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. 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).

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) Centre Hills (950-550ka) 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)

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 Castle Peak dome and English's crater 1995. http://www.caribbeanvolcanoes.com/montserrat/geology.htm

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

Recent Eruption of the active volcano Crater in top of growing dome Oct 1997.  http://www.caribbeanvolcanoes.com/montserrat/geology.htm 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

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

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.indies/soufriere/govt/miscdocs/ofr_index.html