Doc Brown's Chemistry - Earth Science & Geology Revision Notes

for KS4 Science, GCSE, IGCSE & O Level Courses

4. Igneous Rocks - Granite and Basalt

Examples of the formation of the volcanic igneous rocks like granite and basalt are described and explained with the aid of diagrams. Explanation of the effect of slow or fast cooling of the molten lava-magma on the crystal size are also given and the igneous intrusion phenomena and its effect on surrounding rock layers is also described.

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 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.

Fig 6.1 Volcanic activity - eruptions and formation of igneous rocks and metamorphic rocks

See also Fig 6.1

More notes on Fig 6.1 and Fig 8.1

Fig 8.1 Lots going on in the crust!

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.

4(d) You would not expect fossils in igneous rocks because they are formed from molten mixed up magma. Even if a sedimentary rock had fossils in it, they would be destroyed if the rock was re-melted e.g. in a subduction zone - see plate tectonics later.

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• A structured question covering much of the GCSE Earth Science

• Earth Science GCSE study revision notes index (responses to the above Questions)

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above

GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets

Studying Revision for KS4 Earth Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Earth Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry (revise courses equal to US grade 8, grade 9 grade 10)

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