A: Nonexplosive linear eruption
This stage commenced with a crack following to an
intensive extrusion of ash. Then, thick lava comes
out of the crack. After a couple of days, this crack
becomes longer. Each time, a weak explosion occurs
and ash is produced. Then, molten materials extrude
and next the process continues, like an ordinary
The intensity of eruption is different along with the
crack. Therefore, volume of volcanic material is
different along the crack, and sometimes independent
This type of eruption has three stages:
1.Occurrence of a crack relating to an earthquake and
explosion which discharge the exit vent.
2.Abundant lava holding a lot of gas comes out from
crack and produces basalt cap.
3.Throwing scoria and lava pieces producing volcanic
This type of volcano has three parts;
Two ending parts are made of pyroclastic materials and
main middle part having more erupted molten materials.
Re-activation of linear eruptions after a period of
calmness is very exceptional, but if any, it can
appear along the old vents with explosion. Because
previous molten vents solidify and close exit
These eruptions are made of basalt and in a very
liquid form. As a result they scatter, vastly.
Therefore, they are called plateau basalt or flood
It is important to note that molten material does not
flow in form of fluid, but they injected in former
B:Explosive linear eruption
These types of eruption commenced from one point of
volcano, then another point and so on. Next whole
volcanic mount is exploded. After that, a huge
explosion occurs along produced vents.
With regard to the vast development of oceans, these
eruptions are still a lot. But only a couple of them
are observable. When eruption occurs in a
shallow-sea, explosion process boils and throws water
around. It produces a cone of pyroclastic materials
coming out from water level. These materials produce
a small island that is destroyed by waves in a short
period of time. If eruption occurs again, it may
compensate sea devastation and produce more resistant
If pumices throw out by volcanic activity, layers of
wavy pumice are produced which are sent to remote
3.Under-glacier eruptions
Fine ash particles may displace by stratospheric winds
around the Earth and finally deposit. Molten
materials located under the ground coming to the
surface through chimney of volcano are called magma.
After reaching ground surface, it is called lava.
When lava comes out from volcano’s chimney is red,
molten and porous. After cooling and oxidizing, it
converted into dark red and other colors. Very hot
lavas having plenty of gas, Fe and Mg are in form of
fluid and their flow looks like hot tar. Whereas,
cooler lavas bearing less gas, Na and K make a slow
flow on the dip of volcano which is very similar to
Underground molten material which moves upwards
includes crystals, host rock pieces, soluble gases,
oxygen, titanium, silicon, aluminum, iron, magnesium,
calcium, sodium, potassium, titanium and manganese.
Ofcourse, magmas may have other elements but in small
amounts. Inside the ground, cooling process of magma
occurs slowly. Therefore, various mineral crystals
are formed (Bowen series). Finally, whole magma
converts into solid form, and produces magmatic or
These molten materials move upwards through fractures.
In certain circumstances, molten materials are
accumulated inside ground and produce structures, like
Magma is formed in the deeper part of upper mantle
within the continental crust. Different forms of
molten materials may combine with each other and make
Magma has dissolved gases. When it reaches ground
surface, these gases are released due to lack of upper
layers’ pressure. If sufficient gas is available in
the environment, explosive eruption may occur. If
lava is in form of fluid and has low viscosity, then
available gases will release, easily. In this case,
volcano remains active by fluid lava discharging
process. But if lava is thick and has high viscosity,
then gas emission from magma will be difficult.
Hence, gas condensation in lava will cause an
explosion, and explosive volcanoes occur.
Available gases in lava are very similar to drinks’
gases. Assume a finger is put on a drink bottle lid,
and then it is shaked well. When finger is suddenly
taken from the bottles’ lid, bottle contents erupt.
Magma gases have the same conditions. Intensive
separation of gases from magma may produce a rock
called pumice. This rock is very light, and it is
because of high volume of gas bubbles in it.
Therefore, it floats in water. In the most of
explosive volcanoes, the intensity of volcano is so
high that all materials throwing out from volcano
crater to the air may form volcanic bombs, ash and
If magma cools near ground surface will produce fine
crystal or glassy (vitreous) igneous rocks. In this
case cooling process is very fast, and volcanic rocks
are produced that the rock crystals are inserted in a
glassy or fine crystal groundmass. If magma cools
deep in ground and does not reach surface, cooling
process is slow. In these conditions, it produces
After final crystallization and lithification, these
massives under certain factors, such as erosion may
appear on ground after millions of years. Thus,
bigger massives of plutonic rocks may occur. For
instance, the Alvand granite in Hamedan or the Alamkuh
granite is good examples for this case.
