Igneous Rocks

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Igneous Rocks Sills, like dykes, may be multiple and/or composite. Pillow structure is shown by basaltic and andesitic
They are mainly composed of basalt or dolerite. A lavas that are erupted on to the sea floor. Instead of
Subjacent Plutons: massive sill intruded into relatively cold rocks will be parting at with once with the steam incorporated in the
These are major intrusions with no visible floor. Walls completely enveloped in chilled fancies of fine-grained lava, the pillows swell up like a sponge and retain
are generally steeply inclined and they tend to increase rock. In a basic sill, this would be basalt. Inwards the enough steam in the vesicles to drastically reduce the
in size with depth. Rocks involved are mostly quartz- grain would coarsen, but to the slower cooling, firstly density.
bearing such as granite, though basic or gabbroic growing dolerite then gabbro. On eruption, the lava separates into rolling spheres
plutons are reasonably common. The upward steaming of volatiles, mainly water, would which pile up on one another when they come to rest.
be trapped some distance below the upper surface, Their shape indicates the `way upness' of a sequence.
Batholiths: and here the water-enriched fraction would crystallize
These are major, deep sated intrusive magmas found to form a pegmatitic phase or particularly coarse Vesicles:
typically in the great mountain ranges. No granite is grain. Basaltic magma has a low viscosity so that on eruption,
founding oceanic regions. The coast range batholith of Early formed olivine crystals gravitating down to when pressure is reduced the dissolved gases are
Alaska and British Columbia is 1,100 miles long, accumulate as an olivine-enriched layer in the lower largely able to escape. They are in effect `boiled out' f
varying from 8-120 miles wide. The best U.K example part (not the base due to contact) of the sill to give a the lava.
is the S.W Batholith of Devon and Cornwall. layer of peridotite. However, the loss of volatile substances cases a rapid
In the south west, outcrops isolated on the surface are increase in viscosity and many gas bubbles become
linked underground. This forms a cupola. When not Stoping: trapped, particularly towards the top of the flow.
exposed, the existence of a batholith might be This is believed to be the mechanism which accounts The cavities or vesicles are spherical or ellipsoidal, but
suspected if there is extensive mineralization and/or for the emplacement of igneous masses. may be flattened and elongated by flow movements of
metamorphism. Rising magma forces its way along joined and other the lava.
fissures in the brittle zone. This causes blocks
Dykes: become detached (xenoliths). They sink into the Amygdales:
At the time of intrusion, dykes are near vertical sheet- magma, allowing that magma to slowly continue In all recent lavas, the vesicles are gas-filled cavities.
like intrusions which vary in thickness from 1c-100's upwards. In older basalts they are commonly filled with low
metres, though they average less then than 1m. They Depending on its size, the xenolith may or may not temperature minerals such as calcite, quartz and/or
appear as nearly straight lines on geological maps melt. If it does melt it is said to have been assimilated zeolite. Such infilled cavities are called amygdales.
maintaining a uniform direction sometimes for long into the melt, which will than have changed its Amygdaloidal basalts were frequently seen at the
distances. Example would be Cleveland Dyke, 130 composition, or become contaminated. base of a succession. The minerals are associated with
miles long. They are entirely picked out because they superheated groundwater which has been heated
weather differently from the rocks they penetrate. The Structures of Lavas: volcanically.
Dykes are usually injected during times of great stress There are a number of different types of lava flows. Pipe amygdales occur at the base of the flow and have
or tension on the Earth's crust, such as continental the form of long, narrow cones, up to a foot in length,
spilt. Block Lava or AA: perhaps half inch in diameter. They appear to have
This resembles a tumbled mass of clinker or slag. It resulted from the uprise of steam from the moist
Sills: forms when volatiles in the magma are boiled off in the surface over which the lava flowed.
Sills are formed when structures and stresses in the vent before eruption. Mainly basic and cools quickly.
crust make it easier for the magma to spread laterally Columnar Basalt:
rather than vertically. Ropey Lava or Pahoehoe: The columnar structure of some basic lava is caused
Sills are concordant if they do not cross the bedding Here the lava is composed of contorted, snaky folds by contraction due to cooling. The rocks become
planes from one level to another and are discordant caused by a more rapid rise in the vent with less loss of traversed by revocable arranged systems of
is they do. volatiles. As the lava flows away from the vent, the intersecting joints.
Sills may be mistaken for lava flows. The difference volatiles are driven out and the Pahoehoe flow can The columns, which are often long and regular, are
include;- take on a blocky appearance. bounded by 3, 4, 6 or 6 planes. 6 sided prisms are
- Occasionally they transgress to a higher or most common.
lower horizon. This is impossible with a lava Pillow Lavas: The long axes are perpendicular to the retreating
flow. These consist of lava flows which exhibit isolated isotherms during cooling, hence they are vertical. In
pillow-shaped masses piled on one another. The sills and lava flows and they are horizontal in dykes.
- Vein-like offshoots, particularly into the rock intervening spaces are filed with sedimentary material,
above indicates a sill sometimes mud or chert or limestone.
- There are contact relations at the top and Internally, the pillow lavas are characterized by
bottom of a sill. concentrically arranged vesicles and occasionally
there is a central ovoid cavity.


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