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Identify, describe and explain the origin of basic and ultrabasic rocks using silica percentage. Basic (gabbro) = 44 - 52% SiO 2 Ultrabasic = 44%

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Presentation on theme: "Identify, describe and explain the origin of basic and ultrabasic rocks using silica percentage. Basic (gabbro) = 44 - 52% SiO 2 Ultrabasic = 44%"— Presentation transcript:

1 Identify, describe and explain the origin of basic and ultrabasic rocks using silica percentage. Basic (gabbro) = % SiO 2 Ultrabasic = 44%

2 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.

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

4 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 =

5 Gabbro Basic % SiO 2 Mineral Composition Plagioclase = 60% Pyroxene = 10% (no quartz) Coarse grained > 5mm

6 Dolerite Basic % SiO 2 Mineral Composition Plagioclase = 60% Pyroxene = 10% (no quartz) Medium grained 1 - 5mm

7 Basalt Basic % SiO 2 Mineral Composition Plagioclase = 60% Pyroxene = 10% (no quartz) Fine grained <1 mm

8 Peridotite Ultrabasic < 42% SiO 2 Mineral Composition Pyroxene = 50% Olivine = 50% (No quartz) Coarse grained > 5mm


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