A magma in its molten state is less dense than the
surrounding rocks and rises to the surface by buoyancy.
Magma comes out to the surface of the earth through
passageways or vents forming volcanoes. Molten rocks that comes our to the
surface is known as extrusive igneous rocks. Since it comes out through
volcanoes, it is also known as volcanic rocks.
Some magma rises but cools below the surface. Such rock is known as intrusive igneous rocks. Since it does not come from a volcano, it is called plutonic rock.
Intrusive igneous rocks differs from extrusive igneous rocks in terms of their texture, i.e grain sizes of the minerals in them. Intrusive rocks cool slowly and have coarse texture whereas volcanic rocks cool faster and are fine-grained.
Depth of plutonic rock formation is divided into three zones. Those formed very deep in the earth ( > 11 km) is called catazone, intermediate depth (5-11 km) is called mesozone and shallow (0-5 km) is called epizone.
In Peninsular Malaysia, granite plutons forming the Banjaran Titiwangsa (Main Range) is mesozonal while those in the East Coast are mainly epizonal.
Intrusive and extrusive rocks have different textures as they cool at different rates. Extrusive rocks cools faster and have fine texture while intrusive rocks have coarse texture.
Magmatic intrusion is a slow process. During the
intrusion of a magma, the outer margin chills and solidify while the core is
still molten.
If the whole magma then solidify, there is a textural
variation between the core and the margin.
A batholith is a very large igneous intrusive body,
more than 10 square km., formed by the
cooling of a magma. Kajang granite is considered as a batholith
(batolit).
Smaller intrusive body of less than 10 km square is
known as a stock (stok).
Sil (sil) is a flat
tabular body of igneous rock, intruded concordantly along the bedding planes of
sedimentary rocks
Dyke (daik) is a flat
tabular body of igneous rock intruded discordantly at an angle to the bedding
plane. In the East Coast, dolerite dyke is common.
Vein (telerang) is a
tabular body, uniform thickness, consisting normally of a single mineral. E.g.
quartz vein. Is is formed bu deposition by crystallization of minerals in a
fracture. Veins may contain economically important minerals such as gold and
cassiterite.
Lacollith (lakolit) is a
concordant body of igneous rock dome upwards with a flat floor.
Lopolith (lopolit) is a
concordant body of igneous rock with a concave bottom occupying a former basin.
The Bushveld Igneous complex of Southern Africa and structure containing the
nickel deposits of Sudbury, Ontario, Canada are lopoliths.
Hypabyssal (hipabisal) is small
intrusive body of rock found near the surface of the earth