GEOS1211 Lecture Notes - Lecture 7: Igneous Rock, Trachyte, Extrusive Rock
14 Jun 2018
School
Department
Course
Professor
Lectures 7 and 8 learning outcomes
• Describe the differences between intrusive and extrusive igneous rocks
An igneous rock is formed by the cooling and crystallisation of molten rock (magma)
Intrusive igneous rocks- also called plutonic rocks, are those that have solidified below ground. As
they cool slowly, rocks formed characterised by medium to coarse grains. Can only be seen after
erosion has removed overlying rocks to expose them. Intrusive formations-
- Batholith- huge, granitic, thick. Largest igneous intrusions. How is space made for such large
intrusions? Granite magma is ~1 million times as viscous as basaltic magma so they can only
intrude very very slowly. Space is made available by squeezing surrounding rock sideways
and pushing country rock upwards. Main intrusive force is gravity, that is granite magma
lighter than surrounding rock.
- Laccolith- a mushroom-shaped body of igneous rocks with a flat bottom and domed top.
Magma intrudes vertically within a dyke and then between parallel layers of rock at
relatively shallow depths. Low pressure allows magma to dome up overlying rock, so
intrusion becomes a lenticular and mushroom shaped body.
- Sill- magma injected along sedimentary bedding (parallel to layering of country rock).
Because they form by lifting and separating adjacent rock layers, they only form within a few
kilometres of surface.
- Dyke- sheet-like injections cut across existing structures and are commonly steeply inclined.
Because dykes intrude relatively cool rocks, they frequently display a chilled margin. Grain
size becomes coarser towards centre where the rate of cooling has been slower.
Extrusive igneous rocks- also called volcanic rocks, include all products resulting from eruptions of
lava (flows and fragmented debris called pyroclasts). Has fine texture.
Maga is olte or partially olte rok eeath the Earth’s surfae.
A volcano is a vent or chimney that connects molten rock (magma) and associated gasses that erupt
at Earth’s surfae
• Describe the relationship between igneous rocks (and their minerals), structures and
landforms
Forces shaping Australian landforms-
- Volcanoes
o Oig to tesio ad opressio, faultig ad foldig, earth’s rust ay deelop
tears, cracks, fissures, etc. This allows magma to flow toward surface and spread as
lava flow with ash and cinder ejected during violent eruptions forming composite
cone
o Magma to lava via plumes and hot spots
o Volcanic extrusions (lava) and rocks depends primarily on its viscosity (resistance to
flow), slope of ground over which it travels, rate of lava eruption
▪ Basalt- contains least amount of silica, erupts at high temperatures, lowest
viscosity (least resistance)
▪ Trachyte and rhyolite lava tends to pile up around vent to form short, stubby
flows or mound-domes
o South East Australian Cenozoic Volcanic sequence
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lectures 7 and 8 learning outcomes: describe the differences between intrusive and extrusive igneous rocks. An igneous rock is formed by the cooling and crystallisation of molten rock (magma) Intrusive igneous rocks- also called plutonic rocks, are those that have solidified below ground. As they cool slowly, rocks formed characterised by medium to coarse grains. Can only be seen after erosion has removed overlying rocks to expose them. Granite magma is ~1 million times as viscous as basaltic magma so they can only intrude very very slowly. Space is made available by squeezing surrounding rock sideways and pushing country rock upwards. Main intrusive force is gravity, that is granite magma lighter than surrounding rock. Laccolith- a mushroom-shaped body of igneous rocks with a flat bottom and domed top. Magma intrudes vertically within a dyke and then between parallel layers of rock at relatively shallow depths.
