It may not be very apparent, but Geology can be a very contentious science. Geology is a subjective science in that two scientists can find the same evidence but produce two different and viable interpretations of that evidence. One such controversy revolves around a fundamental issue that defines how we view the entire Earth system: do mantle plumes exist?
As most of you will remember from GCSE Geography, there is a wonderful theory called Plate Tectonics in which the planet’s surface is formed of gigantic, rigid plates of rock that only deform (break or bend) at the edges. The movements of the tectonic plates is now known to be driven by the pull exerted from subduction of the boundary of a plate where it is pulled down into the Earth’s mantle by differences in density. Plate Tectonics explains many of the features of the Earth’s surface that would otherwise not exist. Mountain chains form where plates collide and oceans form where plates move apart. According to the theory, volcanoes and earthquakes would only occur on the edges of the tectonics plates, such as in the Pacific Ring of Fire.
There are problems with the Plate Tectonics Model however. Rocks can deform within the tectonic plates, showing that they are not truly rigid. Volcanoes that occur independently of plate tectonic boundaries are the source of the contentious issue. Without a plate boundary to cause melting of either crust or mantle rocks, the source of the magma that is then erupted at these volcanoes is a mystery. Examples of this anomalous magmatism are Hawaii and the Canary Islands which both occur within tectonic plates, whilst Iceland shows anomalously high volcanic eruption rates along only a small section of the Mid-Atlantic-Ridge plate boundary.
In order to account for this anomalous magmatism on the Earth’s surface, a model was proposed that used ‘bottom-up’ principles, in that the deep mantle is seen to affect the surface of the Earth. It was theorised that convection in the mantle included formation of plumes of hot, relatively low density mantle material that would rise to the surface and break through the crust to form volcanoes. This is the Mantle Plume theory. An alternative theory was proposed later, by geoscientists who believed in a ‘top-down’ approach to plate tectonics. In the Plate Theory, the anomalous volcanism seen at volcanoes such as Hawaii is caused by extension in the upper crust and associated melting in sections of the mantle which are more susceptible to changes in pressure and temperature. This would mean that any mantle convection that may occur would not affect the Earth’s surface.
Currently, more geoscientists follow the Mantle Plume theory but there is the unavoidable problem that not a single site of anomalous volcanism perfectly fits the Mantle Plume model. There is always a problem or unexplained feature. There are several criteria that a geoscientist would look for to provide evidence of a mantle plume impacting the Earth’s crust. They include:
- Doming of the crust before the start of volcanic activity
- Geochemical markers in the volcanic rocks that show that they are sourced from the deep mantle
- Anomalously high temperatures
- A time progressive chain of volcanoes that form over the mantle plume and then become extinct as the plate moves away from the mantle plume and the source of the magma is shut off
The example of Hawaii best fits the Mantle Plume Theory because the theory was made based on the features seen at Hawaii. The Emperor-Hawaii Volcanic Chain forms a line of extinct submarine volcanoes that age progressively the further away from the Big Island of Hawaii it is. Hawaii does not fit perfectly with the Mantle Plume, however, as it does not show especially high temperatures or clear geochemical markers from the deep mantle. Iceland is even more problematic, as it has no time progressive mountain chain, has a wide range of temperatures in its magmas and any evidence of doming of the crust is covered by more recent lava flows and volcanic eruptions. The Canary Islands have no time progressive volcanic chain either but rather show a wide area of volcanic activity. There is also a rather rare and strange type of magma in the Canary Islands that is difficult to explain according to either theory.
Scientists have to formulate theories based upon the evidence we see and then we have to try and prove or disprove that theory. The Mantle Plume and Plate Theories have become two divided camps and the scientists on either side have become entrenched in what they believe, rather than what the evidence actually tells us. Neither of the two theories here fit the evidence and it is therefore necessary that we formulate a new theory to explain anomalous volcanism. No matter whoever creates the new theory, it will be a paradigm shift in the geoscience world and will change how we view the entire Earth system.