Intrusive Igneous Rocks

Minor IntrusionsSome intrusions are small with dykes just a few centimetres wide but hundreds of metres long. Many minor intrusions are huge, such as the Palisade Sill, which is 300m thick and outcrops of 60km and underlies thousands of km2. The 100m thick Whin Sill underlies a large area of northeast England and outcrops along the line of Hadrian's Wall: most sills and dykes are made of medium grained, mafic rock dolerite, except along the margins where chilling by the country rocks produces a finer grained basalt. Sills and dykes that have granite as a source will be made of prophyry or microgranite, which are both medium grained silicic rocks. SillsSills are sheet-like intrusions that are condordant and parallel to beds. Some sills occasionally cut across the beds in steps from one bed to another to form transgressive sills: sills are generally irregular with a variable width and a sinuous outcrop they are usually intruded along bedding planes Sills are intruded when the fluid pressure is so great that the underlying magma actually lifts the overlying rocks. This means that sills usually form at shallow depths in the Earth where the weight of overlying rocks is not too great.DykesDykes are sheet-like intrusions that are discordant and often vertical: they range in size from a few centimetres to more than 100m thick, and commonly are 1 to 2 m across. as most sykes are vertical they form straight lines in outcrop, forming long thin features they are often indruded along zones of weakness in the country rock such as faults or joints dykes are commno as feeder vents under volcanoes, so may have radical pattern Major Intrusions BatholithsThe most common form of major intrusion is a batholith, which can be very large: the Peru-Chile batholith in the Andes of South America is 4500km long. the batholith that extends from Dartmoor in Devon to Lands End in Cornwall and to the Scilly Isles is over 235km long most batholiths are discordant where they are intruded into the country rock the outcrop for many batholiths is roughly circular in shape with steep sides batholiths are normally granite in composition although they can be composed of granodiorite or even diorite, all of which are coarse grained they cool very slowly at depths of 5 to 30km as plutonic rocks Formation of Granite MagmasMost batholiths are intruded at convergent plate marins where they form the core of fold mountains, which have resulted from plate collision. High temperature mafic magmas will rise up through the crust and the heat from these will partially melt the continental crust. Partial melting produces silicic magma, which rises up because it is less dense than the rock from which it was melted. Silicic magma is viscous and rises up through the crust slowly. It is difficult for it to mix with other magmas because the viscosities are so different. Batholiths are huge bodies magma usually produced by repeated intrusion of magma in the same area. As the magma moves up it generates more magma by stoping and assimilation.Stoping and AssimilationAt shallow depths of about 10km, the crust is cold and brittle and batholiths may intrude by stoping. Magma moves upwards along joints, faults and bedding planes, separating masses of country rock. Eventually, pieces of country rock were detached and settle into the magma are known as xenoliths. No new space is created during stoping, as the magma simply fills the space formerly occupied by the country rock. As the xenoliths fall into the magma most will be assimilated as they gradually melt to become more magma.

New Page