Transcript Geomorphological evolution of Montserrat (West Indies): importance

Geomorphological evolution of
Montserrat (West Indies): importance
of flank collapse and erosional processes
Authors: A. Le Friant, C.L. Harford. C. Deplus, G. Boudon,
R.S.J. Sparks, R.A. Herd & J.C. Komorowski
What is it essentially?
• The paper looks at the Montserrat island in the
West Indies and then through certain studies of the
morphology of the island and surrounding sea
floor the authors proposed three stages of
evolution for andesitic volcanoes in a marine
setting.
Points covered
Geological setting – where is Montserrat?
Morphology and important observation
Island.
Depositional environment
Discussion and conclusion of the paper
Geological Setting
• Located 16º 45’N and 62º 10’ W
• Forms part of the Lesser Antilles fore arc.
• The N.American Plate subducted under the
Caribbean Plate
Tectonic setting of Montserrat
www.appstate.edu/~abbottrn/e-quake/mp-epcntr0.jpg
Montserrat Position – location of volcanoes in the Lesser Antilles arc
http://www.volcano.si.edu/world/find_regions.cfm
http://www.volcano.si.edu/world/region.cfm?rnum=16
Montserrat. Taken from google earth
Montserrat.
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=6759
The summit of Soufrière Hills volcano towers above the streets of Plymouth,
Photo by Cynthia Gardner, 1995 (U.S. Geological Survey).
• The island is really divided into 3 main
massifs:
– Silver Hills (North)
– Centre Hills (Centre)
– Soufrière Hills
Topographic model of Montserrat: Courtesy of Dr. Ian C.F.Stewart
http://www.caribbeanvolcanoes.com/montserrat/geology.htm
Events occurring:
• Active Volcano – South Soufrière Hills. Andesitic
stratovolvano that is dome forming.
• Silver Hills is an extinct Basalt-Andesitic volcano
(Harford,2002)
• Deposits are characterized by lava domes, lava
talus breccias and tephra falls. Flank and dome
collapse events form pyroclastic flows.
Morphology
• Look at morphology
of each main massif
separately
– Silver Hills
– Centre Hills
– Soufière Hills and
South Soufière Hills
Topographic model of Montserrat: Courtesy of Dr. Ian C.F.Stewart
http://www.caribbeanvolcanoes.com/montserrat/geology.htm
Island Morphology
• Sliver Hills (2600-1200ka)
– Characterized by high amounts of erosion (deeply
eroded)
– Highest peak of 403m
• Centre Hills (950-550ka)
– Characterized by high cliffs – mainly coastal cliffs
– Highest peak of 741m (Kathy Hill)
– 2 Discontinuities that are caused by flank collapse (Sa
& Sb on fig 2b)
– Discontinuities defined by the hydrographic system
Island Morphology
• Soufrière Hills and South Soufrière Hills
– Active volcano
– Less erosion but more faults - unstable
– Main features on the south side of island:
• English’s Crater (1km x 1.6km) due to flank failure
• The Scar Features (Sb. fig2b)
– Forms discontinuities that divides Soufrière Hills and South
Soufrière Hills. All have steep scarps.
• 3 large imbricated horse shoe depression structures all caused
by flank failure (Sc on fig 2 and fig 3)
• Large Debris avalanches (Boudon 2002)
Castle Peak dome and English's crater 1995.
http://www.caribbeanvolcanoes.com/montserrat/geology.htm
Simplified map showing the northern extent of the pyroclastic flow and surge deposits from the June 25,
1997, dome-collapse and explosion event at Soufriere Hills volcano, Montserrat (modified from from MVO
Special Report 03 on June 29, 1997; see anchor below).
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=6759
A. Le Friant (Paper Research pg 150)
Scar Features
• Recent Eruption of the active volcano
– Formed a lava dome at height of 1100m
– Valleys are filled by pyroclastic flows as well as
block and ash flows. These flows are also
responsible for the deltas and fans formed.
Crater in top of growing dome Oct 1997.
http://www.caribbeanvolcanoes.com/montserrat/geology.htm
Delta formed by pyrolaccstic flow.
Google Earth
Ocean Floor Morphology
• Shallow Shelf of 20- 60m depth (Fig 4a,b)
• Canyons and gullies 100m deep (West)
• Gentle slope <4º (East of the island)
– Hummocky structures stretch S-N (750m – 1000m
deep)
• Slope of seafloor along island
– Concave up slope
• More liner towards the active sites of the island
• More concave towards extinct volcanoes (Northwards)
A. Le Friant (Paper Research pg 152)
A. Le Friant (Paper Research pg 153)
Main Instabilities
• Debris Avalanche Deposits
– Characterized by rough hummocky topography
– The avalanche deposits are triggered by flank
failure and dome collapse events
• Flank Collapse Events
– 4 types occur on Montserrat
• English Crater Event
– Due to flow erosion of sediments and forms the Tar River Fan
• South Soufrière Hills - Soufrière Hills events
– Due to submarine and subaerial flank failures
– The events caused the flank scar see as Sc on fig2
• Other Events
– submarine failure
– the collapse of the infill of the horseshoe features
• Submarine embayment
– Submarine failure of accumulated sediments on the shelf
Zoom in of fig 2. Showing the South Soufriere Hills –Soufriere Hills Scar features.
• Pyroclastic flows classified as 2 types
– Pyroclastic flow deposited on land
• Fills Valleys
– Pyroclastic flow deposited in sea
– 80% product eventually ends up in the sea.
• Forms the deltas that can be as big as 1km² in area
• Tar River and White River
– Products of volcanic activity, dome collapse
(e.g 12 July 2003) and also flank failure
Reference : Pyroclastic Flow: http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=6759
Pyroclastic Flow
Video Clip of pyroclastic flow into the ocean: http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/images/051296/
Pyroclastic flows from Montserrat dome collapses have flowed down the White River creating a new delta where they entered the sea. It is
uncertain if this delta will survive or be eroded by seawaves. Photograph copyrighted and provided by Steve O'Meara of Volcano Watch
International.
Discussion
• Long Term Surface
Erosion Rate (ER)
– Using information
form Silver Hills
– The volume loss of
15km³ over 1200ka
– = 0.0125km³/ka
East coast sea cliffs, showing a number of sequences of
pyroclastic flows.
http://www.caribbeanvolcanoes.com/montserrat/geology.htm
Discussion
• Magma Production
Rate
– South Soufrière Hills Soufrière Hills
– Oldest preserved base of
volcano 174ka
– Thought that 50% eruption
products end up in the ocean
(before 12 July 03)
– Vt = Vp + Vm+Ve + Vc
– Minimum time averaged
production rate = 0.17km³/ka
– (400times lower then the
current estimated rate at
70km³/ka)
• Means that production was a
small part in history (Wage &
Isaacs 1988)
• Vt = total produced magma
• Vp = actual preserved subearial
volume of the volcano = 12km³
• Vm = erupted volume into sea
(50% of Vt) = 15km³
• Ve = material that has been
eroded at ER/174ka = 2.2km³
• Vc = collapsed volume = 1km³
• Therefore Vt ~ 30km³ over
174ka
• ER vs. Production rate (PR)
– ER (0.0125km³/ka) = 7% PR (0.17km³/ka)
• Most obviously the PR>ER
• Shows that most erupted material is deposited straight into the
ocean
• South Soufrière Hills - Soufrière Hills attributes 6% of the
Lesser Antilles arc magma production
• Sea Deposits Rate
– The calculated rate of 0.085km³/ka can be modified to
0.11km³/ka due to episodic events such as
• Debris avalanches, flank failure and pyroclastic flows also
affecting land drainage
• This rate shows that over the last 174ka, 19km³ (65%Vt) of
volcanic products are deposited on the sea floor
• Volcanic sediments contributed 80% of volcanic products of
the island arc (Sigurdsson 1980)
Morphological evolution of the island
• Interpreted in 3 stages
– Stage 1
•
•
•
•
Submarine growth
ER has little effect
The PR >> ER
Kick ‘em Jenny (southern part of the arc) at this
stage (Sigurdsson & Sparks 1979)
Kick andJenny
Image by Doug Martin, 1996 (NOAA, courtesy of Seismic Research Unit, University of West Indies).
– Stage 2
•
•
•
•
•
Active subaerial growth
PR > ER
Magma extrusion takes place – subaerial edifice built
South Soufrière Hills - Soufrière Hills is an example
High sediment supply to oceans
– Flank collapse and pyroclastic flows
• The large flank collapse events shape the island
– English’s crater event (4ka)
• The hydrological alteration is the main cause of flank collapse
• Only stage that pyroclastic flows occur
– Stage 3
•
•
•
•
Extinction and erosion
When active volcano becomes extinct
Here PR < ER
Land material is weathered into the ocean which forms a
shallow shelf environment
• Submarine failure and collapse events must be triggered by
tectonic events as there is no volcanic activity for sedimentary
loading
• Silver and Centre Hills of Montserrat are good examples.
(Noting the shelf north of the island)
Fig 10a. Evolution of Montserrat
A. Le Friant (Paper Research pg 159)
Fig 10b. Evolution of Montserrat
A. Le Friant (Paper Research pg 159)
Video Clip of pyroclastic flows into the ocean of Montserrat – Tar River
http://www.geo.mtu.edu/volcanoes/west.indies/soufriere/govt/images/051296/
South Soufriere Hills - Soufriere Hills volcanic eruption at night
: www.islandvulnerability.org/epic.html
Bibliography
• Wikipedia – free online encyclopedia
• GoogleEarth
• http://www.geo.mtu.edu/volcanoes/west.ind
ies/soufriere/govt/miscdocs/ofr_index.html