magma crystallization, which is in turn dependent on the depth at which crystallization occurs Deep = slow cooling = coarse grains

Plutonic rocks The general structure in which plutonic rocks exist is called, not surprisingly, a pluton Types of plutons: batholiths (large), stocks (small), laccoliths, lopoliths, etc.

Volcanoes are complex structures

The diversity of volcanic eruptions Kilauea (Hawaii) quiescent Vesuvius (Italy) explosive

The diversity of volcanic materials Vesicular basalt Banded rhyolite

More terminology Magma = molten rock underground Lava = molten rock above ground New tectonic setting: the “hotspot” -- small, intense mantle convection cell

Connections Tectonic setting leads to type of magma generated leads to type of volcano made leads to behavior of volcano Thus, understanding magma generation is the key

Magma generation at subduction At a subduction zone, the subducted slab loses volatiles (water, principally) and causes the melting temperature of the rocks above it to lower

Magma generation at rift At a rift, as the pressure of overlying layers lessens (crust thins), melting temperature of rock lowers.

But why different rocks? Start with mantle rocks, which are very rich in iron and other metals (like aluminum and magnesium) Magnesium, Aluminum, iron (Fe) = mafic

Modification of magma At subduction zone, the descending slab loses volatiles and partially melts the mafic or ultramafic rock above, some of the slab material (or continental material) can melt and mix with the mafic magma

Modification of magma Fractional crystallization causes higher melting point minerals to crystallize and settle, removing mafic components of magma

Tectonic settings and magma

Magma composition leads to rock name leads to lava behavior Maficintermediate felsic

Volcanic rock names are either due to composition or to texture Basalt, because the rock is mafic Obsidian, because the rock is glassy

Lava viscosity leads to volcano types Low viscosity lava yields low angle slope volcanoes -- shield volcanoes

Higher viscosity = steeper slopes Composite volcano or stratovolcano

Many different volcano types