## 3.5 Convection currents in the Earth´s mantle

##### Video: Convection currents in the Earth´s mantle

(To play the video, please click on the image above)

Photo: Lava lake in Halema‘uma‘u crater Kilauea, Hawai‘i, USA (photo: Ivan Vtorov, 2012, Wikipedia)

## Convection currents in the Earth´s mantle

Convection or convective flow is a physical principle based on isostatic equilibrium. When materials, whether solid, liquid or gaseous, are heated, the intensity of Brownian motion increases as the individual molecules move faster and require a little more space for this movement. This causes the mass to expand slightly and therefore become slightly lighter. The result is that it rises upwards according to the principle of isostatic compensation. We find examples in the household, for example when warm air rises and cold air falls down.

An experiment with heating water shows the principle: If you heat water in a pot at one point, for example with a Bunsen burner (Fig. 3.5.1, animation), heat is supplied above the flame so that the water rises at this point because it becomes lighter. It cools on the surface and sinks back down the sides of the pot. This creates a cycle called convection; the water moves in a convection current.

Fig. 3.5.1: Model for the formation of convection currents (Meschede, unpubl., 2022). (kalt = cold, heiß = hot).

Such convection currents can also be found in the Earth’s mantle and probably also in the Earth’s core. The Earth’s mantle is not liquid like water, but it is mobile, except that the movements occur at extremely low speeds. We measure it in centimeters per year – not centimeters per second like in the water model. A few centimeters per year are the velocities that we know from plate movements on the Earth’s surface and the Earth’s mantle also moves at the same scale. It can flow over periods of many millions of years.

Fig. 3.5.2: Model of convection currents in the Earth’s mantle with large, symmetrical convection cells that sink downwards in a subduction zone and rise under a spreading zone, presented by Wikipedia english (2024).

For plate tectonics, convection currents play an important role in plate movement. Even in the very first models from the 1960s and 1970s, convection currents in the Earth’s mantle were depicted as the main drivers of plate movement. However, most representations that can be found on the Internet or in books in countless variations (example from Wikipedia in Fig. 3.5.2) show the convection currents in the Earth’s mantle as huge convection cells in which cool lithosphere sinks downwards in subduction zones into the deeper mantle where it is heated up and rises up again as a hot flow just below the spreading zone. However, this  model of convection does not work in this simple way.

If convection in the Earth’s mantle really happened like in the Wikipedia representation (Fig. 3.5.2), the convection cells should not change and they would always have to stay in the same place. However, oceans are constantly being re-formed by the creation of new oceanic lithosphere at the spreading zones, but they also disappear by being completely subducted. In this model, however, closing an ocean would not be possible at all. The Atlantic Ocean, for example, does not fit this model at all because, apart from the small areas of the Caribbean and South Georgia, there are no subduction zones on its edge, neither on the American, European or African side. But in the middle between the two plates is a spreading zone that has been active for millions of years.

Fig. 3.5.3: Locally limited extent of convection cells in the area of ​​a spreading zone (Meschede, unpubl., 2022).

The spreading zones are not as important for the mantle flows as previously assumed. Beneath the spreading centers are smaller, localized convection cells located directly beneath the rift, where oceanic crust forms. The large-scale, global mantle flow is only found in deeper areas of the mantle.

Fig. 3.5.4: Model for convection currents in the Earth’s mantle (for abbreviations see Fig. 3.5.6; Meschede, unpubl., 2022, modified after Frisch & Meschede, 2021)

Mantle flows are very large in scale and the term convection cells, which is sometimes used, should be avoided. It is a convection current that contributes to the exchange of cold and warm masses. In the model of Fig. 3.5.4, cool lithosphere moves downward in subduction zones. The main difference to the mantle model in Fig. 3.5.2 is that the flows do not rise under the spreading zones, but are widely distributed beneath rising hotspots.

Fig. 3.5.5: Global distribution of hotspots (from Frisch & Meschede, 2021)

Hotspots are not statistically distributed across the Earth (Fig. 3.5.5). There are two clusters, one around Africa and the eastern Atlantic and another in the central and eastern Pacific, which are related to Earth’s large heat exchange system.

Fig. 3.5.6 Section through the Earth’s body seen from the South Pole. LLSVP = large low shearwave velocity provinces (areas with reduced speed of seismic shear waves), PGZ = plume generation zone (zone in which mantle diapirs are formed), D” = transition zone between sinking oceanic lithosphere and LLSVP, oceanic and continental lithosphere are shown highly exaggerated (from Frisch & Meschede, 2021).

Fig. 3.5.6 shows a section through the Earth’s body parallel to the equator (view from the south pole onto the section). The Earth’s body can be roughly divided into four areas: opposite areas with subduction and areas at approximately right angles to this in which hotspots occur. It is estimated that there are currently around 40-50 such hotspots worldwide. The hotspots Hawai’i, Yellowstone, the Phlegraean Fields near Naples or the Canary Islands with the recently erupted volcano on La Palma are well-known examples.

The two phenomena subduction and hotspot do not occur together, although they are sometimes relatively close, but they do not occur together so that a hotspot would lie directly in a subduction zone. In contrast, there are spreading zones under which there is a hotspot, such as Iceland. There is a hotspot directly underneath the Mid-Atlantic Ridge. The hotspot uplifted the oceanic crust, so that the ridge with the spreading zone can be observed on land today. This is one of the very few places on Earth where this is possible. Normally the spreading zones are located on their mid-oceanic ridges at water depths of more than 2000 m.

Convection currents in the Earth´s mantle