Forms of igneous rocks

Forms of Igneous Rocks
Lavas are extrusive forms of molten
magma which have been poured out
on the surface of the earth.
Lava
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
Md. Yousuf Gazi
Lecturer
Department of Geology
University of Dhaka

A. Extrusive Igneous Forms
Volcanic or extrusive igneous
rocks form when molten elements
erupt from Earth's interior through a
volcanic vent or fissure and cool
rapidly at the surface
B. Intrusive Igneous Forms
Intrusive igneous rock is formed from
solidified magma below the surface of
the Earth.
Magma
Is naturally occurring molten
mobile rock material generated
within the earth.

Extrusive Igneous Forms Intrusive Igneous Forms
Pyroclastic
Deposits
Lava Flow
Agglomerate Lapilli Volcanic Tuff
Concordant Others
Sills Laccoliths Lopoliths Phacoliths
Composite
Intrusion
Multiples IntrusionChonoliths
Disconcordant
Dyke BatholithsVolcanic PipeVolcanic NeckRing Dykes

 Emitted from individual
cones like volcano or from
fissures
 Form tabular bodies, wide
extent
 Form of lava flow depends
on fluidity of magma i.e.
composition, temperature
of eruption, viscosity.
Lava Flows
Extrusive Igneous Forms

Pyroclastic Deposits
Intermittently explosive action that
takes places in volcanic eruption
produced fragmental type of igneous
material.
1.Agglomerate
2.Lapilli
3.Volcanic Tuff/Volcanic Ash

Agglomerate
The fragments of rock associated with lava
flow that are ejected during explosive
volcanic eruptions and distributed in and
around the creator.

Lapilli
The small fragments between the size of walnut
and pea(2 inch-64mm diameter), which are
ejected to a greater distance than the larger
agglomerate.

Volcanic Tuff
The finest of all, the volcanic dusts, ash or sands
may be deposited near the volcano or over the
surrounding country.

Intrusive Igneous Forms
Two main types are distinguished
1. Concordant
 The injected strata remain more or less horizontal or
parallel to the country rocks over wide area. Example:
Sill, Laccoliths, Lopoliths etc.
2. Discordant
 Cut across the bedding of the rocks in which they intrude
 Vertical to steeply inclined and sheet like body (extensive
in lateral dimension)
 Thickness vary widely from an inch up to hundred of feet
 Injected through fractures, joints, and weak planes

Relatively thin tabular sheets of
magma
 Paralleled along approximately
horizontal bedding
 Sills spread to a distance
dependent on the hydrostatic force,
their temperature, degree of
fluidity and weight of the block of
strata.
Sills

 Flat bottom and convex upward
dome shaped igneous body
 When viscous magma is injected
rapidly along the bedding, as it cannot
spreads it pushes up the overlying
layers and keep on piling up.
 It causes folding of the overlying rock
layers
Laccoliths

 These are basin or saucer shaped
concordant bodies with top nearly flat and
convex bottom.
 These are very huge body with diameter 150
miles(app). (Duluth Gabbro Mass, Minnesota,
USA).
Lopoliths

 These are concordant bodies that
occurs along the crests and troughs of
the folded sedimentary strata.
 In folding, the crest and trough are
regions of weakness and tension,
magma often intruded into these
trough and crest.
Phacoliths

 Intrusion of magma into more or less vertical
fissures i.e. joints, fractures, faults.
 Cut across the bedding or other structures.
 Narrow, elongated, parallel sided wall of igneous
rock.
 Thickness may vary from 1 inch to hundreds of
feet and length may few yards to many miles.
Dyke

Ring dykes form a cylinder around a subsided
block of country rock, filling the ring-shaped
fracture with magma.
Ring Dyke

These are the igneous masses which seal up the vents of ancient volcanoes. A volcanic neck forms inside a
volcano when the heat and pressure from below are reduced so that the magma inside the volcano's vent
solidifies. Over a period of a few hundred thousand years, the surrounding softer rock erodes away,
leaving the volcanic neck standing alone. (Shiprock Volcanic Neck, New Mexico, USA)
Volcanic Neck

A vertical conduit below
a volcano through which
magma has passed and
that has become filled
with solidified magma,
volcanic breccia, and
fragments of older rock.
Volcanic Pipe

Batholiths are the largest bodies of Igneous rock,
irregular in shape and occupies large area. Their
occurrence is commonly associated with the
mountain-building process. These are either
granites or granodiorites in composition(Sierra
Nevada Batholoiths, USA)
Batholiths
Stocks
Stocks are irregular masses of batholiths habit of
smaller dimension.

Others Intrusive Igneous Forms
1. Chonoliths
This term is applied to all other intrusive igneous
bodies with irregular shape, i.e. the body with no
specific shape.
2. Multiple Intrusion
When a magma of a uniform composition has been
injected along a single channel in two or more
successive pulses, the result is a multiple intrusion.
3. Composite Intrusion
A dike or sill composed of two or more varieties of
igneous rock presumably resulting from successive
injections.

Label the Diagram

Forms of igneous rocks

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