Mantle Plumes – Alan Grey
hawaiin islands

Monmouth U3A Geology Group meeting 12th April 2023 at Bridges Centre
“Intraplate volcanism and its geological implications”
Our convenor Jim Handley introduced Professor Alan Gray to 16 members who had come out on a wet and cold April day.
The theory of plate tectonics has revolutionised geological thought in recent decades although the first publication suggesting the idea of continental drift was by Alfred Wegener in 1915 based on common glacial, rock and fossil evidence suggesting existing continents were once part of a common landmass.
Alan talked about the Kurile and Aleutian trenches in the Northern Pacific. Tectonic plates move at between 1 and 10 cms each year. Subduction occurs when the edge of one plate is forced under another and this is related to changes in the asthenosphere and to volcanic activity.
He demonstrated several rock formations which he had seen on Big Island, Hawaii. These included lava “tumulus”, lava tubes and landslip scars which are all seen when the lava rock flows freely. The summit of the peaks may be more than 4000 metres from their base under the sea.
This led on to an explanation of the Plume hypothesis first put forward by John T Wilson and W. Jason Morgan in 1971.
Plumes of hot (but solid) material may push up from the mantle through the zone of convection and acquire the shape of a mushroom head. The plumes are static but the tectonic plates continue to move over these “hot-spots”. As the plates continue their slow but inexorable movement volcanic activity may occur over the hot-spot plume giving rise to linear ranges of mountains associated with a single static hot spot.
This idea would explain the Hawaiian chain: as the tectonic plate has moved West and North a line of volcanoes has been formed between Midway and Hawaii. Midway which would have been formed first is dormant but Hawaii remains active
The hypothesis may also explain other volcanic chains such as seen at Samoa, Louisville, Yellowstone and the Canary Islands.
Alan went on to show how there may be a link between plumes and features like the breakup of continental masses, eg in the development of the current rift valley in eastern Africa, and the occurrence of flood basalt lava eruptions, eg in Siberia and India.
The fascinating thing is that there still a dispute about the very existence of plumes that originate from the core mantle boundary. Our speaker illustrated this by introducing to two leading geologists on opposite sides of the argument. Barbara Romanowicz suggests there is evidence that such plumes do arise from that depth whilst the Durham Professor Gillian Foulger argues firmly against this suggesting that any features are the result of magma movements only with the top 1000k of the mantle as part of the normal plate tectonic convection cells.
The main problem in weighing this all up is the absence of sufficiently clear seismic evidence from the mantle region. The geological jury is still out!
We enjoyed a full and fascinating afternoon buoyed up by the enthusiasm and knowledge of Alan Gray



Wed 12 Apr 2023


2:00pm - 3:30pm
Bridges Centre, Drybridge Park, Monmouth NP25 5AS
Jim Handley

Accessibility at Bridges Centre

Members’ monthly meetings are held at Bridges Community Centre, Drybridge Park. Some group meetings and activities also take place at Bridges. Off street parking is available here outside the building, and disabled parking is adjacent to the building entrance. There are no external steps or slopes, and the entrance doors are automatic. The ground floor is fully accessible and level throughout, and there is space for wheelchairs. There is a lift to the first floor, and accessible toilets on both floors. There is a hearing induction system in the Agincourt room where the monthly meetings are held.

Accessibility at Ty Price

Some group activities and meetings are held at Ty Price, St Thomas Community Hall, St Thomas’s Square. There is no off street parking here. The approach on foot is a gentle slope to double entrance doors. The ground floor of the building is fully accessible and there is a disabled toilet. The stairs to the first floor are wide and well-lit with a handrail on both sides, but there is no lift. There is a hearing induction system on the ground floor.