Eruption of vulcano

The word volcano is derived from the name of the ancient Roman island of Vulcano which lies off the southwest coast of Italy. The Romans believed that Vulcan, the god of fire and the maker of weapons, used the volcano on that island to forge his weapons.

Volcanoes are not alive but scientists use human terms to talk about volcanoes, such as active, alive, dormant, resting, sleeping, extinct, dead, lifetime, and restless.

The island in the middle of the picture is Vulcano. The island was formed by Vulcanian eruptions, which are eruptions of hot gas and steam followed by ejections of thick and pasty lava.

The term Volcano has two definitions;

-           An opening in the crust of the Earth in which molten rock called magma and gases can escape to the surface.

-           The mountain that is formed from volcanic eruptions.

This is a photo of the volcano Paricutin (Pear-A-Koo-Teen). Paricutin's cone formed from nine years of almost constant eruptions. Red hot cinders exploded from the main vent and landed near it building the cone higher and higher. This type of cone is called a cinder cone. You will learn more about the types of volcanic cones in the eighth lesson, "Volcanic Cones and Eruptions".

Volcanoes actually build themselves into a mountain with repeated eruptions. In 1943 a farmer in Mexico noticed that some cracks (fissures) in his corn field were growing wider and wider. The next day his field was engulfed by a growing volcanic cone (Light Green). During the week the cone grew 500 feet taller (Dark Green). Within a year (Dark Gray) Paricutin was over 1200 feet higher than the surrounding landscape. During the next eight years the volcano did not grow much taller but the cone's base grew wider and wider (Light gray). Paricutin stopped erupting in 1952 almost as fast as it started. The mountain has been silent since.

Volcanoes can build themselves into high mountains one day and in the case of Mt. St. Helens erupt violently blowing their top off the next day. Mt. St. Helens lost over 1300 feet of its summit during the eruption and simultaneous landslide of 1980.

Volcanoes are classified as active, dormant, and extinct. Active volcanoes are either currently erupting or have erupted in recorded history. There are over 500 volcanoes on Earth that fit this category today. Dormant or resting volcanoes are not currently erupting but are considered likely to do so. Mt. St. Helens had been dormant for one hundred twenty-three years before it erupted in 1980. Extinct or dead volcanoes have not erupted in recorded history and are not expected to erupt again.

The photo above is of beautiful Mt. St. Helens before it erupted on May 18, 1980. Mt. St. Helens was one of the most beautifully symetrical stratovolcanoes in the world. It was called "the Fuji of the west". Mount Fuji, in Japan, is the most photographed mountain in the world. The next card will show you what this mountain looked like shortly after the eruption. The lake in the foreground changed. The lake's level is now 150 feet higher because the landslide and eruption filled the bottom of the lake with rock, soil, and pyroclasts

This is Mt. St. Helens four months after the eruption. Notice the loss of over 1300 feet of the summit. Also notice the total devastation of the beautiful forests and how Spirit Lake rose. Spirit Lake's surface was completely filled with trees that were blasted into the lake by the force of the eruption. The lake is now much more shallow, wider, and longer than before the eruption. Huge trees still float across the lake today.

The eruption left a crater over a mile wide and over 2000 feet deep. The mountain is still active today spewing small whisps of steam. A lava dome is growing in the bottom of the huge crater.

lava dome is a steep mass of very thick and pasty lava that is pushed up from the main vent. The lava is so viscous (thick and pasty) that it does not flow but slowly rises higher with each movement of magma in the conduit. Think of toothpaste that is slowly squeezed and then stopped and then squeezed again from the tube. This is how the lava dome in Mt. St. Helen's was formed.

The dome's exterior surface is very rough with chunks of lava that were formed from small eruptions that broke the cooled and hardened surface into blocks.

The dome slowly "grew" larger and larger over a seven year period. An earlier dome started to form one month after the famous eruption when very thick lava (dacitic lava) rose into the crater from the magma chamber below. This dome was destroyed by an explosive eruption just a month later.

The large dome that is very visible today is over 900 feet tall (taller than an 80 story building) and over 3000 feet wide (10 football fields). As large as the lava dome is, it is still dwarfed by the huge crater that was the result of the 1980 eruption. Steamy whisps of steam are still visible from the dome telling us that the volcano's magma is filling the conduit, making the volcano still active today.

You are looking at the inside of a volcanic crater. The steep walls were produced be many eruptions ejecting very liquid lava. This lava then lands on the crater walls building them higher and higher. The lava in the main vent is extremely hot (probalbly about 1800 degrees F.) The lava on top cools and hardens because the air that it is in contact with is so much cooler than the lava. This hardened lava will then be dragged back down under the surface and remelted. You probably noticed the same process if you have ever heated soup on the stove. If you did not keep stirring the soup it formed a "scum" on top.

Predicting exactly when a volcano will erupt is next to impossible. Today geologists are becoming much more accurate in making the public aware that a volcano is showing signs that it may erupt in the near future.

In the months before Mt. St. Helens erupted geologists knew the mountain was getting restless. A magnitude 4.1 earthquake was recorded on March 20 (about 2 months before the large eruption). Many shallow earthquakes were recorded over the next seven weeks. Magma moving higher and higher inside the mountain was causing these earthquakes. As the magma rose it formed a large bulge on the north flank. This bulge was growing daily and the geologists knew that an eruption was soon to be.

What the authorities did was evacutate most of the people in and near the mountain. Some decided to stay. Almost everyone that was near the eruption was instantly killed. In all, 57 people died. Without the evacuation perhaps as many as 30,000 deaths would have been attributed to Mt. St. Helens fury.

The geologists in the photo are measuring a growing fissure near the lava dome in Mt. St. Helens crater. As magma rises the fissure will grow wider telling the geologists that the magma is rising again.

Scientists can not stop a volcano from erupting but with constant monitoring they can warn and evacuate people and save lives.

Many volcanoes erupt in very consistant patterns, while other volcanoes have no eruption pattern at all. This makes forecasting eruptions difficult.

What makes predicting eruptions even more difficult is the fact that many volcanoes start with one type of eruption pattern and then change eruption patterns as they grow older.

Some of the most powerful eruptions in recorded time have come from volcanoes that have been dormant for hundreds and even thousands of years.

Here we have geologists studying a tilt meter. A tilt meter is used to measure the growth of the lavadome in the foreground. The tiltmeter will show a different angle as the dome grows. With careful study the geologists can tell if magma is on the rise and that an eruption may occur in the near future.

The letter A represents a magma chamber. Magma is molten rock that is located under the surface of the Earth. A magma chamber is usually located far beneath the surface of the Earth where an oceanic plate is driven down into the mantle by a continental plate. The oceanic plate melts as it desends into the upper layer of the mantle. Some ocean water gets trapped with the oceanic plate and is turned into steam by the intense heat.
The magma is less dense and under extreme pressures that force it up toward the surface. This molten rock and gas collects in a magma chamber until it can escape to the surface.

Igneous Rock

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Geomorphology of Igneous Terrains
I. REVIEW OF IGNEOUS PROCESSES
A. Basic Terminology
1. Igneous Rocks- a rock (or agglomeration of one or more minerals) that results from
the cooling of magma, or molten rock. As the magma cools, minerals crystallize
from the molten rock.
2. Magma - molten or hot liquid rock, originates beneath the earth's surface (up to
120 miles beneath), composed of elements found in silicate minerals, water vapor,
and gases.
3. Lava - magma that is extruded onto the earth's surface via volcanic eruptions (hot
magma is confined at depth beneath surface, relatively lighter than confining rock,
rises upward, may eventually erupt onto earth surface).
4. Extrusive Igneous Rocks or Volcanic Ig. Rocks - rocks which solidify from lava (or
were extruded onto earth's surface)
5. Intrusive Igneous Rocks or Plutonic Ig. Rocks - rocks which solidify from magma
beneath the earth's surface.
B. Magma Crystallization Process
1. Hot, molten magma: ions of elements are moving freely in a fluid, unordered state,
as magma cools, the ions slow and begin to form atomic bonds, arranging
themselves in orderly patterns --- known as process of crystallization.
2. Rates of cooling strongly influence size of mineral crystals that develop from
magma/lava.
a. Slow Cooling - few centers of crystal growth develop, ions allowed to
migrate over larger distances - results in rather large mineral crystals .
b. Fast Cooling - many centers of crystal growth, ions readily bond together,
results in smaller mineral crystals.
c. Very rapid cooling - if magma is quenched instantly, not sufficient time for
ions to bond, results in randomly distributed ions frozen - referred to as glass
similar to manmade glass.
C. Naming Igneous Rocks - Based on composition and texture of igneous rock.
1. Mafic Rocks (from Mag and Fe) - generally darker colored rocks relatively high in
iron, magnesium, calcium and low in silicon. Associated with high temp. end of
Bowen's Reaction Series.
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E.g. Gabbro = plutonic = phaneritic = mafic composition
Basalt = volcanic = aphanitic = mafic composition (Ca-rich plag., and Pyroxene).
2. Felsic (from feldspar and silica) Rocks - generally lighter in color, high in silica, Na,
Potassium - consist mainly of quartz, K-feldspar, and Plagioclase
e.g. Granite = plutonic = phaneritic = felsic composition
Rhyolite = volcanic = aphanitic = felsic composition
3. Intermediate - admixtures of both felsic and mafic, dominated by amphibole,
intermediate plagioclase feldspar, biotite.
e.g Diorite = plutonic = phaneritic = intermediate composition
Rhyolite = volcanic = aphanitic = intermediated comp.
4. Ultramafic = very rich in iron and mag., olivine and pyroxene, Ca-rich plagioclase
e.g. intrusive variety only = Peridotite - common in upper mantle
D. Volcanism
1. Volcanism- process by which magma, gas, and water are released from the
interior of the earth. Volcanic processes and eruptions often result in the spewing
and build up of volcanic material about a volcanic center, constructing a volcanic
edifice commonly referred to as a volcano.
a. Status of Volcanic History
(1) Active: volcano observed in eruption during historic time
(2) Dormant: volcano with no historic record however show evidence
of geologically recent activity
(a) volcanic deposits
(b) hydrothermal activity
(3) Extinct: no historic record and no evidence of geologically recent
activity
2. Nature of Volcanic Activity
a. Style of volcanism; i.e. Explosive vs. Quiescent is determined by the
composition of the magma, its temperature, and amount of dissolved gases
contained within. All of which influence the magmas viscosity, or resistance
to flow.
(1) > viscosity, > violent nature of the eruption
(2) < viscosity, < explosive nature of the eruption 112 b. Factors affecting viscosity (1) temperature: the higher the temperature the lower the viscosity, i.e. the more kinetically active the magma is at atomic level, the less resistance to flow it possesses. (a) Temp range of lavas: 1000-2700 F (540-1480 C) i) Basaltic Lavas: 2000 F melting pt. a) quiescent eruptions ii) Rhyolitic Lavas: 1200 F melting pt. a) explosive eruptions (2) chemical composition: the SiO or silica content of the magma also 2 influences the viscosity. (a) Rhyolitic/Granitic magma = 70% silica i) Very "sticky" explosive eruptions (b) Andesitic/intermediate magma = 60% silica, (c) basaltic magma = 50% silica. i) In general > silica content, > viscosity of magma,
believed to result from tendency of complex silica
anions to form long chains of molecules before
crystallization begins. Thus granitic magmas are more
viscous than basaltic magmas.
(3) Gas content:
(a) dissolved gasses tend to < viscosity of magma, (b) gasses also exert pressure on magma resulting in explosive eruption of magma from vent i) "Degassing" as magma rises towards surface, confining pressure decreases, and temperature slightly decrease resulting in expansion of gasses and POW! a) Product: frothy gaseous lava b) pumice and glass shards ii) Fluid basaltic lavas easily allow gases to escape, often resulting in lava fountains such as in Hawaii, generally quiescent eruptions 113 (4) Phreatic State: (a) external occurrences of groundwater, surface water, snow and ice i) can create large steam explosions and increase explosivity of eruption 3. Products of Volcanic Eruptions a. Lava Flows- lava may be produced from any composition magma, but in general flowing or molten lava is commonly associated with low silica, basaltic composition magmas (e.g. Hawaii) (1) Hawaiian-type lavas- basaltic composition (60% silica), slowly flow down slopes, cooling to form basalt. (a) Pahoehoe flows-lava flows with a smooth congealed skin, a whipped or ropy appearance. (b) aa flows- more blocky in appearance, flows are rough jagged blocks with angular edges. i) These are generally lower temperature lavas than Pahoehoe flows, thus tend to be cool, blocky, and thick. b. Escaping Gases from Lava- (1) Magmas hold dissolved gases within them, as these magmas are extruded as lava, gases begin to escape. (a) Gases estimated to compose 1-5% of total wt. of lava, i) most of which is water vapor (70%), ii) Lesser amounts of carbon dioxide (15%), iii) Sulfur oxide and nitrogen oxides (<5%), also hydrogen, chlorine, and argon. c. Pyroclastic Materials - fragments of pulverized rock and lava ejected from a volcano. These ejecta range in size from very fine dust or ash to sand sized volcanic ash, to house-sized volcanic bombs and blocks. (a) Pyroclastic eruptions are commonly associated with highly viscous rhyolitic-magmas with high-pressure buildup of gases (b) May also be associated with basaltic magmas (c) Tephra- airborne volcanic material of any size (1) fine ash- result of gas-filled frothy magma, gases expand and blast semi-cohesive lava into tiny pieces to form ash. (a) Tuff- deposits of ash 114 i) welded tuffs- glass shards in ash heat-fuse after deposition. (2) Pumice- sand to gravel sized fragments of cooled lava with many air voids. (3) Lapilli- walnut sized pyroclastic ejecta. (4) Cinders- pea-sized basaltic particles (5) Blocks and bombs- pyroclastic fragments larger than lapilli, blocks = comprised of ejected hardened lava, bombs=ejected as molten lava. Bombs are semi-molten when ejected and attain a stream line shape as result of aerial shear forces exerted on it as follows its trajectory through the atmosphere. (6) Composition of Pyroclastic Material (a) Crystals- single mineral crystals of varying size (b) Lithic Fragments: volcanic rock fragments (c) Vitric Fragments: glassy shards from rapid cooling of magma I. TYPES OF VOLCANIC ERUPTIONS A. Hawaiian Type 1. Type Area: Hawaiian Islands 2. Eruptive Style: a. low-viscosity, fluid lava eruptions (1) basaltic composition (a) 2000 F temp. b. Quiescent eruptions c. elongate fissure vents to domal volcanic centers d. large volume lava eruptions (1) lava curtains (2) lava fountains (3) lava rivers e. lava-dominant, low amounts of pyroclastics/tephra B. Icelandic Type: Fissure Eruptions 1. Type Area: Iceland, Mid-Atlantic Ridge 2. Eruptive Style: a. Elongate, laterally extensive fissure eruptions (1) lack of central vent/volcanic cone 115 b. hot, fluid basaltic lavas c. large volume eruptions (1) fluid lava: flows laterally extending 100's of miles (2) sheet geometry to lava flows (3) Lava fountains/lava curtains 3. Other Localities a. Columbia Plateau of Oregon/Washington/Idaho (1) Columbia River Basalts (a) >500,000 sq. km area
(b) Fissure eruptions with vents in NE Oregon
(c) cumulative thickness of >14,000 Ft
b. Snake River Plain of Idaho
c. Deccan Plateau of India
d. Parana Region of South America
C. Strombolian Type
1. Type Area: Volcano Stromboli, Sicily
2. Eruptive Style:
a. Basaltic Magmas
(1) Cooler and more viscous than Hawaiian
(2) Modest to high explosivity
(3) Pyroclastic Fragments common
(a) blocks, bombs, breadcrust
D. Vulcanian Type
1. Type Area: Vulcano- volcanic island off coast of Sicily
2. Eruptive Style
a. Basaltic to Andesitic Magmas
(1) Of higher silica content compared to Strombolian
(a) > viscosity
(b) > explosivity
b. Tendency toward to congealed vent plugs/crusts
(1) Explosive plug blow-out
c. Pyroclastic Eruptions common
(1) blocks, bombs, ash, scoria, pumice
E. Plinian Type
1. Type Area: Mt. Vesuvius, mainland Italy
a. Major eruption, 79 A.D.
b. Destruction of Pompeii and Herculaneum
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c. First hand account by Pliny the Elder and Pliny the Younger
2. Eruptive Style
a. Andesitic magmas of increased silica content
b. Very violent/explosive eruptions
(1) Gas and tephra
(a) Pumice and ash eruptions
(2) Huge tephra clouds to altitudes of 200,000 Ft
c. Post-eruption lahars and/or mudflows common
F. Pelean Type
1. Type Area: Mt. Pelee, Martinique, West Indies (Carribbean Plate)
2. Eruptive Style
a. Explosive Eruptions
(1) high viscosity magma
(2) vent plugging followed by gas-charged eruption
b. Mixtures of lava, gas and hot pyroclastic material
(1) Nuees Ardentes: "fire clouds"
(a) Process: hot, glowing ash clouds
i) searing ash and gas mixtures
ii) density clouds with D>Air, rolling mixture laterally down
volcanic slopes
iii) Gas thrust upward into eruptive column
iv) Base Surge: Lateral blast of ash cloud
(b) Ignimbrites: welded nuee ardent deposits
i) glowing ash flow sheet
ii) welded tuffs
c. Plinian: high-cloud eruptions associated with deep gas build-up
d. Peleean: low-cloud Nuee Ardents associated with shallow gas build-up
G. Krakatoan Type
1. Type Area: Krakatoa a volcanic island between Java and Sumatra in southeast
Asia
a. Most violent eruption in recorded history
b. Ramifications felt world-wide in 1883 eruption
2. Eruptive Style:
a. Caldera Collapse
(1) Calderas- blowing apart of volcanic cone or collapse of volcanic
cone into central vent
b. Highly explosive eruption
(1) abundance of pyroclastic material/ash
(2) ash encircled the earth
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(a) "nuclear winter" phenomena for 3 years
i) blockage of solar radiation
ii) below normal temperatures
H. Mt. St. Helens
1. Type Area: Mt. St. Helens, Washington Cascades
2. Eruptive Style:
a. Andesitic magma
(1) High viscosity, explosive eruptions
b. Magmatically induced seismic activity
c. seismically induced landslide
(1) reduction of lithostatic pressure
d. violent pyroclastic eruption
I. Solfataric Eruptions
1. gas emission only from volcanic center
2. Fumaroles: submarine gas vents
J. General Relations
1. With increasing explosivity; increasing height of eruptive column
a. Hawaiian Column Height: 2 km
b. Strombolian: 10 km
c. Subplinean: <30 km d. Ultraplinean: < 55 km II. VOLCANOGENIC LANDFORMS A. Volcanic Edifices 1. Volcanoes-mountainous accumulations of volcanic material 2. Anatomy of Volcano a. Crater- steep-walled depression at summit of volcano, < 1 km in diameter b. Calderas - large craters > 1 km.
c. Magma Chamber - magma center located beneath volcano, source of
magma/lava.
d. Central vent or pipe- conduit leading from magma chamber to crater or
opening of volcano
e. Flank Eruption- eruption of volcanic materials from side of volcano, not
through central vent.
f. parasitic cone- smaller secondary volcanic buildup on side of volcano, via
flank eruptions
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B. Volcano Morphology
1. Shield Volcanoes
a. broad, shield shape, gentle slopes, low profile
b. associated with basaltic lavas (low viscosity lavas).
c. Hawaiian-type volcanoes
(1) Fissure eruptions from volcanic edifice
(2) layered basalt lava flows
(3) little pyroclastic debris
(4) Upwards of 30,000 Ft vertical relief from ocean floor
2. Composite Cones
a. Composite cones or Strato Volcano- volcanoes comprised of a mixture or
alternating layers of lava and pyroclastic material,
b. Generally form large volcanoes, often associated with violent eruptions
(e.g. Mt. St. Helens) and andesitic magmas (sl. more siliceous than basalt).
(1) Composite cones produce:
(a) nuee ardents - glowing clouds of hot volcanic ash.
i) hot ash flows
ii) gas-driven
(b) lahars - debris flows formed of water saturated volcanic
debris.
i) cold-state debris flows
3. Plug Domes
a. Steep-faced dome composed of obsidian and pumice
b. very viscous magma, extremely plastic and sticky
(1) rhyolitic-dacitic compositions
4. Cinder Cones
a. Cinder cones- composed of ejected lava fragments or pyroclastic material
(1) Avg. diameter: < 1.5 km (2) Volume: 104-107 cu. m. b. Steeper sides to volcano than shield type (1) Accumulated pyroclastic debris at angle of repose (30-35 degrees) c. Cones occur singly or in clusters (1) Cone fields 119 5. Spatter Cones a. Parasitic cones and vent eruptions b. Ejected lava (1) bombs and spatter 6. Table Mountains a. Flat-topped volcanic cones with steep sides b. Sub-glacial volcanism/melt-water quenching (1) pillow-lavas (2) glassy accumulations 7. Calderas a. Extremely large craters > several km in diameter
(1) e.g. Crater Lake in Oregon
(a) Remanent caldera of Mt. Mazama
i) Explosion dated at 6000-7000 yrs ago
ii) "Mazama Ash" deposit throughout western interior
b. Causes
(1) Explosion of summit area
(2) volcanic collapse
(a) stoping of magma chamber
(b) collapse under weight of overburden
8. Maar- Phreatomagmatic crater with steep to vertical inner slopes surrounded by
tuff ring
a. explosive craters formed by interaction of magma and water saturated
depression (sub-aqueous lake eruptions)
(1) characterized by volcanic breccia in central vent with collapsed
margins
9. Landform Evolution of Volcanoes
a. < in relief of constructional landform b. > drainage density
c. > soil development on slopes
d. > weathered sediment apron surrounding core
III. LAVA FLOW MORPHOLOGY
A. Hawaiian Types- low-viscosity basaltic lava flows
1. Aa- blocky, rough lava flows
a. higher viscosity than Pahoehoe
b. crumbly flow advance, crusting process
2. Pahoehoe- ropy, smooth lava flows
a. lower viscosity than Aa
b. fluid lava flows
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B. Pressure Ridges- crenulated flow lines on lava flow, perpendicular to direction of flow
C. Lava Tubes- solidified exterior/crust forming
1. continued molten flow through tube
D. Flow Processes
1. Lobate flow form
2. lava streams
3. levees
4. Water Cooled lava
a. Chilled glassy rinds
b. Pillow lavas
E. Differential Erosion and Lava Flows
1. With erosion process over time
a. Lava flows commonly form resistant cap rocks and serve as ridge formers
IV. INTRUSIVE IGNEOUS LANDFORMS
A. Intrusive Activity and Phenomenon (Plutons: Cooled from below)
1. Intrusive- refers to injection and cooling of magma beneath the earth's surface
a. Dikes - planar bodies of igneous intrusive rock that resulted from the
injection of magma across strata or layers of rock, i.e. discordant sheet-like
intrusive bodies.
(1) Feeder Dikes: dikes may represent feeder fissures supplying
magma to form surface volcanic lava flows
(2) Radial Dike Swarms: series of dikes radiating outward from volcanic
center
(3) Ring Dikes: concentric sets of ring-shaped dikes
(a) Dikes may dip inward toward central feeder point
(4) Differential Erosion: dikes commonly form resistant, tabular bodies
that stand high in relief on landscape
b. Volcanic Necks - column-shaped landforms comprised of congealed
magma
(1) solidified plumbing conduit of volcanic center
(a) differential erosion leaves neck standing high in relief
i) e.g. Shiprock New Mexico, volcanic neck with ring dike
complex
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c. Sills - planar bodies of igneous intrusive rock that resulted from the injection
of magma parallel to strata or layers of rock, i.e. concordant sheet-like
intrusive bodies.
(1) May stand high in relief; resistant to erosion
(a) Mesas- flat-topped mountains
(b) Cuestas
(c) Hogbacks
d. Laccoliths - inverted lense-shaped igneous intrusive bodies (convex side
up), analogous to a sill, but much larger and result in upwarping pre-existing
strata or rock layers (e.g. Black Hills of South Dakota).
(1) Intrusive process results in doming of surrounding rock strata
(a) Cuesta-hogback relations surrounding central laccolithic
intrusion
e. Batholiths- large intrusive bodies of igneous rock that are > 100 sq. km. in
diameter, in reality magma chambers that have cooled and solidified
beneath the earth's surface (e.g. Sierra Nevada Mountains of California,
Idaho Batholith)
(1) Commonly form granitic cores of mountain ranges
(a) stand high in relief
(b) commonly form exfoliation domes
f. Stocks- massive igneous rock intrusions < 100 sq. km. in diameter

How Igneous Rock Is Formed

How to Processed Magmatic can reased: Igneous rocks are called fire rocks and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth becomes trapped in small pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks. Igneous rocks are also formed when volcanoes erupt, causing the magma to rise above the earth's surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.

Vulkanologi

AKTIVITAS MAGMA

Indonesia merupakan salah satu negara dengan jumlah gunung apinya yang terbesar di dunia. Kira-kira 179 gunung api yang terdapat di negeri ini dan 129 diantaranya masih aktif sampai sekarang. Karena hal inilah maka hampir setiap tahun paling sedikit satu gunung api melakukan erupsinya.
Aktivitas gunung merupakan pencerminan dari aktivitas magma yang terdapat di dalam bumi.

Aktivitas Volkanik
Aktivitas volkanik pada umumnya digambarkan sebagai proses yang menghasilkan gambaran yang menakjubkan, atau kadang menakutkan dari suatu bentuk struktur kerucut yang secara periodik melakukan erupsinya. Erupsi dari gunung api ini kadang –kadang merupakan letusan yang sangat gebat (eksplosif), tetapi kadang-kadang berlangsung dengan tenang. Faktor utama yang mengontrol macam erupsi gunung api adalah komposisi magma, temperatur magma dan kandungan gas yang terdapat dalam magma. Faktor-faktor tersebut sangat mempengaruhi mobilitas dari magma , atau sering disebut viskositas (kekentalan) magma. Semakin kental magma, semakin sulit magma untuk mengalir.
Komposisi kimia magma telah diuraikan pada bab sebelumnya dengan klasifikasi batuan beku. Satu faktor utama yang membedakan antara bermacam-macam batuan beku dan juga antara macam magma asala ialah kandungan unsur silika (SiO2). Magma pembentuk batuan beku basaltik mengandung kira-kira 50% silika. Batuan beku granitik mengandung sekitar 70% silika, sedang batuan beku menengah mengandung sekitar 60% silika. Jadi dapat dikatakan bahwa viskositas magma sangat berhubungan dengan kandungan silikanya. Semakin tinggi kandungan silikanya, maka magma semakin viskos dan aliran magma akan semakin lambat. Hal ini disebabkan karena molekul-molekul silika terangkai dalam bnetuk rantai yang panjang, walaupun belum mengalami kristalisasi. Akibatnya, karena lava basaltik mengandung silika yang rendah, maka lava basaltik cenderung bersifat encer dan mudah mengalir, sedangkan lava granitik relatif sangat kental dan sulit mengalir walaupun pada temperatur tinggi.

Tabel. Bermacam-macam sifat magma karena perbedaan komposisi.

Sifat Magma Basaltik Andesitik Granitik
Kandungan silika Kecil (+50%) Menengah (+60%) Tinggi (+70%)
Viskositas Rendah Menengah Tinggi
Kecenderungan Membentuk Lava Tinggi Menengah Rendah
Kecenderungan Membentuk Piroklastik Rendah Menengah Tinggi
Titik Lebur Tinggi Menengah Rendah

Kandungan gas dalam magma juga akan berpengaruh terhadap mobilitas dari magma. Keluarnya gas dari magma menyebabkan magma menjadi semakin kental. Keluarnya gas ini dapat pula menyebabkan tekanan yang cukup kuat untuk keluarnya magma melalui lubang kepundan. Pada waktu magma bergerak naik ke atas mendekati permukaan pada gunung api, tekanan pada bagian magma yang paling atas akan berkurang. Berkurangnya tekanan akan mengakibatkan lepasnya gas dari magma dengan cepat. Pada temperatur tinggi dan tekanan yang rendah, memungkinkan gas untuk mengembangkan volumenya sampai beberapa kali dari volumenya mula-mula. Magma basaltik yang kandungan gasnya cukup besar, memungkinkan gas tersebut untuk keluar melalui lubang kepundan gunung api dengan relatif mudah. Keluarnya gas tersebut dapat membawa lava yang disemburkan sampai bermeter-meter tingginya. Sedangkan pada magma yang kental, keluarnya gas tidak mudah, tetapi gas tersebut akan berkumpul pada kantong-kantong dalam magma yang menyebabkan tekanan meningkat besar sekali. Tekanan yang besar ini akan dikeluarkan dengan letusan yang hebat dengan membawa material yang setengah padat dan padat melalui lobang kawah gunung api. Jadi besarnya gas yang keluar dari magma akan sangat mempengaruhi sifat erupsi gunung api.

Material Erupsi Gunung Api
Material yang dikeluarkan oleh gunung api pada waktu erupsi bisa berupa lava, gas ataupun material piroklastik. Tiap gunung api mempunyai karakteristik tersendiri mengenai material yang dikeluarkan selama erupsinya.
Aliran Lava
Pada umumnya aliran lava terjadi pada lava bsaltik yang bersifat cair karena kandungan silikanya relatif kecil. Lava basaltik akan mengalir dengan mudah pada daerah yang luas atau kadang-kadang menyerupai bentuk lidah. Adakalanya aliran lava basaltik bisa mencapai puluhan kilimeter dengan kecepatan aliran antara 10 sampai 300 meter per jam. Sebaliknya aliran lava yang kaya silika sangat lambat sekali.
Aliran lava basaltik, kadang-kadang menghasilkan permukaan yang halus, tetapi juga kadang-kadang menghasilkan permukaan yang berkerut seperti bentuk tali. Bentuk lava yang demikian disebut dengan pahoehoe lava atau ropy lava. Bentuk lain yang juga umum terjadi adalah permukaan yang kasar, berbentuk blok-blok dengan tepi yang tajam, disebut dengan blok lava atau aa lava. Aliran dari aa lava biasanya tebal dan dingin, dengan kecepatan aliran sekitar 5 sampai 50 meter per jam. Blok lava ini terjadikarena bagian luar lava yang relatif cepat membeku, tetapi di bagian dalamnya relatif masih cair dan terus mengalir. Akibat aliran lava di bagian dalam ini akan menyebabkan bagian luar yang sudah membeku terpengaruh oleh aliran ini sehingga mengalami retakan dan membentuk blok-blok. Selain pada permukaannya juga terbentuk lubang-lubang bekas keluarnya gas.
Gas
Magma mengandung bermacam gas yang jumlahnya kira-kira 1 sampai 5% dari berat total, dan sebagian besar merupakan uap air.meskipun persentasenya kecil, tetapi jumlah gas yang dikeluarkan bisa mencapai ribuan ton per hari. Komposisi gas yang dikeluarkan dalam aktivitas gunung api mengandung 70% uap air, 15% karbon diosida, 5% nitrogen, 5% sulfur dan sisanya terdiri dari klorida, hidrogen dan argon.
Material Piroklastik
Material padat dan setengah padat yang dikeluarkan oleh gunung api pada waktu erupsinya disebut material piroklastik. Material fragmental ini mempunyai ukuran dari sangat halus sampai diameter beberapa meter. Sebagian besar material yang dikeluarkan ini diendapkan disekitar kawah, sehingga membentuk struktur kerucut gunung api.
Karena material piroklastik mempunyai ukuran fragmen yang sangat bervariasi, maka material piroklastik dapat dikelompokkan berdasarkan ukurannya. Partikel-partikel yang berukuran sangat halus disebut debu vulkanik (volcanic ash). Material ini terbentuk bila lava banyak mengandung banyak gas di dalamnya. Bila gas yang panas ini dieksplosifkan keluar, maka lava akan terurai menjadi partikel-partikel yang halus. Hal semacam ini bila dikeluarkan dalam ukuran yang relatif besar akan membentuk pumis. Bila debu volkanik yang panas ini jatuh di permukaan bumi, akan membentuk welded tuff, yang dicirikan adanya glass shard.
Partikel yang berukuran seperti kacang disebut lapilli, sedang partikel atau material piroklastik yang berukuran lebih besar dari lapilli disebut block bila dikeluarkan dari gunung api dalam keadaan padat, sehingga bentuknya meruncing. Sedang bila dikeluarkan dalam keadaan setengah padat sehingga bentuknya relatif membundar disebut bomb.

Gunung Api dan Erupsi Gunung Api
Erupsi gunung api yang berkelanjutan, akan menghasilkan material-material yang terkumpul di sekitar pusat erupsinya dan membentuk gunung api (volkano). Pusat erupsi gunung api yang biasanya terletak pada puncaknya disebut crater (kawaH0, berhubungan dengan dapur magma melalui semacam pipa. Beberapa gunung api mempunyai kawah yang sangat besar sampai beberapa kilometer diameternya yang disebut kaldera. Tidak semua gunung api mengeluarkan hasil erupsinya melalui lubang yang terpusat, tetapi kadang-kadang melalui suatu celah yang memanjang pada lerang gunung api tersebut. Aktivitas magma pada lereng gunung api membentik parasitik cone.
Setiap gunung api mempunyai sifat dan tipe erupsi yang berbeda-beda, sehingga masing-masing mempunyai bentuk yang berbeda pula. Berdasarkan sifat dan tipenya, maka gunung api dapat dibedakan menjadi tiga yaitu gunung api shield, cinder cone dan composit cone.
Kaldera diperkirakan terbentuk pada waktu terjadi erupsi yang sangat besar, sehingga dapur magma kosong. Kemudian karena kosongnya dapur magma, puncak gunung api tersebut runtuh ke dalam dapur magma sehingga membentuk lubang kawah yang sangat besar.
Erupsi celah (Fissure Erupsions)
Aktivitas erupsi gunung api melalui celah yang memanjang disebut fissure. Erupsi yang demikian akan menyebabkan penyebaran material volkanik sangat luas. Apabila material yang dikeluarkan merupakan lava basalt yang encer, akan membentuk flood basalt, yang dapat mengalir sampai berkilometer jauhnya.
Apabila lava yang dikeluarkan banyak mengandung silika, maka akan menghasilkan aliran piroklastik (pyroclastic flows) yang terdiri dari debu volkanik dan pumis.

Aktivitas Magma Dalam Bumi
Seperti telah diketahui dan dipercaya oleh sebagian besar orang, bahwa sebagian besar magma berada pada tempat yang sangat dalam. Mempelajari aktivitas magma di dalam bumi merupakan hal yang penting bagi ahli geologi seperti mempelajari aktivitas gunung api. Ada beberapa tipe dari bentuk tubuh batuan beku instrusif yang terbentuk pada waktu magma mengkristal di dalam bumi. Bentuk-bentuk tubuh tersebut ada yang tabular, dan ada pula yang masif. Selain itu sebagian tubuh batuan beku tersebut ada yang memotong perlapisan batuan sedimen dan ada pula yang menerobos diantara perlapisan batuan sedimen. Mengacu pada perbedaan-perbedaan tersebut, maka tubuh batuan beku dalam dapat digolongkan berdasarkan bentuknya apakah tabular atau masif, dan orientasinya terhadap batuan disekitarnya. Batuan beku dalam yang memotong batuan sedimen disebut diskordan, sedang yang sejajar dengan perlapisan batuan sedimen disebut konkordan.
Batuan beku intrusif mempunyai variasi ukuran dan bentuk yang sangat besar. Dike adalah batuan beku diskordan yang dibentuk oleh magma yang menerobos melalui retakan yang memotong perlapisan batuan sedimen. Tubuh batuan yang berbentuk tabular ini mempunyai ketebalan dari beberapa sentimeter sampai lebih dari satu kilometer, dengan panjanh dapat sampai beberapa kilometer. Umumnya dike lebih resisten terhadap proses pelapukan daripada batuan disekitarnya.
Sill adalah batuan beku yang tabular yang berbentuk ketika magma menerobos melalui bidang perlapisan batuan sedimen. Pada umumnya batuan beku sill mendatar, tetapi sebenarnya kedudukan sill sangat tergantung pada kedudukan perlapisan batuan sedimen disekitarnya. Dari ketebalannya yang seragam dan penyebarannya yang luas, maka sill dipercaya bahwa terbentuk dari magma yang sangat encer. Jadi pada umumnya sill disusun oleh magma basaltik. Selain itu sill pada umumnya terbentuk pada tempat yang relatif dangkal dimana tekanan yang dibentuk oleh batuan sedimen yang diterobosnya relatif kecil.
Lakolit merupakan batuan beku konkordan seperti sill yang terbentuk pada lingkungan dekat permukaan. Tetapi magma yang membentuk lakolit lebih kental. Tubuh lakolit terbentuk seperti lensa cembung ke atas. Lakolit pada umunya merupakan inti dari struktur kubah yang akan tersingkap apabila batuan sedimen yang menutupi diatasnya tererosi.
Batolit merupakan tubuh batuan beku diskordan yang sangat besar, dengan diameter lebih dari 40.000 km2. Batuan yang menyusun batolit biasanya mempunyai komposisi mineral yang mendekati tipe granitik. Batolit yang besar merupakan hasil dari kejadian yang berlangsung sangat lama lebih dari jutaan tahun, tetapi tubuh batolit yang relatif kecil umumnya disusun oleh satu tipe batuan beku. Batolit biasanya merupakan inti dari suatu sistem pegunungan. Atap batolit bentuknya tidak teratur. Bagian atap batolit yang cekung dinamakan roofpendant.

Aktivitas Magma dan Plate Tectonic
Asal magma merupakan topik yang sangat kontroversial dalam geologi. Pertanyaan-pertanyaan yang selalu muncul adalah bagaimana magma yang mempunyai komposisi berbeda terbentuk ? Mengapa gunung api yang berada di dasar samudera mengeluarkan lava basaltik, sedang yang berhubungan dengan palung laut menghasilkan lava andesitik ? Masih banyak lagi pertanyaan yang berkaitan dengan aktivitas magma terutama yang muncul ke permukaan. Untuk menjawab semua pertanyaan tersebut akan dibahas pertama kali asal-usul dari magma.
Asal Usul Magma
Seperti yang telah diketahui bahwa magma terbentuk apabila batuan dipanaskan hingga mencapai titik leburnya. Pada kondisi permukaan, batuan dengan komposisi granitik mulai melebur pada temperatur sekitar 750oC, sedangkan batuan basaltik mencapai temperatur 1000oC. Karena batuan mempunyai komposisi mineral yang sangat bervariasi, maka batuan akan melembur dengan sempurna dengan perbedaan temperatur sampai beberapa ratus derajat dari pertama kali batuan mulai melebur. Cairan yang pertama terbentuk pada waktu batuan mengalami pemanasan yang tinggi adalah mineral yang mempunyai titik lebur terendah. Bila pemanasan berlangsung terus, maka proses peleburan akan berlangsung terus mengikuti masing-masing titik lebur mineral yang menyusun batuan tersebut, sampai komposisi cairan mendekati komposisi batuan asalnya. Tetapi kadang-kadang proses peleburan ini tidak berlangsung sempurna. Proses peleburan yang bertahap ini disebut partial melting. Hasil yang signifikan dari proses partial melting adalah dihasilkannya cairan magma dengan kandungan silika yang lebih tinggi daripada batuan asalnya.
Darimana sumber panas yang melebur batuan ? Salah satu sumber panas yang berasal dari peluruhan mineral radioaktif yang terkonsentrasi pada mantel bumi bagian atas dan kerak bumi. Pekerja-pekerja tambang bawah tanah juga sudah lama mengetahui bahwa temperatur meningkat dengan bertambahnya kedalaman.
Jika temperatur merupakan satu-satunya yang menentukan apakah batuan akan meleleh atau tidak, maka bumi merupakan suatu bola pijar yang dilapisi oleh lapisan padat yang tipis. Tetapi ternyata tekanan juga bertambah besar sesuai dengan kedalaman. Karena batuan mengembang pada waktu dipanaskan, maka diperlukan tambahan panad untuk melelehkan batuan yang ditutupinya untuk mengatasi efek dari tekanan disekitarnya. Titik lebur batuan akan meningkat dengan meningkatnya tekanan.
Di alam, batuan yang dalam akan melebur oleh salah satu sebab dari dua faktor, yaitu pertama, batuan akan melebur karena temperatur naik melebihi titik lebur batuan tersebut. Kedua tanpa kenaikan temperatur, pengurangan tekanan disekitar batuan akan menyebabkan titik lebur batuan turun. Kedua proses tersebut merupakan faktor yang memegang peranan penting dalam proses pembentukan magma.
Penyebaran Aktivitas Magma
Sebagian besar dari lebih 600 gunung api aktif yang telah diketahui terletak disepanjang busur pertemuan lempeng konvergen. Beberapa gunung api aktif terletak disepanjang pemekaran samudera. Ada tiga jalur gunung api aktif yang berhubungan dengan aktivitas tektonik global, yaitu disepanjang pematang oceanic, palung oceanic dan pada kerak oceanicnya sendiri.
Volkanisme pada sperading center. Batuan voklanik sebagian besar terbentuk disepanjang pematang benua dan pemekaran benua sangat aktif. Karena adanya pemisahan kerak samudera, maka tekanan pada mantel bagian atas berkurang. Berkurangnya tekanan ini menyebabkan turunnya titik lebur batuan. Partial melting batuan ini menghasilkan magma basaltik yang mengalir keluar melalui rekahan tadi.
Volkanisme pada zona subduksi. Aktivitas volkanisme pada daerah ini menghasilkan batuan yang berkomposisi andesitik sampai granitik, dan terbentuk disepanjang tepi kerak samudera. Sebagian besar volkanisme yang menghasilkan magma andesitik dijumpai di daratan atau pulau-pulau dekat dengan jalur palung laut. Jalur gunung api Meriterane dan Pasifik merupakan jalur gunung api yang dihasilkan pada zona subduksi.
Volkanisme pada kerak bumi. Proses aktivitas volkanik pada kerak yang tegar biasanya sangat sulit terjadi. Aktivitas volkanisme ini dapat menghasilkan lava basaltik, maupun lava granitik. Lava basaltik dapat terbentuk baik pada kerak benua maupun oseanik. Lava basaltik kemungkinan berasal dari partial melting batuan mantel bagian atas.
Lava granitik dan debu volkanik dengan komposisi granitik umumnya terbentuk pada daratan tepi benua. Lava jenis ini kemungkinan berasal dari pelelehan kerak benua