Download flood basalts - G

Adam C. Simon
Ph.D., University of Maryland, 2003
Research Associate
Department of Geology
University of Maryland
College Park, MD 20742
p: 301 405 0235
f: 301 314 9661
e-mail: [email protected]
Volcanism
Volcanoes affect our lives in many ways.
Today we talk about their construction.
Eruption of Mt. Etna in Sicily sends a plume of ash visible from space.
Planetary Volcanism
As we have seen, volcanic activity, or volcanism, is an
important factor in shaping other worlds in our solar system.
Olympus Mons, Mars
Io, moon of Jupiter
Igneous Rock Classification
PERIDOTITE
increasing
viscosity
increasing
melting
temperature
Volcanic Products
When magma reaches the Earth’s surface,
we call it lava.
Aside from lava, volcanoes may eject a variety of
common products:
-- volcanic gases
-- pyroclastic debris
Volcanic Gases
Much more gas can be
dissolved in a liquid at high
pressure than at low
pressure.
Liquids that are rich in
gaseous components will
tend to degas (lose their gas
component) as they
decompress.
Here on Earth,
decompression occurs as
magma travels from deep
sources to shallow regions
(like the surface).
Gas-Related Textures
Gas leaving decompressing
magma may not be
successfully escape.
Gas trapped in rapidlycooling lava forms vesicles,
or void spaces in the rock.
vesicular basalt
Extreme examples of
vesicular rocks are pumice
and scoria: these frothy
rocks are lightweight, since
most of their volume is air,
not rock.
pumice
Pyroclastics
All of the solids ejected from a volcano is collectively called
pyroclasts or pyroclastic debris.
The smallest
pyroclastic material
is ash. Realize that
volcanic ash is
silicate (glass),
unlike ash from
burning wood.
Pebble-size bits of
quenched lava and
ash are called
lapilli.
Pyroclastics
“Fire fountaining” (as shown
here from the 1969 eruption of
Mauna Ulu, Hawaii) produces
abundant pyroclastic material,
including tiny glass beads
(Pele’s tears).
Pyroclastics
Larger chunks of lava, from grape size up to practically car size,
are called bombs.
Note that on impact these do not explode in flaming carnage, as
seen in certain extremely-unrealistic Hollywood productions.
Volcanic/Sedimentary Rocks
Pyroclastic materials can form their own rocks, with hybrid
volcanic and sedimentary components.
A tuff is a rock formed from pyroclastic debris.
Welded tuffs are hard rocks, which form when pyroclastic
material is still very hot when it accumulates on the surface.
Molten components cause pyroclasts to ‘weld’ together.
Poorly-welded tuffs form from airfall material far from the
volcanic source. They carry little thermal energy, since they
cool during long travel in the atmosphere.
Lava Flows
Smoothly-oozing, low viscosity basalt flows commonly
form ropy structures, called pahoehoe (a Hawaiian word).
Lava Flows
Basalt lava that has lost much of its gas will be more viscous.
This lava will form rough, blocky flows called aa.
Volcano Types
The type of volcano that forms depends on:
viscosity of lava, proportion of lava/ash, lava flux.
increasing viscosity
increasing violence
-- flood basalt
-- shield volcano
-- cinder cone
-- composite volcano (stratovolcano)
-- lava dome
Large Igneous Provinces
At various points in Earth history massive basaltic eruptions
have taken place, producing what are sometimes called
flood basalts, or large igneous provinces.
These eruptions have left lasting marks of the Earth’s surface
and may have had significant effects on global climate.
Why these massive eruptions occur is
poorly understood, but they are
fundamentally like hot spot
activity on Hawaii, except
that much greater volumes
are involved and eruptions
last for much shorter time
periods.
Columbia River
Ontong-Java plateau
Deccan Traps
Columbia River
Flood Basalts
This is the largest
large igneous province in
North America.
Deccan Traps, India
Flood Basalts
512,000 cubic km of lava (Mt. St. Helens erupted ~1 km3).
The eruption about 65 Myr ago is strangely coincident with a
global extinction event (i.e., dinosaurs).
Shield Volcano
Mauna Kea on Hawaii is 10.2 km above the seafloor: the
highest mountain on Earth (Everest is only 9.2 km high!).
-- low silica, basaltic lava: low viscosity, flows readily
-- gentle slope
-- non-explosive (in general)
Cinder Cones
-- low to moderate silica lava
-- steep slopes, generally
symmetrical
-- dominantly pyroclastic material
Fernandina, Galapagos, 1991
Cerro Negro, Nicaragua, 1968
Composite
Volcanoes
-- moderate to high silica lava: high viscosity
-- steep slopes, generally symmetrical
-- layered lava flows and pyroclastic material
(stratovolcanoes)
-- explosive!
Fujiyama, Japan
Mayon, Philippines
Volcanic Landscapes
This satellite image shows how volcanoes can dominate the
landscape in some areas. The large ones are stratovolcanoes.
Kluchevskoi volcano, Kamchatka, Russia
Wilson Butte, CA
Domes
Panum Crater, CA
Lava domes are
usually small features,
constructed of lava
flows of high viscosity.
Many volcanoes return
to life after hiatuses
and form resurgent
domes.
Novarupta, Alaska
Phreatic Eruptions
A phreatic eruption is one that is
triggered or aided by groundwater or
surface water.
When this water infiltrates the hot
volcanic system, it converts to steam,
thus expanding, leading to explosive
circumstances.
How to make a volcano
more destructive than it is
on its own?
Just add water!
The 1883 eruption of Krakatoa in
Indonesia was a famous
phreatic eruption.
Plinian Eruptions
In 79 AD Pliny the Younger
carefully noted the destructive
eruption of Vesuvius in Italy
(which, among other things,
destroyed Pompeii and
Herculaneum,
and killed Pliny the Elder).
Violent, ash-rich eruptions have
since become known as plinian
eruptions.
Comparison of Scales
Diatremes
Features such as
Ship Rock (NM)
and
Devil’s Tower (WY)
are diatremes:
the eroded remains
of extinct volcanoes.
Extinct Volcanoes
In spite of their volcano-esque shapes, they actually are part
of the volcanic conduit that was underground when the
volcano was active.
Airfall
Distribution
6850 yr ago,
Mt. Mazama, OR,
erupted, sending
a recognizable
layer of ash
throughout the
northwest.
We now know
this volcano as
Crater Lake.
Big Pyroclastic
Eruptions of
North America
This illustration
shows the Bishop
Tuff, which was
deposited
760,000 yr ago,
the most
widespread ash
layer of the last
hundred million
years in North
America.
The Bishop Tuff
This eruption scatter ash hundreds of kilometers away,
and the area close to the eruption was treated to
a thick blanket of hot ash!
Ash Layers
Distant deposition
of volcanic material
is important to
dating sedimentary
rocks, as we will
see next class.
10-100,000 yr old lake
sediments at Mono Lake,
CA, showing several distinct,
regionally-recognized ash
layers.
ash layers
Waiting for the Big One
There are three particularly
good choices for which area
will have the next big
volcanic eruption in the
continental US.
Area one is the Pacific
Northwest, from northern
CA into BC.
Continued oblique
subduction sustains lava
supply at depth.
Let’s not forget what
happened in 1980.
The Next
‘Big One’?
The second choice is
Long Valley, CA.
This was the source of the Bishop Tuff
and has seen cinder cone construction
as recent as ~250 years ago.
caldera
north
Long Valley in Cross-Section
This interpretation was produced by examining surface geology,
deep drill holes, and seismic data.
The Next
‘Big One’?
Choice three is
Yellowstone, WY.
This caldera has blasted
out gigantic pyroclastic
eruptions in the last
couple million years.
(honorable mention:
Valles caldera, NM)
We will talk about the
implications of volcanic
eruptions for society later
in class.
Credits
Some images in this presentation come from:
Plummer, McGeary and Carlson, Physical Geology, 8th
ed.; NMNH, Global Volcanism Project; Geological
Society of America (Geology); USGS; Hamblin and
Christiansen, Earth’s Dynamic Systems, 8/e; D Swanson,
USGS; EOS; Univ. of North Dakota’s Volcano World