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Image: Mauna Kea, Hawai’i, Big Island (photo: Meschede, 2024).
Solid Earth
4.6 Absolute and relative plate motion
Video: Absolute and relative plate motion
Chapter 4.6
Absolute and relative plate motion
In this chapter I would like to talk about the way plate movements on Earth can be perceived and described.
Fig.4.6.1: Relative plate motion velocities at plate boundaries. Modified from Meschede & Warr (2019 ) and Frisch, Meschede & Blakey (2022 ).
At divergent plate boundaries, two plates move away from each other because new oceanic crust is being formed at the mid-oceanic ridge along the plate boundary. And at convergent plate boundaries, two plates move towards each other, with one plate subducting beneath the other and thus being returned to the Earth’s mantle. These movements are relative, i.e. they relate to the plate boundary being considered. In the global context, however, the question arises as to which plate moves where.
At the western plate boundary of the South American Plate (Fig. 4.6.1), the Nazca Plate is subducted beneath the South American Plate. The question here is whether the Nazca Plate is moving eastwards, as can be seen in the red arrows that indicate the plate motion velocity, or whether the South American Plate is moving westwards. A combination of both is also possible. The result, namely that the Nazca Plate is pushed beneath the South American Plate and thus subducted, is the same in every case.
In order to be able to determine the direction and velocity of movement, we need a reference system. In the physical sense, a movement can only be defined with reference to another point or object. The arrows and also the double arrows in Fig. 4.6.1 show the relative movements of the plates among each other. This means the movement between two adjacent plates. The reference system in this case is one of the two plates. In the case of a subduction zone (such as off the west coast of South America), this is a single arrow that shows the velocity at which the respective plate (here the Nazca plate) is subducted beneath the overriding plate (here the South American plate) at exactly the location where the arrow is shown. This velocity applies exactly at the position of the arrow; slightly above, for example off the coast of Ecuador, another velocity arrow is shown on the Nazca plate, although this only shows a velocity of 7 cm/year. The relative velocity between two plates changes along the plate boundary. This connection was already discussed in chapter 4.3, which dealt with plate movements and rotations on Earth. Every plate motion on Earth can be described as the rotation of this plate and accordingly the plate motion velocity becomes slower towards the pole of rotation, with the maximum velocity being found at the rotation equator. Recommendation: Repeat chapter 4.3 .
Abb. 4.6.2: Age of the oceanic crust (Meschede, unpubl., 2022, modified after Frisch, Meschede & Blakey, 2022).
Fig. 4.6.3: Age of the oceanic crust (Meschede, unpubl., 2022, modified after Frisch, Meschede & Blakey, 2022 ) with the extension of the oceanic crust formed in the last 20 million years at the East Pacific Rise (3000 km) and the Mid-Atlantic Ridge (500 km).
What about the directions and plate motion velocities in a global context?
The age structure of the oceanic crust can be determined from the magnetic stripe pattern (Fig. 4.6.2; see Chapter 3.8 ). The wider the stripes, the more oceanic crust was produced per unit of time at the mid-oceanic ridge and the faster the plates are moving away from each other. From the width of the stripes, it is possible to calculate fairly accurately how quickly the plates have moved relative to each other in the overview map. This requires simple rule of three calculations.
Example of the East Pacific Rise (Fig. 4.6.3): the red stripe shows the extent of the oceanic crust formed during the last 20 million years, which is approximately 3000 km wide here. 3000 km corresponds to 20 million years (300 million cm / 20 million years = 15 cm / 1 year). This corresponds to an increase of 15 cm per year, at a divergent plate boundary this is 7.5 cm on each side.
The red stripe at the Atlantic spreading center is, however, much narrower. For the Mid-Atlantic Ridge (Fig. 4.6.3), 500 km corresponds to 20 million years (50 million cm / 20 million years = 2.5 cm / 1 year). This corresponds to an increase of 2.5 cm/year.
In this way, relative plate motions can be easily determined. However, to determine plate motions in global interaction, we need a reference system that is independent of plate boundaries and applies equally to all plates. The hotspots that come into question here are considered to be stationary. Unlike the lithospheric plates, they do not change their position in relation to the Earth as a whole.
Fig. 4.6.4: Animation of an ascending mantle diapir with the formation of a volcanic chain and a flood basalt (Meschede, unpublished, 2024).
Under hotspots, hot mantle rock rises from great depths and collects below the lithosphere in a diapir head (Fig. 4.6.4). From there, magmas penetrate upwards into the crust and form magma chambers. These in turn feed volcanoes on the Earth’s surface. If the hotspot rises beneath an ocean, volcanic islands can form over the magma chambers over time. Huge volcanic systems often form under continental crust, such as the Yellowstone hotspot, as evidenced today by the Yellowstone Caldera.
In the animation you can see how the lithospheric plate moves over the hotspot. Where the volcanoes rise up, a chain of volcanoes forms. The volcanoes form only above the rising hotspot and when the plate with the volcano moves away from the hotspot’s sphere of influence, it takes the volcano with it, which then dies out and leaves a chain of volcanic islands on the earth’s surface. In some cases, such hotspot volcanoes have produced so much material in a short time that it becomes a so-called flood basalt or large basalt province. These are the LIPs, LIP stands for large igneous province. The topic of flood basalts will be discussed in more detail in a later video.
The classic example of a chain of volcanoes that formed over a hotspot is the Hawaiian island chain (Fig. 4.6.5). The animation shows how the island chain has developed over the last 2 million years. The volcanoes always form over the hotspot, in several places, but in principle always more or less over the center of the hotspot. With the movement of the plates, the volcanic islands formed over the hotspot are gradually transported out of the hotspot’s zone of influence, so that the volcanoes then become inactive.
Fig. 4.6.5: Animation of the development of the Hawai’i island chain over the last 2 million years (Meschede, unpublished, 2024)
Fig. 4.6.5a: 3D block image of the Hawai’i island chain (Meschede, unpublished, 2024)
Fig. 4.6.6: Hawai’i island chain with age data (Meschede, unpublished, 2024. Topographic/bathymetric base map 2024 created with GeoMapApp / CC BY / CC BY ( Ryan et al., 2009 ).
Today, the active volcanoes of Hawai’i are located on Big Island (Fig. 4.6.6). These include Kilauea, which is currently one of the most active volcanoes on Earth, Mauna Loa, which last erupted in 1984, and Hualalai, which erupted several times in the 18th century. Off the southern coast, in less than 1000 m of waterdepth, there is the active volcano Lo’ihi, which may grow to the sea surface in a few thousand years. The chain of volcanoes of Hawai’i instructively traces thedirection of plate motion of the Pacific Plate across the hotspot (Fig. 4.6.6).
Fig. 4.6.7: Hawai’i-Emperor Seamount Chain with age data (Meschede, unpublished, 2024. Topographic/bathymetric base map 2024 created with GeoMapApp / CC BY / CC BY ( Ryan et al., 2009 ).
The Pacific Plate is moving from the youngest active volcano towards the older, now long extinct volcanoes. The volcanic chain extends with islands and seamounts through the entire North Pacific to the subduction zone off Kamchatka, where the seamounts have been determined to be over 80 million years old (Fig. 4.6.7). A kink can be seen in the island-seamount chain where the Hawai’i island chain merges into the Emperor seamount chain.
Not only the direction of plate motion and its change can be deduced from the course of the island seamount chain, it is also possible to determine the velocity of plate motion across the hotspot. Here, too, the absolute plate motion velocity according to the so-called hotspot reference system can be calculated using a simple rule of three. The distance from the active volcanoes on Big Island to the Daikakuji Seamount is almost exactly 3500 km. The Daikakuji Seamount was about 43 million years ago where Big Island is today. This means that the plate has traveled 3500 km in 43 million years, which corresponds to an average velocity of 8.14 cm/year.
Fig. 4.6.8: Global distribution of hotspots and hotspot traces ( from Frisch, Meschede & Blakey, 2022 ).
There are a whole series of such volcanic chains on Earth, from which the absolute plate movements can be dewtermined (Fig. 4.6.8). On the Pacific plate, for example, it is noticeable that the volcanic chains emanating from the Macdonald Seamount or the Pitcairn Islands have exactly the same bend as the Hawai’i-Emperor chain. This is because they lie on the same plate, namely the Pacific plate. The bent hotspot traces will be examined in more detail in one of the next videos.
Fig.4.6.9: Absolute plate motion velocities with respect to the hotspot reference system. Modified from Meschede & Warr (2019 ) and Frisch, MEschede & Blakey, (2022 ). Plate motion velocities according to Gripp & Gordon (2002) .
Figure 4.6.9 shows the absolute plate motion velocities calculated using the plate motion calculator at Rice University (Gripp & Gordon, 2002) .
Absolute und relative Plattenbewegungen
Absolute and relative plate motions, part 2, volcano chains and transform faults (in prep.)
