G148 Volcanic monitoring Techniques:
Seismic Monitoring
· Almost all volcanic activity is preceded by some seismic activity
· ~200 of the worlds volcanoes are actively monitored with seismographs
· Very important technique for forecasting eruptions
· Earthquake activity on volcanoes often follows a sequence of events
1) Background noise: small vibrations on seismograph but no pattern
2) High frequency swarms: caused by shearing of rock and implies deep magma movement
3) Quiet period: this can last weeks
4) Low frequency events: caused by fluids and vapors in lavas forming gas bubbles
5) Volcanic tremors: can last for minutes or days and are often called harmonic tremors.
Represents movement of magma
6) ERUPTION
7) Deep quakes after eruption as settling occurs
Ground Deformation
The onset of volcanism can cause rocks and surrounding area to uplift as magma and gasses push
up creating swelling and tilting. Deformation is monitored by:
-Leveling: using survey equipment to check for changes in elevation
-Global Positioning System (GPS): Uses satellites to locate a position and to measure
horizontal changes in features
-Synthetic Aperture Radar (SAR): Radar images collected from satellites over time look at the
same point and compare any changes in topography over time. Very good for showing
changes in elevation. This is the technology that was/is used to discover the bulge on South
Sister (more later)
Gravity Changes
Normal/average gravitational acceleration on earth is 9.8m/s2. This value can vary depending on
mass. The greater the mass or the more dense it is, the greater the value. As magma moves
into a volcanic region, mass in that region is increased and so is the gravitational attraction.
After the eruption the gravitational acceleration for the region decreases.
Magnetic Properties of Rocks
This technique uses a device known as a magnetometer. The idea is that as magma enters a
system, the magnetic properties of the existing rocks will change due to the temperature
increase. These changes can be recorded.
Gasses/Volatiles
Volcanic gasses include carbon dioxide, water, hydrochloric acid, sulfates, hydrogen sulfide etc…These gasses escape before, during and after volcanoes erupt and are sampled to give information about the magma source, how active the system is, and if the system is getting ready to erupt. Often times the composition of the gasses will change before eruption as well as the quantity of the gas. Gas can be gathered through direct measurement or using remote sensing techniques
Thermal Monitoring
Gathered by either direct measurements or through remote sensing. Increasing temperature
implies magma is moving closer to the surface.
Volcanic Activity in Oregon (past and present)
Oregon and the Pacific NW has experience all types of volcanic activity from very explosive to very effusive. This activity extends from approximately 50 million years before present (mybp) to present. Some of the volcanic areas in the state of Oregon are
Area Silcic Basaltic
John Day x
Steens Mountain and Harney Basin x x
Newberry Volcano x x
Crater lake (Mount Mazama) x
Cascade Mountains x x
Columbia River Basalts x
Boring Lavas x
Juan De Fuca ridge x
John Day Area
Two main formations or groups of rocks make up this area. John Day formation (~30 mybp) and
the Clarno formation (~50mybp)
Clarno Formation: Related to subduction zone activity and is comprised of ash deposits,
rhyolite and andesite. Many of these deposits have been reworked by water and there has been
many fossils found in this formation
John Day Formation: Represents a change in the subduction zone and the activity associated
with that change. Volcanic activity extends over ~18 million years and during this time a
change in climate occurs that helps created different sediments and deposits
-Red Ash (lower deposit): oxidized layer of ash with few fossils
-Green layer: Clay rich deposit with lots of fossils
-Buff color (upper layer): Ash layer that is rich is fossils
Often in the John day formation, the ash was deposited by pyroclastic flows and this activity is
associated the development of the Strawberry Mountains as well as Smith Rock
Columbia River Basalts (CRB's)
These deposits represent flood basalts that erupted between 17-11 mybp. It's believed the basalt
was erupted from fissure vents in E. OR and WA. Finding the actual vents has been difficult due
to later volcanic deposits covering them over. There are many individual flows that make up the
Columbia river basalts and they have been broken into four major groups. On average a flow
occurred every 35,000 years and each flow consisted of approximately 100 cubic miles of basalt.
Flow thickness is variable but several are ~200' thick. These deposits create many intercanyon
flows and lots of GREAT jointing!
Steens Mountain:
Part of the Basin and Range and dates to ~16 mybp
Bi-modal volcanismà both rhyolite and basalt but NEVER AT THE SAME TIME
-Basin and Range faulting creates extension and normal faults
-Thinned crust lowers pressure on the mantle and creates magma
-Magma in chamber evolves through assimilation and crystallization and evolved magma
(rhyolite and andesite) occupies the top of the chamber and the lower chamber is occupied by
new basalt that is intruding. They stay separate from each other due to density differences.
-As eruption begins, the rhyolite erupts first as the original eruption then subsequent eruption
are less evolvedà andesite, basaltic andesite and finally basalt
REMEMBER: the different types of magma are not erupted during the same event. Different
eruptions and different products
Harney Basin
Collapsed caldera feature that occurred approximately 5mybp (very young feature). Eruption
created a very large ash deposit covering ~4300miles2
Diamond Craters:
Created by extension that formed the Basin and Range. Extension of the crust caused it to thin
which allowed magma to form. This region began developing ~16mybp and the Basin and Range
is still extending today.
Cascade Mountains
The Cascade Mountains have been built through three distinct eruptive episodes over the past 20 million years. The current mountains have only really existed for ~3 million years.
The most recent Cascade event begun as basaltic volcanism and then evolved over time to erupting andesite. The early eruption of basalt for the current Cascades was created by a rift that developed approximately 5 million years ago. The current Cascades are sitting in a grabben (think back to the discussion on the basin and range for the definition) which was created by this extension. The shift to andesite began as magma was generated by the subduction zone and found its way to the surface.
The different eruptive episodes listed below for the Cascades represent times when the
subduction zone was shut down periodically or underwent a change in the angle of seduction.
Cascade eruptive episodes:
-Western Cascades (20-15mybp): Erupted west of the current chain. Andesites and dacites
primarily with many of the deposits no longer visible due to later volcanism. Many ash layers have converted to clay creating a "slip" surface for landslides
-Plio-cascades or Ancestral Cascades: Developed ~7mybp and are buried under current
Cascade mountains. Remnants are found in the Gorge such as beacon rock and wind
mountain
-High Cascades: current Cascade chain. Started developing ~3 million years ago and began
eruption andesite ~ 1 million years ago.
Currently: South sister has a bulge on the East Flank that continues to grow. Not noticeable to the unaided eye but can be seen using remote sensing (SAR) technology. Rate of inflation ~4 cm/year
Newberry Volcano
A very young shield volcano with the most recent eruption occurring ~1300 years ago.
Bimodal volcanism with the majority of the basaltic volcanism now concentrated on the flanks of
the shield and the summit eruption rhyolite and obsidian (this is the most recent eruption).
Crater Lake (Mt. Mazama)
Most recent eruption (extremely large) occurred ~7000 years ago. Created a collapse of the
central vent creating a caldera on the summit. Primarily rhyolite composition though a small
cinder cone forms wizard island. This was believed to be created by the same type of magma
chamber layering that occurred in the Steens mountain region
Fort Rock, Big Hole, Hole in the Ground
These three features are all phreatic eruption features created when a lake or ground water came
in contact with magma. These features date to approximately 15,000 years ago. At this time a
small ice age was on going and this area was very wet with many lakes. Each of these locations
have tuff rings created by the eruption.
Boring Lavas
This episode of volcanism got its name from Boring OR which is the Eastern most edge of this
series of volcanoes. The Boring lavas consist of several cinder cones and shield volcanoes
(Larch Mountain) which get younger with age as you move to the southwest. The furthest south
they extend is between Woodburn and Salem. The time period for these eruptions was between
1-4 mybp.