Doc Brown's Chemistry - Earth Science Revision Notes

4. Igneous Rocks - Granite and Basalt

Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  (revise courses equal to US grades 9-10)

based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz

ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets

4. Igneous Rocks form the fastest, and last the longest!

4(a) Igneous rocks are formed from hotter less dense (than surrounding rock) molten rock called magma, welling up and pouring out from the mantle and sometimes from re-melted crust (see 6. and 7.)  The rising 'plumes' of magma break through the crust from volcanoes and mid-ocean ridges (see Fig 6.1 and Fig 8.1) and cooling to solidify to form igneous rocks. Sometimes the magma does not break through the surface and cools within the crust (see igneous intrusion below). Most igneous rocks consist of interlocking crystals from cooled magma and are physically hard and relatively dense and do not erode easily.

Note (i): There is quite a variety of mode of formation though e.g. some volcanic rocks are very hard and 'glassy', others form from ash deposits from volcanic eruptions. They sometimes occur as intrusions into other pre-existing rocks (see below) and the crystal size and type of igneous rock also depends on the rate of cooling.

Note (ii): You often see the lava bubbling as dissolved gasses under pressure in the mantle are released into the atmosphere - sometimes with explosive force!

4(b) The igneous rock granite is formed by the slow cooling of magma in the crust or perhaps inside a volcano after it stops erupting and the top becomes plugged. It is called an intrusive rock because it is formed 'inside' the crust and not on the crust surface. The crystals are relatively large due to slow cooling and 'speckled' as different minerals of different colours crystallise out within the rock structure. Granite tends to be lighter in colour than basalt (see 4(d) below). Granite type rocks are sometimes called course-grained rocks because of the mixture of interlocking larger crystals.

Fig 4.1 An igneous Intrusion

4(c)(i) An igneous intrusion is where a mass of very hot 'plastic' magma from the mantle rises and 'bulges' up into the crust and cools to form igneous rock. This is often granite because it will cool very slowly as the surrounding rocks act as an insulator. The intrusion may 'push' up through many layers of previously formed sedimentary rock (see section 5. and section 6.).

4(c)(ii) If these sedimentary rocks are then weathered away, the harder wearing granite remains as a hill or mountain.

4(c)(iii) The igneous intrusion rock must be younger than the surrounding sedimentary rock because it is formed by the magma cooling in the previously existing rock layers.

4(d) The igneous rock basalt is formed much more quickly than granite and in several locations e.g.

• molten lava from undersea volcanoes and mid-ocean ridges is rapidly quenched by the cold water.
• molten lava pouring out of volcanoes onto land and cooled by air (or pouring out from land into water).

Basalt is described as an extrusive rock because it 'extrudes' out into air or water to cool and form the solidified rock. It is formed by the fast cooling of magma and the crystals are relatively small because of the fast cooling. It consists of interlocked microscopic crystals which are darker in appearance compared to granite. This situation is found when lava/magma cools rapidly when flowing out into air or water. Basalt rocks are sometimes called fine-grained rocks because of the mixture of interlocking tiny crystals.

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