Download LECTURE 15 - Volcanic Rocks 2

Volcanoes and Volcanic Deposits 2
IN THIS LECTURE
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Stratovolcanoes
Intermediate-silicic centres
Rhyolitic volcanoes
Submarine spreading ridges and seamounts
Intra- or subglacial volcanoes
Facies models for ancient volcanic successions
Classification of volcaniclastic deposits
Shield Volcanoes - Hawaiian
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Hawaiian Shield Volcanoes
– Summit calderas and major rift zones marked by spatter cones,
spatter ramparts, collapse craters (pit craters), scoria cones and
smaller superimposed monogenetic shields
– Shape usually controlled by eruptions from the rift zones
– Eruptions within the calderas occur slightly more frequently than on
the rifts but the eruptions from the lateral rifts that give the
shields their elongate form.
– Calderas range from 5 to 20kms in diameter
– Shields are built by lavas and minor pyroclastics as well as high level
intrusives which may be present in the summit caldera walls.
– Compositional differences occur as the shield volcano evolves
changing from tholeiitic to progressively more alkalic
– More explosive activity accompanies the eruptions of alkaline
magmas.
– Eruption frequency decreases with time
Hawaiian Volcanic Chain
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The two most active shields on Hawaii are
Kilauea and Mauna Loa.
Mauna Loa is the world’s largest active
volcano
– Rises nearly 9km from the pacific ocean
floor to its summit of 4169m above sea
level
– Total volume of 40,000km3
Combined growth rate of ~0.1 km3 per year
indicates both Kilauea and Mauna Loa could
have been built in less than 1 Ma
Large portion of the base of both volcanoes
made up of pillow lava formed by subaqueous
extrusions
Gravity sliding and slumping along normal
faults is common on the flanks and occurs in
response to oversteepening caused by
addition of lava flows and intrusion of magma
into the summit.
Mauna Loa
Snow-covered Moku`aweoweo Caldera atop Mauna Loa shield volcano
(Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep,
and is estimated to have collapsed between 600-750 years ago. Several
pit craters along the upper southwest rift zone of Mauna Loa (lower
right) also formed by collapse of the ground.
For more information on the
world’s largest volcano visit
http://hvo.wr.usgs.gov/maunaloa/
Shield Volcanoes - Icelandic
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Icelandic shield volcanoes
– Smaller – Ws < 15 km
– Symmetrical
– Almost entirely built up by effusive eruptions from a central summit
vent
– Summit crators usually < 1 km across and often have raised rims of
spatter
– Few radial fissures or lines of parasitic cones
– Generally composed of large numbers of thin pahoehoe flows
– Mostly monogenetic and usually constructed in less than 10 years.
Shield Volcanoes - Galapagos
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There is a third type of shield volcano known as the Galapagos type.
Very similar to Hawaiian shield volcanoes but the shape of the upper summit is
different
Gentle lower slopes that rise to steeper central slopes that flatten off around
spectacular summit calderas.
Usually more alkaline than Hawaiian volcanoes
Three-deminsional Space
Shuttle Image of the Alcedo
Shield Volcano, Galapagos -- The
near circular caldera of the
Alcedo shield volcano on the big
island of Isabela is a feature
common to many of the Galapagos
shield volcanoes. The image, taken
by the Space Shuttle Endeavor,
covers an area of about 75 km by
60 km. The oblique view was
constructed by overlaying a
Spaceborne Radar Image on a
digital elevation map. The vertical
scale is exaggerated by a factor
of 1.87.
Stratovolcanoes
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Stratovolcanoes or composite volcanoes are the characteristic volcanic
landform found at subducting plate margins
They represent the most abundant large volcano on the Earth’s surface
Stratovolcano morphology results from repeated eruptions of
pyroclastics and relatively short lava flows from a central vent.
Volcaniclastic deposits (pyroclastic and epiclastic) are usually very
important volumetrically and can make up more than 70% of the volcanic
succession the rest being lavas.
At destructive plate margins, stratovolcanoes are built by eruptions of
calc-alkaline magmas that are usually broadly andesitic or basalticandesite in composition.
Alkaline magmas generate stratovolcanoes which are on average larger
than their calc-alkaline counterparts.
Average slopes on stratovolcanoes range from 15° to 33°.
Most active stratovolcanoes are less than 100,000 years old and have
repose periods of up to 10,000 years
Stratovolcanoes
Mount Mageik volcano viewed from
the Valley of Ten Thousand Smokes,
Katmai National Park and Preserve,
Alaska. Mageik's broad summit
consists of at least four separate
structures built above different
vents.
Mount St. Helens is the youngest
stratovolcano in the Cascades and
the most active. Geologists have
identified at least 35 layers of
tephra erupted by the volcano in the
past 3,500 years. This picture is
prior to the 1980 eruption
Stratovolcanoes
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Stratovolcanoes are composed of a wide variety of primary volcanic
products
– Various lava types from basaltic through to rhyodacitic
– Pyroclastic flows
– Welded air-fall tuffs
– Ash deposits
– Ignimbrite deposits
– Pumice fall deposits
This variety of volcanic products arises because the generation,
evolution and type of magma erupted from these volcanoes is complex
and could represent magma chambers on different levels with complex
conduits between them and replenishment by different batches of
primary basaltic magma rising through the system.
The preservation of these primary volcanic products is complicated by
the mass wastage and epiclastic processes that are common on the
flanks of stratovolcanoes
Stratovolcanoes
Other Types of Volcanic Vents
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Lava and tephra can erupt from vents other than these three main
volcano types. A fissure eruption, for example, can generate huge
volumes of basalt lava that make up continental flood basalts
Other types of volcanic edifices include
– Flood basalts
– Maars and tuff rings and cones
– Rhyolitic volcanoes
– Interediate or silicic multi-vent centres
– Inter- or glacial volcanoes
Flood Basalts and their Source Vents
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The source vents to flood basalts are not central or point-source
volcanoes
They usually have high discharge rates up to 106 m3 per second
Flood basalts represent the largest single eruptive units known and
usually have flowed great distances from their source.
Flood Basalts built up by repeated eruptions forming a vast lava plateau
which may cover areas > 106 km with slopes generally less than 2-3°
Often closely associated with the initiation and early development of
rifted margins
Dominantly tholeiitic but alkali basalts are also common
Many of the larger flows must have formed vast lava lakes that took
many years to solidy as indicated by the well-developed massive
columnar jointing preserved in many flood basalt provinces
Columnar jointing is often two-tiered related to cooling fronts
propagating inwards from both the top and bottom of the lava flow.
Examples of Flood Basalts
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Mid-Miocene Columbia River Plateau
or Basalts
– Occur in Washington, Oregon
and Idaho
– Deposited within 2-3 Ma
– Cover 220,000 km2 and have an
estimated volume of 195,000
km3
•Mid-Tertiary Ethiopian-Yemen plateau
•Cretaceous Deccan Traps Northwestern India, 500,000 km2 and volume of
more than 1 million km3
•Cretaceous Parana-Etendeka province of southern Brazil-Uruguay-Namibia
•Jurassic Karoo in South Africa
•Jurassic Ferrar in South America
‘Local’ Continental Flood Basalts
Maars and Tuff Rings and Cones
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Volcanic craters that are usually monogenetic and produced by phreatomagmatic
and phreatic eruptions
Second only to scoria cones in abundance
Maar is a general term for broad, low-rimmed volcanic craters that form when
rising magma explosively interacts with ground water or surface-derived water
below the original topographic surface and contain little or no juvenile magma
Tuff rings have craters that lie on or above the pre-eruption surface and form
when rising magma interacts explosively with abundant water close to or at the
ground surface and contain a higher proportion of juvenile magma. Tuff rings are
usually basaltic but more acidic one are also common
Tuff cones differ from tuff rings by having smaller craters and larger height to
width ratios and form in areas where surface water is located above the vent.
Maars, tuff cones and tuff rings consist of pyroclastic deposits of stratified and
cross-stratified ash.
These types of volcanic centres often show a progression from phreatomagmatic
to strombolian or hawaiian activity reflecting a decrease in the degree of
magma-water interaction during eruption.
Duration of eruptions is thought to be fairly short from a few days to a few
weeks
Maars and Tuff Rings and Cones
Distinguishing characteristics of maar-type volcanoes
Urinrek Maars, Alaska
Aerial view toward N of Ukinrek Maars,
Alaska; Lake Becharof at top of photo.
Water partially fills the eastern maar and
completely covers a lava dome that was
erupted in the 100-m deep crater during a
10-day eruption in 1977. Maar is about 300
m in diameter.
Eruption column generated
by phreatic and magmatic
explosions rises from the
larger east maar.
Rhyolitic Volcanoes
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Rhyolitic volcanic centres are some of the largest volcanic landforms on the
Earth’s surface
– Usually polygenetic, multivent centres
– Usually consist of multiple eruption points or volcanoes
– Usually found in extensional tectonic regimes such as rifts, grabens and
marginal basins.
– Typically lack a topographically impressive cone cf stratovolcanoes
– Sometimes form large broad volcano-tectonic depressions called inverse
volcanoes of which Lake Taupo in NZ is the type example
– Typically consist of a collection of low rhyolitic hills composed of rhyolite
domes, coulees and pumice cones, rising from gently sloping ignimbrite
sheets which may contain more than one ignimbrite sheet
– Largest rhyolitic caldera known to exist is Lake Toba in Sumatra which has
rim dimensions of 100 x 35 km.
– Eruption rates are typically very low on the order of thousands of years and
the period of repose may be quite long as much as one million years
indicating that some rhyolitic volcanoes may have quite long lifespans.
– Lake Taupo has been active for 0.6 Ma, while Yellowstone has been active
for 2 Ma.
Rhyolitic Volcanoes
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Ignimbrite forming eruptions are generally associated with major
structural changes to the volcano
Caldera collapse occurs during or after the eruption, around a circular
ring fracture formed above the drained or draining magma chamber.
Later volcanic activity is concentrated in this ring fracture.
Explosive phases precede the eruption of rhyolite domes and flows but
rhyolite lavas do not travel far from the vent.
Rhyolitic volcanoes are thought to go through an evolutionary cycle with
the following seven stages
– Regional tumescence and generation of ring fractures
– Ignimbrite eruptions
– Caldera collapse
– Pre-resurgence volcanism and intra-caldera sedimentation
– Resurgent doming
– Major ring-fracture volcanism and
– Terminal fumarolic and hot spring activity.
Large Volume Rhyolite Lavas??
Felsic magmas will either (1) erupt explosively to produce extensive deposits of
tephra, or (2) nonexplosively to produce degassed, viscous lava (domes, coulees,
or obsidian flows) which advance only short distances from their vents. There
has been a significant amount of controversy, therefore, over rare rhyolite lavas
that appear to occur as large-volume flows (10-100 cubic kilometers).
Most such flows occur near continental hotspots. The best known examples are
those associated with (1) the Yellowstone hotspot track near the Idaho-Oregon
border, and (2) the Ethiopian hotspot in northeastern Africa. These large-volume
felsic volcanic rocks have outcrop, hand specimen, and thin section
characteristics typical of lava flows. However, many volcanologists suspect that
they are not lava flows at all, but rather rheomorphic ignimbrites. These are
densely welded pyroclastic flows of pumice and ash, which were thick and hot
enough to flow downslope and obliterate primary pyroclastic structures. They
suggest that the original pumice and ash fragments have been streaked out like
toffee strands so that the pyroclastic nature of the flow becomes
unrecognizable.
Is this the case with the large rhyolite lavas of the Lebombo Monocline?
Intermediate-silicic multi-vent centres
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These types of volcanic centres are similar to Rhyolitic volcanoes but
have lavas that are andesitic to dacitic in composition and often
alkaline.
Normally involve a caldera and caldera collapse processes after
explosive eruption activity
Often surrounded by large ignimbrite sheets similar to rhyolitic
volcanoes
Intra- or subglacial volcanoes
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The type locality for these eruptions is Iceland.
Compositionally all lavas types may occur including basaltic, andesitic,
dacitic and rhyolitic.
Typically form steep sided ridges called Tindas or steep circular table
mountains called tuyas
Basaltic subglacial volcanoes consist principally of masses of pillow
lavas, palagonitised hyaloclastite breccias and sideromelane fragments.
Silicic eruptions beneath ice are likely to initially be explosive similar to
subaerial silicic eruptions.