Tag Archives: North American Plate

Caribbean, Famous Eruptions, Plate Tectonics

Mt. Pelée, La Soufrière St. Vincent And A Quick Tour Of The Plates Surrounding The Caribbean Plate

Good Morning!

On 4 December 2020 the alert level for Mt. Pelée was raised to yellow due to increasing seismicity above background levels; and, on 29 December 2020 the alert level for La Soufrière St. Vincent was raised to orange following an increase in seismic activity, changes seen in the lake and fumaroles and a new growing lava dome emerging in the summit crater.  

This led us to look into what drives volcanism in the area, notably the interaction of the Caribbean Plate with its surrounding plates.

Fig 1: Mt. Pelée on the left with St. Pierre, photo by Lee Siebert, 2002 (Smithsonian Institution); Soufrière St. Vincent on the right, photo by William Melson, 1972 (Smithsonian Institution).

Mt. Pelée is famous for destroying the town of Saint- Pierre and its inhabitants plus visitors – a total of 29,000 people – in a matter of minutes on the morning of 8 May 1902 in a pyroclastic flow during a VEI 4 eruption.  La Soufrière St. Vincent also erupted around the same time.  Both volcanoes are located in the Lesser Antilles on the Caribbean Plate. 

Mt. Pelée

Mt. Pelée is located on Martinique.  She is a 1,400m high stratovolcano located in the caldera of an earlier volcano; edifice failures have breached the south west section of the caldera. Her lava types are andesite, basaltic andesite, dacite and basalt, picro basalt.

54 Holocene eruptions are recorded by GVP. Her historic eruptions include 1792 (VEI 1), 1851 (VEI 2), 1902 (VEI 4) and 1929 (VEI 3).  Two lava domes were emplaced in the summit crater, l’Etang Sec, during the 1902 and 1929 eruptions. 

1902 Eruption

Prior to the 1902 eruption, Mt Pelée’s known eruptions had been mild.  Activity at the volcano started to ramp up gradually with fumaroles in the summit crater in 1889 to March 1902.  From 23 April 1902 phreatic activity cleared out old rock, starting with minor explosive activity. By 4 May 1902 ash was raining down on Saint- Pierre. On 5 May, 23 people were killed when near boiling water from the crater heated by rising magma overran a distillery in a lahar in the Rivière Blanche valley.

On 6 May 1902, new lava emerged creating a lava dome.  During 7 May 1902, small parts of the dome collapsed.  At 07:50 on 8 May 1902, explosions were heard and a large black cloud seen to emerge and flow down the volcano, engulfing Saint-Pierre and some of the ships in the harbour.  Most of the casualties were killed by hot gases and dust from the blast.  Several pyroclastic flows followed: on 20 May 1902 a second pyroclastic current swept over Saint-Pierre destroying several of the remaining buildings; the Rivière Blanche valley saw several PDCs over the ensuing months; and, Morne Rouge was destroyed and 2,000 people killed by a pyroclastic current on 30 August 1902.  During this activity a 300m lava spine emerged. After this eruptive activity continued until 1903. The lava spine has since been eroded.

Saint-Pierre had not been evacuated prior to 8 May 1902 for a couple of reasons: it was not known at the time that the volcano produced pyroclastic flows so the danger was not understood; and, an election was due on 11 May 1902, which politicians were keen should go ahead.  No evacuation order was given. 

When activity ramped up again prior to the 1929 eruption, people were evacuated in time.

La Soufrière St. Vincent

La Soufière St. Vincent can be found on St Vincent Island.  She is a 1,234m high stratovolcano with crater lake and lava domes.  The 1.6km wide summit crater is located on the south west edge of a 2.2km wide Somma crater; slope failure caused a breach in the Somma crater.  Her lava types are andesite, basaltic andesite and basalt, picro basalt.

22 Holocene eruptions are recorded by GVP. Her historic eruptions include 1718 (VEI 3), 1812 (VEI 4), 1902 (VEI 4), 1971 (VEI 0) and 1979 (VEI 3).  The 1902 eruption occurred on 6 May 1902, killing 1,680 people. The 1812 eruption produced a new crater, cutting through the summit crater.  1971 eruption extruded a lava dome in the summit crater, which erupted explosively in 1979 to be replaced by another dome. 

Tectonic Setting

As noted above, both Mt. Pelée and La Soufrière St. Vincent are located on the Caribbean Plate in the Lesser Antilles. The Caribbean Plate is thick oceanic crust located between the North American and South American Plates.  The northern boundary of the Caribbean Plate is a transform boundary with the North American plate, running from Central America to the Virgin Islands. The Gonâve microplate and Puerto Rico Trench form part of the northern boundary. At the eastern boundary, the South American Plate subducts under the Caribbean Plate in the Lesser Antilles. At the western boundary, the Cocos Plate subducts under the Caribbean Plate, forming the Central American Volcanic Arc.  The southern boundary with the South American plate is a complex, comprising a convergent margin with the Panama Plate, a subduction zone with the North Andes Plate and a transform boundary with the South American Plate.  The main plates velocities relative to the African Plate are noted below.

PlateDirectionVelocity
North American PlateWest25 mm per year
Cocos PlateNorth east67 mm per year
Caribbean PlateNorth west10 mm per year
Panama PlateNorth west19 mm per year
Coiba and Malpelo PlatesEast 
North Andes PlateNorth west23 mm per year
Nazca PlateNorth east40 -53 mm per year
South American PlateWest27 – 34 mm per year

The origins of the Caribbean Plate are debated.  There are two main theories which attempt to explain why the less dense but thicker crust of the plate overrides the Cocos and South American Plates. It may have evolved millions of years ago from the Caribbean large igneous province, formed at the Galapagos hotspot, drifting to its current location as the plates moved to accommodate the widening of the Atlantic Ocean.  Alternatively, it may have formed from an old hotspot in the Atlantic.  These theories are based on the relative motions of the plates.  The first theory works on the basis that the Caribbean Plate is moving eastward compared to the North and South American Plates, whereas the latter uses the actual westward motion of the Caribbean Plate.

Recent Seismicity

Yes, we’ve downloaded earthquakes for the region from USGS’s earthquake search, taking a larger area than the Caribbean Plate in order to pick up the subduction zones.  In this case, it was not really necessary as most subduction is beneath the Caribbean Plate, but it was fun to find several microplates in the process: the Gonâve, Panama, Coiba and Malpelo Plates.  The Malpelo Plate was first identified as late as 2017 by Tuo Zhang, Richard G Gordon et al of Rice University.

The coordinates selected were: 3.760°S, 107.051°W to 26.838°N, 48.867°W for earthquakes with magnitudes of 2.5 or more between 1 January 1975 and 5 January 2021.  This picked up 80,751 earthquakes.

Fig 2: Earthquakes plotted by the author using data downloaded from USGS (see sources below).  Green dots denote earthquakes with magnitude between 2.5 M and 4.5 M, yellow dots earthquakes between 4.5 M and 6.0 M, orange stars, earthquakes between 6.0 M and 7.0 M and red stars, earthquakes over 7.0 M.  Some volcanoes are shown, these are denoted by blue triangles. Mt. Pelée and Soufrière St.Vincent are shown as yellow triangles. © Copyright remains with the author; all rights reserved, 2021

From Figs 2, 3 and 4 below, we can see the plate boundaries and the subduction zones on the western and eastern margins of the Caribbean plate are well marked by earthquakes and volcanoes; the subduction of the Caribbean Plate under the North Andes Plate is also visible (lower centre of the depth plot); and, the Puerto Rico Trench is also tectonically very active.  The Puerto Rico Trench has produced some large earthquakes and tsunamis.

Fig 3 Scatter plot of earthquakes round the Caribbean plate: latitude v longitude on the left and a depth plat on the right.  Earthquake with magnitude less than 4.5 are not shown in the depth plot to reduce noise.  Colour key as before. © Copyright remains with the author; all rights reserved, 2021
Fig 4: Scatter plot of the earthquakes at the Puerto Rico Trench: latitude v longitude on the left and a depth plat on the right. Colour key as before. © Copyright remains with the author; all rights reserved, 2021

Fig 4 shows subduction of the North American Plate on the right of the depth plot.  But there is also a line of earthquakes on the left of the plot which appears to indicate another subduction zone.  Neither subduction zone here, despite being seismically active, has active volcanoes associated with it.  It’s possible that there is another microplate here, but this is conjecture on our part until we can find an explanation or confirmation.

If you are interested or concerned by the alert statuses of Mt Pelée and La Soufrière St Vincent, you can find more information at L’Observatoire Volcanologique et Seismologique de Martinique, the National Emergency Management Organisation (NEMO) or GVP.

Hope you enjoyed our little tour.  We will be looking in more detail at points of interest in the future.

The Armchair Volcanologist

© Copyright remains with the author; all rights reserved, 2021.

Sources & Further Reading

Raw earthquake data from USGS Earthquake Catalogue Search: https://earthquake.usgs.gov/earthquakes/search/

L’Observatoire Volcanologique et Seismologique de Martinique: http://www.ipgp.fr/fr/ovsm/lobservatoire-volcanologique-sismologique-de-martinique-ovsm-ipgp

National Emergency Management Organisation (NEMO): http://nemo.gov.lc/

The Smithsonian Institution’s Global Volcanism Program (GVP): https://volcano.si.edu/

Caribbean Plate – Wikipedia: https://en.wikipedia.org/wiki/Caribbean_Plate

Zhang, Tuo; Richard G. Gordon; Jay K. Mishra, and Chengzu Wang. 2017. The Malpelo Plate Hypothesis and implications for nonclosure of the Cocos-Nazca-Pacific plate motion circuit, 1. AGU Fall Meeting, New Orleans. Accessed 2018-06-06.

“Volcanoes”, Peter Francis and Clive Oppenheimer, Oxford University Press, Second Edition, 2004.

La Soufrière (volcano) – Wikipedia: https://en.wikipedia.org/wiki/La_Soufrière_(volcano)

Plate Tectonics, The Aleutian Arc

An Introduction to the Aleutian Arc

Good Morning!

Today we are looking at plate tectonics, giving a brief introduction to the Aleutian Arc, which marks the boundary between the North American Plate and the Pacific Plate. The Arc is both seismically and volcanically active. Volcanic activity is of interest because the Arc lies under very busy North American and Asian aviation routes .

Fig 1: The Aleutian Arc.  Map base from Google Maps.  Names of some geological features added by the author.

The Pacific Plate is moving north westward at a rate of 59mm per year in the north east and 92mm per year in the north west. It subducts orthogonally under the North American Plate on its north-eastern margin, obliquely further westward, and parallel to the North American plate at the transform boundary on its north-western margin.  The subduction zone comprises the island arc, the Aleutian Trench, and distinct Wadati-Beniof zones, which extend down to around to between 100 km and 250 km. 

The Aleutian Arc is 3,000 km long from the junction with the Fairweather Fault (an extension of the Queen Charlotte Fault) in the east to the triple junction of the Ulakahan Fault, Aleutian Trench and Kuril-Kamchatka Trench in the west.  The arc includes a 2,500 km long chain of basaltic andesitic stratovolcanoes and calderas.  Behind the arc are basaltic lava fields in the Bering Sea. Most volcanoes are in uninhabited or sparsely populated areas so they pose more of a risk to aviation.

Origins of the Aleutian Arc

In the early Cenozoic, 60 million years ago, the Farallon Plate and Kula Plates covered what is now the northern Pacific Ocean; the Kula Plate moved northwards, while the Farallon Plate moved eastwards.  There were active continental margins: in the north west there was in island arc; and, in the north east there was subduction of oceanic crust beneath the Bering Shelf volcanic belt.  The Kula-Pacific Transform Fracture Zone may have separated the north west Pacific from the north east. 

In the middle of the Eocene (50 million years ago to 47 million years ago) the movement of the Pacific Ocean plates changed from the NNW subduction of the Kula Plate, followed by ridge subduction, to the NW subduction of the Pacific Plate. This change in motion resulted in the formation of the Aleutian Arc and its back-arc basin in the Bering Sea. The Shirshov Ridge and Bowers Ridge originated from the ongoing displacement of the North American Plate in relation to the Eurasian Plate in the mid Eocene.  Subduction beneath the Kamchatka margin is associated with the late Cenozoic. The Komandorsky Basin formed in the Miocene.

The oblique subduction of the Pacific Plate towards the centre of the Aleutian Arc has caused clock-wise rotation of island arc blocks and breaches in the island chain, notably at the Near, Buldir, Amchitka and Amukta Straits.  This motion moved the Komandorsky block from its location at the subduction zone near the junction with the Bowers Ridge to its current location at the transform boundary; Eocene volcanicsnin the block ceased after its movement away from the subduction zone.  Recent tomographic studies have shown a possible slab under the Bering Sea that may be a remnant of the Kula Plate.

Recent Seismicity

The current Aleutian Arc is seismically very active; it has produced several large earthquakes with magnitudes in excess of 7.0M.

We looked at the earthquakes between 1 January 1975 to 30 November 2020 with a magnitude greater than 4.5M from 47.04°N, 142.559°W to 66.548°N, 198.984°W downloaded from USGS Earthquake search (see Sources below).  This includes the triple junction at the west of the Arc and the junction with the Fairweather Fault in the east, and the Bering Sea.  It also picks up some of the subduction zone to the west at the northern end of the Kuril-Kamchatka Trench.  We also identified seismic swarms in the period; for this purpose, swarms are defined here as more than 30 earthquakes per day (normally there are less than 10 per day).

Fig 2a:  Plot by the author of earthquakes with magnitude greater than 4.5 from 47.04°N, 142.559°W to 66.548°N, 198.984°W downloaded from USGS Earthquake search.  Coloured dots indicate the seismic swarms identified; gold stars denote earthquakes greater than 6.0M and red stars denote earthquakes greater than 7.0M. Black triangles denote Holocene volcanoes.  © Copyright remains with the author; all rights reserved, 2020.
Fig 2b:  Geodensity plot by the author of earthquakes with magnitude greater than 4.5 from 47.04°N, 142.559°W to 66.548°N, 198.984°W downloaded from USGS Earthquake search.  Black triangles denote Holocene volcanoes.  © Copyright remains with the author; all rights reserved, 2020.

Figs 2a and 2b show that most of the action is at the southern section of the Arc where the subduction of the Pacific Plate changes from orthogonal to oblique.  The geoscatter plot shows that most of the swarms are occurring here, confirmed by the geodensity plot.  There is very little seismic activity behind the arc, except for a swarm in Kamchatka north of the arc. 

Earthquakes with magnitudes greater than 7.0M occur round the arc, with the exception of the region near the Fox Islands; the lack of earthquakes here greater than 7.0M may be due to the nature of the crust, or, the comparatively short time period selected (45 years is a short time in geological terms).

If we look closer at sections of the arc (see figs 3a to 3e below), we can see that that earthquakes tend to follow a block pattern with gaps in between; the gaps may be gaps between crustal blocks or they may be areas likely to have swarms in the future.  We can also see the well-defined Wadati-Benioff Zones which extend to a depth of 250 km in the Eastern and Central Blocks (blocks here are the sections of the arc that we have selected to plot and are not intended to represent crustal blocks).

Fig 3a: Plot by the author of the earthquakes over 4.5 M between 1 January 1975 and 30 November 2020 on the Eastern Aleutian Arc between 54.00°N, 203.75°E and 66.0°N, 214.45°E.  This shows the Wadati-Beniof Zone and also the complexity of the junction between the Arc and the fault systems of Alaska and northern Canada.  Green dots denote earthquakes ≤ 6.0. yellow dots, earthquakes between 6.0 and 7.0, red stars, earthquakes > 7.0M, and blue triangles, Holocene volcanoes. © Copyright remains with the author, all rights reserved, 2020.
Fig 3b: Plot by the author of the earthquakes over 4.5 M between 1 January 1975 and 30 November 2020 on the Central Aleutian Arc between 52.00°N, 193.75°E and 59.0°N, 203.75°E.  This shows the Wadati-Beniof Zone.  Green dots denote earthquakes ≤ 6.0. yellow dots, earthquakes between 6.0 and 7.0, red stars, earthquakes > 7.0M, and blue triangles, Holocene volcanoes. © Copyright remains with the author, all rights reserved, 2020.
Fig 3c: Plot by the author of the earthquakes over 4.5 M between 1 January 1975 and 30 November 2020 on the Central Aleutian Arc between 50.00°N, 187.55°E and 55.0°N, 193.75°E.  This shows the Wadati-Beniof Zone.  Green dots denote earthquakes ≤ 6.0. yellow dots, earthquakes between 6.0 and 7.0, red stars, earthquakes > 7.0M, and blue triangles, Holocene volcanoes. © Copyright remains with the author, all rights reserved, 2020.
Fig 3d: Plot by the author of the earthquakes over 4.5 M between 1 January 1975 and 30 November 2020 on the Central Aleutian Arc between 49.5°N, 174.0°E and 54.0°N, 188.0°E.  This shows the Wadati-Beniof Zone.  Green dots denote earthquakes ≤ 6.0. yellow dots, earthquakes between 6.0 and 7.0, red stars, earthquakes > 7.0M, and blue triangles, Holocene volcanoes. © Copyright remains with the author, all rights reserved, 2020.
Fig 3e: Plot by the author of the earthquakes over 4.5 M between 1 January 1975 and 30 November 2020 on the Western Aleutian Arc between 48.00°N, 161.02°E and 60.0°N, 174.0°E.  This shows from west to east: the Wadati-Beniof Zone in the west at the Kuril-Kamchatka Trench, the transform zones and the edge of the subduction zone at the Aleutian Trench. Green dots denote earthquakes ≤ 6.0. yellow dots, earthquakes between 6.0 and 7.0, red stars, earthquakes > 7.0M, and blue triangles, Holocene volcanoes. © Copyright remains with the author, all rights reserved, 2020.

The lack of an active Wadati-Beniof Zone under the western segment of the Aleutian Arc explains why there is little current volcanism there.  Active volcanism in Kamchatka is south of the Aleutian arc, driven by the subduction of the Pacific Plate under the Okhotsk Plate.  There is some seismicity in Kamchatka, north of the Aleutian Arc; this does not appear to be connected to an active subduction zone, although looking at Google Maps there may be an old trench in the area.

Clearly, the western and eastern segments of the Aleutian Arc are complex junctions and deserve a closer look. We will examine this in more detail when we discuss volcanic activity.

Season’s Greetings

The Armchair Volcanologist.

19 December 2020

© Copyright remains with the author, all rights reserved, 2020.

Sources & Further Reading

Map base for Fig 1: Google Maps

Raw earthquake data from USGS Earthquake Catalogue Search: https://earthquake.usgs.gov/earthquakes/search/

The Aleutian Arc, Wikipedia: https://en.wikipedia.org/wiki/Aleutian_Arc

“Volcanoes of the World”, Third Edition, Lee Siebert, Tom Simkin, and Paul Kimberly, Smithsonian Institution, 2010, University of California Press

“Cenozoic Geodynamics of the Bering Sea Region”, V. D. Chekhovich, A. N. Sukhov, O. G. Sheremet, and, M. V. Kononov, Geotectonics, 2012, Vol. 46, No. 3, pp 212-231.

“Bowers Ridge (Bering Sea): An Oligocene – Early Miocene Island Arc”, Maren Wanke, Maxim Portnyagin, Kaj Hoernle, Reinhard Werner, Folkmar Hanff, Paul van den Bogaard, and Dieter Garbe-Schönber, Geology (2012) 40 (8): 687–690.