Impacts of extrusive igneous activity

Impacts of Extrusive
Igneous Activity

A2 Geographical Research

Learning Objectives
• Look at the various aspects of extrusive
igneous activity
• formation of volcanic cones and fissures
• The formation of Lava plateaux

What is extrusive igneous
activity?
• In simple terms extrusive igneous activity
refers to when magma reaches the surface
of the earth and becomes known as lava.
• The lava flows once cooled form landforms
such as volcanoes

3 Forms of activity
• Major extrusive activity can come as a gas, liquid or solid:

• Gas - sulphur, hydrogen, carbon dioxide and hot
steam(geysers).: When uprising magma decreases in pressure
suddenly, gases within magma explode to the earth's surface
and cause destruction.

• Solid - pyroclast: Mainly composed of: fragments of country-
rock, solidified lava and fine materials of volcanic ash and dust. It
can be classified in term of size into volcanic bombs, volcanic
blocks, lapilli, volcanic ash and volcanic dust.

• Liquid – lava, hot springs.

Extrusive Landforms
• There are several types of extrusive landforms
whose nature depends on how gaseous or
viscous the lava is when it reaches the earth’s
surface.
• Lava produced by the upward movement of
material from the mantle is Basaltic and tends
to be located along mid-ocean ridges, over hot
spots and along rift valleys.

• Lava that results from the process of subduction is described
as andesitic (after the Andes) and occurs as island arcs or at
destructive plate boundaries where oceanic crust is
destroyed.

Lava types have a major impact on
landforms

Volcano Classification
• There is no universally accepted method of
classification. The two most quoted groupings
are:
- According to shape
- Nature of the eruption.

Fissure Eruptions
• When two plates move apart lava may be ejected
through fissures rather than a central vent.
• The Heimaey eruption of 1973 in Iceland began
with a fissure of over 3km.
• This is small compared to Laki also in Iceland in
1783 where a fissure exceeding 30km in length
opened up.
• The basalt may form a large plateaux, filling
hollows rather than building up into a typical cone-
shaped volcanic peak.
• The columnar jointing produced by the slow
cooling of the lava provides tourist attractions e.g.
Giant’s Causeway

Basic or shield volcanoes such as Mauna Loa
in Hawaii have lava flowing out of a central
vent and can spread over a wide area before
solidifying. The result is a cone with long
gentle sides made up of many layers of lava
from repeated flows.

Acid or dome volcanoes – acid lava quickly
solidifies in the air and this produces a steep
sided, convex cone as most lava builds up
near to the vent. In the case of Mt Pelee the
lava actually solidified as it came up the vent
and produced a spine rather than flowing
down the sides.

Ash and cinder cones (e.g. Paricutin) form when
ash and cinders building up into a symmetrical
cone with a larger crater.

Composite cones – many of the larger, classically
shaped volcanoes result from alternating types
of eruption in which first ash and then lava
(usually acidic) is ejected. Mt Etna is a result of a
series of both violent and more gentle eruptions.

Calderas - when the build up of gases
becomes extreme, huge explosions may
clear the magma chamber beneath the
volcano and remove the summit of the
cone. This causes the sides of the crater
to subside, thus widening the opening to
several kilometres in diameter. In the case
of both Thira and Krakatoa, the enlarged
crater or caldera has been flooded by the
sea and within the resultant lagoons, later
eruptions have formed smaller cones.

Minor Extrusive Landforms

Minor extrusive landforms are often associated with, but not
exclusive to, areas of declining volcanic activity. They include
soltfatara, fumaroles, geysers and mud volcanoes.

The Nature of Eruptions
• Icelandic - lava flows gently from the
fissure, usually on flat slopes
• there is no central crater.
• Giant cracks open in the ground and expel
vast quantities of lava that spread far and
wide to form huge pools that can cover
almost everything around.
• When these pools of lava cool and
solidify, the surface remains mostly flat.
• Since the source cracks are usually
buried, there is often nothing "volcano-like"
to see--only a flat plain.
•

Hawaiian type (VEI 0-1)
• there is a small vent through which small amount
• fluid basaltic lava is thrown into the air in jets from a vent or
line of vents (a fissure) at the summit or on the flank of a
volcano.
• The jets can last for hours or even days, a phenomenon
known as fire fountaining.
• The spatter created by bits of hot lava falling out of the
fountain can melt together and form lava flows, or build hills
called spatter cones.
• Lava flows may also come from vents at the same time as
fountaining occurs, or during periods where fountaining has
paused.

Hawaiian type
• Because these flows are very fluid, they can travel miles
from their source before they cool and harden.
• Hawaiian eruptions get their names from the Kilauea
volcano on the Big Island of Hawaii, which is famous for
producing spectacular fire fountains.
• Two excellent examples of these are the 1969-1974
Mauna Ulu eruption on the volcano’s flank, and the 1959
eruption of the Kilauea Iki Crater at the summit of
Kilauea.
• In both of these eruptions, lava fountains reached heights
of well over a thousand feet

Strombolian type (VEI 1-2)
• These are distinct bursts of fluid lava (usually basalt or
basaltic andesite) from the mouth of a magma-filled
summit conduit.
• The explosions usually occur every few minutes at regular
or irregular intervals.
• The explosions of lava, which can reach heights of
hundreds of meters, are caused by the bursting of large
bubbles of gas, which travel upward in the magma-filled
conduit until they reach the open air.

Strombolian type
• This kind of eruption can create a
variety of forms of eruptive products:
• Spatter, or hardened globs of glassy
lava.
• Scoria, which are hardened chunks of
bubbly lava; lava bombs, or chunks of
lava a few cm to a few m in size; ash
• Small lava flows (which form when hot
spatter melts together and flows
downslope).
• Products of an explosive eruption are
often collectively called tephra.

Strombolian type
• Strombolian eruptions are often associated with
small lava lakes, which can build up in the
conduits of volcanoes.
• They are one of the least violent of the
explosive eruptions, although they can still be
very dangerous if bombs or lava flows reach
inhabited areas.
• Strombolian eruptions are named for the
volcano that makes up the Italian island of
Stromboli, which has several erupting summit
vents.
• These eruptions are particularly spectacular at
night, when the lava glows brightly

Vulcanian type (VEI 2-3)
• A Vulcanian eruption is a short, violent, relatively small
explosion of viscous magma (usually andesite, dacite, or
rhyolite).
• This type of eruption results from the fragmentation and
explosion of a plug of lava in a volcanic conduit, or from
the rupture of a lava dome (viscous lava that piles up
over a vent).
• Vulcanian eruptions create powerful explosions in which
material can travel faster than 350 meters per second
(800 mph) and rise several kilometres into the air.
• They produce tephra, ash clouds, and pyroclastic
density currents (clouds of hot ash, gas and rock that
flow almost like fluids).

Vulcanian type
• Vulcanian eruptions may be repetitive and
go on for days, months, or years, or they
may precede even larger explosive
eruptions.
• They are named for the Italian island of
Vulcano, where a small volcano that
experienced this type of explosive eruption
was thought to be the vent above the forge
of the Roman smith god Vulcan.

Vesuvian type
• Typified by the eruption of Mount Vesuvius
in Italy in A.D. 79, great quantities of ash-
laden gas are violently discharged to form
cauliflower-shaped cloud high above the
volcano
• It has a long period of inactivity but right
after it erupts with enormous power

Peléan eruption (VEI 3-4)
• They can occur when viscous magma, typically of rhyolitic
or andesitic type, is involved, and share some similarities
with Vulcanian eruptions.
• The most important characteristics of a Peléan eruption is
the presence of a glowing avalanche of hot volcanic ash, a
pyroclastic flow.
• Formation of lava domes is another characteristic feature.
Short flows of ash or creation of pumice cones may be
observed as well.
• The initial phases of eruption are characterized by
pyroclastic flows.

Peléan eruption
• The tephra deposits have lower volume and
range than the corresponding Plinian and
Vulcanian eruptions.
• The viscous magma then forms a steep-
sided dome or volcanic spine in the
volcano's vent.
• The dome may later collapse, resulting in
flows of ash and hot blocks. The eruption
cycle is usually completed in few years, but
in some cases may continue for decades.
• The 1902 explosion of Mount Pelée is the
first described case of a Peléan
eruption, and gave it its name.

Krakatoan or Plinian (VEI 4-8)
• They are caused by the fragmentation of gassy magma, and are
usually associated with very viscous magmas (dacite and rhyolite).
• They release enormous amounts of energy and create eruption
columns of gas and ash that can rise up to 50 km (35 miles) high at
speeds of hundreds of meters per second.
• Ash from an eruption column can drift or be blown hundreds or
thousands of miles away from the volcano.
• The eruption columns are usually shaped like a mushroom (similar to
a nuclear explosion) or an Italian pine tree; Pliny the Younger, a
Roman historian, made the comparison while viewing the 79 AD
eruption of Mount Vesuvius, and Plinian eruptions are named for him.

Point of Note
• Vesuvian and Plinian are often to referred to as one and the
same but differentiations are made within certain textbooks
• Plinian eruptions are extremely destructive, and can
even obliterate the entire top of a mountain, as occurred
at Mount St. Helens in 1980.
• They can produce falls of ash, scoria and lava bombs
miles from the volcano, and pyroclastic density currents
that raze forests, strip soil from bedrock and obliterate
anything in their paths.
• These eruptions are often climactic, and a volcano with
a magma chamber emptied by a large Plinian eruption
may subsequently enter a period of inactivity.

Eruptions and the VEI
• VEI was proposed in 1982 as a way to describe the
relative size or magnitude of explosive volcanic
eruptions.
• It is a 0-to-8 index of increasing explosivity. Each
increase in number represents an increase around a
factor of ten.
• The VEI uses several factors to assign a number,
including volume of erupted pyroclastic material (for
example, ashfall, pyroclastic flows, and other ejecta),
height of eruption column, duration in hours, and
qualitative descriptive terms

Eruptions and the VEI
The classification of the eruption shows some similarity to the Volcano Explosivity Index
(VEI) developed in the USA.

•In the figure, the volumes of several
past explosive eruptions and the
corresponding VEI are shown.
• Numbers in parentheses represent
total volume of erupted pyroclastic
material (tephra, volcanic ash, and
pyroclastic flows) for selected
eruptions; the volumes are for
uncompacted deposits.
• Each step increase represents a ten
fold increase in the volume of erupted
pyroclastic material.

What Determines Eruption Type?
• The crystal and gas content and temperature of a magma
help determine a volcano’s eruption style.
• Crystals in magma make it more viscous, so magma with a
high crystal content is more likely to explode than flow.
• Gases create explosions if they cannot easily escape from
viscous magma, but they can also be released without
explosions (or with only minor ones) from fluid magma.
• High-temperature magmas usually erupt effusively, while
low-temperature magmas cannot flow easily and are more
likely to erupt explosively.

Lava Domes
• Are mounds that form when viscous lava is erupted slowly
and piles up over the vent, rather than moving away as a
lava flow.
• The sides of most domes are very steep and typically are
mantled with unstable rock debris formed during or shortly
after dome emplacement.
• Most domes are composed of silica-rich lava which may
contain enough pressurized gas to cause explosions
during dome extrusion.

Your task
In pairs you have to choose one of the two most devastating
eruptions of the last 100,000 years:
- Mount Tambora 1815 VEI 7
-Mount Toba 73,000 BP (Before Present 1950) VEI 8

You must then put together a short presentation covering the
following key areas:
- Location (geographical and geological)
-Type of volcano (fissure, composite cone etc.)
- Events leading to the eruption
- Effects of the eruption (immediate, short-term, long-term) at a
number of different scales – local, regional, global.

TASKS
• Students are to separate into groups of three and
research the effects earthquakes can have on the
landscape and give a presentation on one of the following:
– Rift Valleys
– Ground Displacement
– Dip slips and Strike Slip
– Definition and Example of S and P waves
– Normal Fault, Reverse Fault, Oblique-slip fault
– Block Mountains or Horst
– Fault Scarps
– Fold Landforms – Anticline, Recumbent Fold, Fold Mountains

Impacts of extrusive igneous activity

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