GCSE Earth Science: Examples of igneous rocks

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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!

doc b's Earth Science Notesdoc b's Earth Science Notes

All igneous rock is made from ANY molten rock material that cools and solidifies. The type of igneous rock is very dependant on how fast the magma or lava cools and its chemical composition. It is a complex mixture of minerals whose crystals all interlock together to give a hard wearing rock.

4(a) Igneous rocks are formed from hotter less dense (than the surrounding rock) molten rock called magma, welling up and pouring out from the mantle and sometimes from re-melted crust. 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 very hard and relatively dense and do not erode easily like chalk and limestone do. Igneous rocks are complex mixture of minerals and also sorts of randomised crystals of varied colour can be seen under a microscope.

Fig 5. Volcanic activity - eruptions and formation of igneous rocks and metamorphic rocks

doc b's Earth Science Notes

Some volcanoes erupt with magma that is rich in iron minerals, this magma is very fluid so the lava is very hot (~1200oC), very runny and cools to form an iron-rich basalt. Volcanoes producing such magma tend to be fairly safe because they are less explosive.

However, if the magma is silica rich, like rhyolite, it gives a very thick lava that does not flow easily (very viscous), and the eruptions can be very explosive. If the pressure builds up sufficiently, the whole of the top of the volcano can blow apart with enormous destructive force.

See also 6. Metamorphic Rocks and 8. Tectonic plate theory, Wegener's theory, evidence for continental drift

Fig 2. Lots going on in the crust!

doc b's Earth Science Notes

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 Fig 6. notes below) and the crystal size and type of igneous rock also depends on the rate of cooling.

Extrusive igneous rocks are formed from the 'extrusion' of molten magma or lava into cool air or water, which rapidly cools giving small crystals e.g. basalt or rhyolite.

Intrusive igneous rocks are formed when magma or molten lava cools slowly underground, so cooling occurs much more slowly because of the insulating effect of surrounding rock, the result is much larger crystals of a courser rock e.g. granite or gabbro.

Fast cooling gives small crystals and slow cooling gives bigger crystals.

This crystallisation 'rule' applies to any chemistry preparation in the laboratory!

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!

doc b's Earth Science Notes 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.

Granite rock is very durable material used in building construction, hard wearing steps and gravestones!

doc b's Earth Science Notes Fig 6. 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 forming surrounding metamorphic regions. (see section 5. Sedimentary rocks. and section 6. Metamorphic Rocks).

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.

doc b's Earth Science Notes 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.

Any organism, plant or animal falling into magma, would be totally destroyed.

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.



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