Magmatic rocks or magmatites
(rocks derived from melting)

The ori­gin of every mag­ma­tic rock is mol­ten rock mass. For the result, the kind of rock which was mol­ten is an important fac­tor, as it pro­vi­des the che­mi­cal mate­ri­al which can then be re-used. Sim­pli­fied, the­re are ‘aci­dic’ mag­ma types which are simi­lar to gra­ni­tes and ‘basic’ ones which che­mi­cal­ly resem­ble basalt. The terms ‘aci­dic’ and ‘basic’ do not have any­thing to do with the pH value, but with the amount of sili­ca (SiO₂): the more sili­ca, the more ‘aci­dic’. Aci­dic lava (mag­ma which has rea­ched the sur­face) tends to be very vis­cous, but other fac­tors such as tem­pe­ra­tu­re, con­tent of fluids (water, CO₂ and others) also play an important role. 

Most mag­mas cool down way below the sur­face to form Plu­to­ni­tes; lar­ge rock bodies such as gra­ni­te which can com­pri­se many km³. 

Some­ti­mes, smal­ler volu­mes of mol­ten rocks fill weak zones such as cracks within the pre-exis­tent rocks, forming so-cal­led intru­si­ons. Basal­tic intru­si­ons are com­mon in many are­as and are visi­ble as dark stripes.

If mag­ma comes to the sur­face, a vol­ca­nic erup­ti­on will be the result. The­re is a lar­ge num­ber of dif­fe­rent erup­ti­on mecha­nisms and an end­less choice of rocks which can be for­med as a result; they are coll­ec­tively cal­led vol­ca­ni­tes or sim­ply lava.

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