Volcanoes
Form 4
Volcanoes
Volcanoes erupt along lines of weakness, much like earthquakes, when
rocks are heated deep beneath these areas, magma rises towards the
surface creating volcanic features
• The Hawaiian island chain are examples of hotspot volcanoes
• Volcanism is mostly focused at plate margins Pacific Ring of Fire
Volcanoes can be formed at:
• Destructive plate margins
• Divergent plate margins (spreading)
• Fissures – vertical cracks
• Hotspots – rising plumes in the middle of plates
Volcanoes are formed at areas of Subduction,
Rifting and Hot Spots
3 Types of Volcanoes
• ACTIVE VOLCANO – a volcano that has erupted within the last 100
years.
• DORMANT VOLCANO – a volcano which have not erupted in the last
100-1000 years
• EXTINCT VOLCANO – a volcano that has not been active for over 1000
years.
Structure of a Volcano
• The structure of a volcano is conical with a depression/crater on-top
in the middle. This depression takes the shape of a funnel
• Below/beneath the funnel shaped crater is the vent (similar to a pipe)
via which the volcanic material is ejected
• Volcanoes have a conical shape due to previous eruptions which have
caused material ejected from the vent to accumulate after it runs
down the sides and solidifies
Volcano
• A volcano is a natural vent or fissure in the earth’s crust through
which hot gases, molten rock and solid materials may be released
unto the surface
• Magma is termed as lava when it reaches the surface
• The magma chamber is the area under the volcano where magma
accumulates
Volcanic Features
Intrusive and Extrusive
Volcanic Features
• Lava is magma which is emitted to the surface and forms extrusive
volcanic features
• Magma that does not reach the surface and solidifies underground
forms intrusive land features
• Plutons is another name for Intrusive volcanic features
- Intrusive features may get exposed to the surface after time due to erosion of
the overlaying rock
Types of Intrusive Volcanic Features
• Batholiths
• Laccoliths
• Dykes
• Sills
• Plug
Batholiths
• Large dome shape reservoir of
magma located deep beneath
the earth’s surface
• Occurs where molten magma
has been ejected upwards into
the surrounding rocks
Batholiths
• Example: The base of the Coast Range Mountains of British Colombia
is formed from Batholiths. In the centre of Tobago lies a large
batholith
• The smaller versions are known as stocks (less than 100km²)
Definition: Batholiths are formed when a large underground reservoir
of molten rock cools and hardens below the surface
Laccoliths
• A laccolith is sheet intrusion that has
been injected between two layers of
sedimentary rock
• The pressure of the magma is high
enough that the overlying strata is
forced upward, giving the laccolith a
dome or mushroom like form
Dyke
• A vertical sheet of magma/igneous rock which rises vertically and cuts
across the bedding planes of the rocks and solidifies
Dykes
• Dykes are typically small (20-30km long)
• Exception: the Great Dyke of Zimbabwe 485km long and 8 km wide
Dykes
• Dykes may be singular or in a large group
• When exposed to the surface over time, dykes may form ridges,
waterfalls or springs due to their greater resistance to weathering and
erosion than the surrounding rocks
• They can also become steep cliff walls
Dykes
Sills
• Are formed by magma flowing horizontally between the bedding
planes of the rocks and cools
Sills
• Sills are uniformed in thickness and consist of basaltic rocks
• They also form cliffs, scarps and waterfalls when the overlaying
surface is eroded
Plug
Plug
• A vertical pipe of rock that is formed when the molten material in the
vent of the volcano cools and hardens.
• The plug may become exposed if the overlaying body of the volcano is
removed after years of erosion. The plug will appear to be a steep
sided rock outcrop
Summary
Extrusive Features
• Classification of volcanoes is based on their shape, size and nature of
their eruptions
Lava
• Lava can be Basic or Acidic, the nature of the lava influences the
shape of the cone forming the extrusive landforms
- Acidic lava: has a high silica content (65%), is viscous, relatively
immobile, cools and solidifies quickly upon reaching the surface.
Forms steep sided cones
- Basic Lava: low silica content (40%), very mobile as it flows almost like
a liquid, it can cover long distances as it flows in a sheet-like manner.
Forms gently sided slopes
Lava
Acidic/Viscous Lava Basic Lava
• Comes to the surface at • Flows close to the surface.
convergent plate margins Associated with divergent plate
(Subduction Zones) margins
• Contains gas which cannot • Volcanoes of Hawaii and Iceland
escape easily, which is the cause are produced with basic lava
for the violent explosive
eruptions
Lava Plateau
Lava Plateau
• Sometimes basaltic lava is poured out from long fissures and not
vents. Spreading out over large distances forming broad plateau
• Examples: Deccan Plateau in India and Basalt Plateau in Iceland
• A point to note is that as the lava flows out of the fissure, the ends of
the flow can be dissected by streams
Shield Volcano
Shield Volcanoes
• When basic lava accumulates, shield volcanoes are formed
• Because the lava is thin and flows, the sides of the volcanoes
are broad and gentle with the inclines they form being 7° or
less.
• The lava has a low Viscosity and the craters are wide
• The world’s largest volcano, Mauna Loa is formed this way
• Mauna Kea, also in Hawaii is the most active volcano
Ash and Cinder Cones
Ash and Cinder Cones
• This is the simplest type of volcano
• Forms when pyroclastic material is ejected from a single vent
• From the violent eruption, fragmented material such as ash, cinder,
dust and chunks of lava falls back, cools and solidifies, when this is
repeated layers are formed
• The slopes of the volcanoes are inclined between 10-30°
• Their shape is symmetrical due to the fast cooling, slow moving acidic
lava that solidifies
• Example Mt. Misery in Nevis
Ash and Cinder Cones - Differences
• There is a difference in the two:
• Cinder cones originate from explosions within the magma pipe
• Their crater is wider than that of ash cones
• Ash cones poses gentler gradients
• The materials at ash cones are finer than that of cinder which has finer fragments
Composite
Composite/ Strata Volcanoes
• Have alternating layers of lava flows, ash, cinders, blocks and bombs
• They may rise to over 2500 meters. Also symmetrical on both sides
and have alternating eruptions of ash and lava
• Example: Soufriere in St Vincent
• In many cases the main vent is blocked by molten rocks that cool and
solidifies which results in the formation of secondary cones/vents
know as parasitic cones
• Composite Volcanoes are known to form at Convergent Margins (Ring
of Fire and the Eastern Caribbean
Craters and Calderas
Craters and Calderas
• A crater is an opening or depression at the top of a volcano
• Caldera Formation:
- Pressure builds up within the volcano when viscous lava plugs the vent of
composite cones, causing violent eruptions which cause the crater to explode
- The top of the volcano now subsides into the magma below and forms a
greater depression known as a caldera
- A caldera can also be formed by the collapse of a magma chamber beneath a
volcano
- Rather than one vent, Calderas usually have several smaller vents
Craters and Calderas
• Water can collect over time within
craters and calderas to form lakes,
example Gran Etang in Grenada
• A crater is classified as a caldera
when it is greater than 1 km in
diameter
Word search
• Tephra
• Pyroclastic flow
• Parasitic volcano
• Basic Lava
• Acidic Lava
• lahar
