1. 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

2. 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.

3. Points covered Geological setting – where is Montserrat?
Morphology and important observation
Island.
Depositional environment
Discussion and conclusion of the paper

4. 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

5. Tectonic setting of Montserrat

6. Montserrat Position – location of volcanoes in the Lesser Antilles arc

7. Montserrat. http://earthobservatory. nasa
Montserrat. Taken from google earth
The summit of Soufrière Hills volcano towers above the streets of Plymouth,
Photo by Cynthia Gardner, 1995 (U.S. Geological Survey).

8. 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

10. 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.

11. 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

12. Island Morphology Sliver Hills (2600-1200ka) Centre Hills (950-550ka)
Characterized by high amounts of erosion (deeply eroded)
Highest peak of 403m
Centre Hills ( ka)
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

14. 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)

15. 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).
Castle Peak dome and English's crater 1995.

16. A. Le Friant (Paper Research pg 150)
Scar Features

17. Recent Eruption of the active volcano
Crater in top of growing dome Oct 1997. 
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.
Delta formed by pyrolaccstic flow.
Google Earth

18. Ocean Floor Morphology
Shallow Shelf of m 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)

19. A. Le Friant (Paper Research pg 152)

20. A. Le Friant (Paper Research pg 153)

21. Main Instabilities Debris Avalanche Deposits
Characterized by rough hummocky topography
The avalanche deposits are triggered by flank failure and dome collapse events

22. 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

23. Zoom in of fig 2. Showing the South Soufriere Hills –Soufriere Hills Scar features.

24. 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

25. Reference : Pyroclastic Flow: http://earthobservatory. nasa

26. Pyroclastic Flow
Video Clip of pyroclastic flow into the ocean:

27. 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.

28. Discussion Long Term Surface Erosion Rate (ER)
Using information form Silver Hills
The volume loss of 15km³ over 1200ka
= km³/ka
East coast sea cliffs, showing a number of sequences of pyroclastic flows.

29. 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

30. 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)

31. 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)

32. Kick andJenny
Image by Doug Martin, 1996 (NOAA, courtesy of Seismic Research Unit, University of West Indies).

33. 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

34. 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)

35. Fig 10a. Evolution of Montserrat
A. Le Friant (Paper Research pg 159)

36. Fig 10b. Evolution of Montserrat
A. Le Friant (Paper Research pg 159)

37. Video Clip of pyroclastic flows into the ocean of Montserrat – Tar River

38. South Soufriere Hills - Soufriere Hills volcanic eruption at night:

39. Bibliography Wikipedia – free online encyclopedia GoogleEarth