Identify, describe and explain the origin of basic and ultrabasic rocks using silica percentage. Basic (gabbro) = 44 - 52% SiO2 Ultrabasic = 44%

Classification by mineral composition Basic and ultrabasic rocks can be characterised by having certain proportions of minerals. In the diagram the gabbro represents which type of rocks, basic or ultrabasic? What does ultramafic represent? Using the diagram estimate the mineral proportions for basic (gabbro) and ultrabasic (peridotite) rocks.

Ultrabasic (Peridotite): Basic (Gabbro): Plagioclase = 60% Pyroxene = 10% (no quartz) Ultrabasic (Peridotite): Pyroxene = 50% Olivine = 50% (No quartz)

Identify basic and ultrabasic rocks via colour This is sometimes called the colour index. Can use the terms: Leucocratic = light coloured Mesocratic = medium coloured. Melanocratic = dark coloured. What gives the rock it’s colour? The minerals. What controls which minerals form? Composition. Basic = Ultrabasic =

Gabbro Basic 52 - 44% SiO2 Mineral Composition Coarse grained > 5mm Plagioclase = 60% Pyroxene = 10% (no quartz) Coarse grained > 5mm

Dolerite Basic 52 - 44% SiO2 Mineral Composition Plagioclase = 60% Pyroxene = 10% (no quartz) Medium grained 1 - 5mm

Basalt Basic 52 - 44% SiO2 Mineral Composition Fine grained <1 mm Plagioclase = 60% Pyroxene = 10% (no quartz) Fine grained <1 mm

Peridotite Ultrabasic < 42% SiO2 Mineral Composition Pyroxene = 50% Olivine = 50% (No quartz) Coarse grained > 5mm