There are two main forms of igneous rocks:
1) Extrusive rocks form from lava erupted at the Earth's surface and cool rapidly. They include lava flows, pyroclastic deposits like volcanic ash and tuff.
2) Intrusive rocks form from magma that cools below the surface. They can be concordant, forming sheets and domes parallel to layers, like sills and laccoliths, or discordant and cutting across layers, like dikes, batholiths, and volcanic necks.
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Forms of igneous rocks
1. 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
2. Forms of Igneous Rocks
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.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
3. 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
Forms of Igneous Rocks
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
4. 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
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
5. 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
Extrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
6. Agglomerate
The fragments of rock associated with lava
flow that are ejected during explosive
volcanic eruptions and distributed in and
around the creator.
Extrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
7. 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.
Extrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
8. Volcanic Tuff
The finest of all, the volcanic dusts, ash or sands
may be deposited near the volcano or over the
surrounding country.
Extrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
9. 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
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
10. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
11. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
12. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
13. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
14. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
15. Ring dykes form a cylinder around a subsided
block of country rock, filling the ring-shaped
fracture with magma.
Ring Dyke
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
16. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
17. 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
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
18. 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.
Intrusive Igneous Forms
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
19. 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.
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)
20. Label the Diagram
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)