Initial introduction about Igneous Petrology subject.

1. Saumendra Kumar, Sonu
Geology Department , University of Delhi,Delhi-110007
Introduction about Igneous rocks

2. Igneous rocks
The word igneous is derived from the Latin word ignus, meaning fire . The
term igneous applies to natural process relating to the formation,
movement, and cooling of molten materials—magma (hot, molten
material underground), and lava.
In geology, the word igneous applies to materials that have solidified from
molten rock material, it also applies to the processes associated with the
movement of molten material underground or erupting or the surface

3. When magma reaches the surface it becomes lava.

4. Heat convection from the mantle is the source of heat driving
volcanic activity. This heat is remnant from its period of Earth's
formation and the decay of radioactive elements

5. Igneous rocks are classified by their unique properties and characteristics which
are related to composition of their host melt and the environmental setting
where they form, underground or on the surface. Volcanoes form where molten
material erupts on the surface. Regions where volcanoes occur, both modern
(active or dormant) or ancient (extinct) have a unique variety of landforms.
There are at least 500 active volcanoes around the world, of which about 25 are
actively erupting, spewing out lava, rock, ash, and noxious gases.

6. Origin of igneous rock
The Earth stores vast quantities of energy in the form of kinetic heat. Portions
of the Earth's interior are molten, such as the outer liquid core. However,
rocky materials in the overlying mantle are thought to be under too much
pressure to melt. However, in many places, heat flow near the Earth's
surface is high enough and confining pressure is low enough for rocks to
melt. This molten material may find zones of weakness, such as along fault
zones, to inject, melt, or under great pressure, find means to migrate to
the surface.
The Geothermal Gradient
In most regions, the average temperature increases an average of 20 to 30
degrees C per kilometer with increasing depth in the upper crust. This
temperature gradient (called the geothermal gradient) varies considerably
with depth and location, depending on geologic settings.

7. What causes rocks to melt?
Rocks will melt (generating magma) if heat flow increases in an area to the point that minerals reach their melting
points. Other factors that cause melting include the introduction of hot volatile fluids (water and gases) into
rocks under pressure, or if there is a decrease in pressure confining hot rocks (such as the release of pressure
caused by a great earthquake).
Melting ranges from only partial melting of select low-temperature minerals to complete melting under higher
temperature conditions. As heat flow increases, more materials will potentially melt.
“Igneous Rocks Are Rock Formed From Molten Material”
Rocks formed from magma cooling and crystallizing underground are called intrusive igneous rocks. Intrusive igneous rocks form in
naturally insulated settings (rock is a poor conductor of heat) so that minerals crystallize slowly, forming large, visible crystals.
Rocks formed from cooling lava on or very near the surface are called extrusive igneous rocks. When lava cools rapidly it crystallizes
quickly, preventing visible crystals from forming. Extrusive rocks include lava flows and pyroclastic materialsuch as volcanic ash,
cinders, etc

9. How hot is magma/lava?
The temperature of magma/lava plays perhaps the most important
factor influencing igneous activity (both underground and on the
surface).
Temperature influences:
• How minerals either melt or crystallize,
• How much volatile content magma/lava can absorb, hold, and
release.
• How it influences the character of the volcanic eruptions.
• the composition of rocks that can form when the material cools.
• The texture of the rock that forms with the molten material
solidifies
• The kinds of volcanic landscape features eruptions can produce.

10. The table below illustrates the temperature of lava sampled
from a variety of volcanoes around the world.
Lava/Rock
composition
(place)
Geologic setting Type of volcanic
eruptions
Temperature
(C)
sample
(estimate
range)
Basalt hotspot - shield
volcano,
divergent zone -
rift zone
hot, fluid eruptions
with extensive lava
flows, rare explosive
eruptions
1,170
Andesite convergent zone -
arc - composite
cone
explosive eruptions
and pyroclastic flows
1,075
Dacite
convergent zone-
arc - composite
cone
both explosive
eruptions and lava
flows, lava domes,
pyroclastic flows
(800 to 1,100)
very large, catastrophic

11. General Characteristics of Intrusive and
Extrusive Igneous Rocks
The factors the help produce the texture and appearance of
igneous rocks include:
1) the amount of dissolved gases in magma,
2) the rate at which magma cools, and
3) the amount of silica (SiO2) present.
Intrusive igneous rocks form in naturally insulated settings
(rock is a poor conductor of heat) so that minerals
crystallize slowly, forming large, visible crystals.
Extrusive igneous rocks that form from rapidly cooling
magma (or lava) near or on the surface, crystallize quickly,
preventing visible crystals from forming.

12. Intrusive rock(Granite)
Extrusive rock(Ryolite)

13. Economic Value of Igneous Rock Resources
Most of the world's most precious metals, rare-earth elements, and gemstones come
from rocks of igneous origin. Most gold, silver, copper deposits around the world are
associated with igneous source rocks, or the sedimentary placer deposits derived from
them.
• Gemstones From Igneous Rock
quartz as an example.
• The Igneous Origin of Diamonds
Kimberlite a special kind of intrusive igneous rock associated with some diatremes that sometimes contain
diamonds. Kimberlites are typical coarse grained rocks with an bluish in color. Diamonds are xenoliths.
• Building stone

14. References
• Text book of geology- P.K. Mukhargee
• Introduction of Igneous and Metamorphic
petrology- winter
• Myron G. Best.
• Internet.

15. Thanks