Plate Tectonic History
A Unique Subaerial Vision into Hotspot and Mid Ocean Ridge InteractionsIceland is an island unlike any other in the world in its geology and tectonics. Here, we discuss the plate tectonic history of Iceland and how it came to be such a unique geological wonder.
Iceland's tectonic evolution started around 150 million years ago, when the Earth's plates began to diverge; two large continents broke apart, Pangea (Gondwanaland) and Laurasia. Then, around 80 million years ago, Laurasia broke apart further, Europe (traveling on the Eurasian Plate) drifting to the East and North America (traveling on the North American Plate) drifting to the west. Greenland followed behind North America. These continents spread apart due to a process called rifting (10).
Rifting is a term used to talk about the movement of plate tectonics. Rifting is caused due to a divergent plate boundary where two tectonic plates are moving away from each other, and as a result less dense magma floats to the surface. In a divergent plate boundary, magma is formed by decompression melting and produces really mafic lava, often in the form of basalt. There are many places where ocean lithosphere diverges under the ocean; we call places mid ocean ridges (Figure 1).
Iceland falls directly between two smaller ridge systems which make up a larger mid ocean ridge, called the Mid Atlantic Ridge. The Mid Atlantic Ridge spans the entirety of the Atlantic Ocean. Surrounding Iceland are the Reykjanes Ridge to the south and the Kolbeinsey Ridge to the north (6). The boundary where Iceland is located separates the North American and the Eurasian plates (Figure 8). The two are spreading at a rate of 2 cm/year (6). Around 16 million years ago, the magma formed at the Mid Atlantic Ridge accumulated and broke the surface, creating Iceland. These subaerial rocks are very young compared the age of the Earth (1). The subaerial exposure of this magma is so unique because it is the only place on Earth where we can see a mid ocean ridge without using a submarine.
The processes accumulating this magma are a little bit more complicated than the dynamics in a static system; in fact, systems on Earth are never static. The plates that make up the upper layer of the Earth are parts of a fully dynamic system in which oceanic lithosphere cycles and recycles through the deeper layers of the Earth. This is called the Wilson Cycle.
Within this dynamic system, the North American and Eurasian plates have a net movement west. However, the spreading center at the Mid Atlantic Ridge remains relatively stationary. With each plate jump westward, the boundary appears to be jumping east. This process is called rift jump, and it piles new rock on top of older rock formed between the old and new rift locations relative to the landmass (2,3,6). This is shown in Figure 2 (6).
This lava is interacting with older rock to form complicated folds along the island. More magma is then added from dikes and sills, trailing from the magma chamber to the surface (7). Thus far, we have discussed how the spreading from the Mid Atlantic Ridge causes magma buildup, contributing to Iceland’s subaerial exposure. However, there is more to the story. Iceland is not only located on a mid ocean ridge, it is also located directly on top of a hotspot.
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65 million years ago, hot magma from the mantle rose and came in contact with oceanic lithosphere, melting rock along the way, eventually becoming exposed at the surface (6). This process continued, forming a mantle plume, or hotspot, ~2500km in diameter (4). This stationary hotspot affected many different landforms throughout time as the plates moved west (Figure 3).
For example, in the Paleocene, the hotspot was located under Greenland (4). As the plate moved west, the plume continued interacting with the surface, forming underwater volcanoes. But while this magma may have created an underwater chain of volcanoes, when it comes to Iceland, we are interested in the subaerial expression of magma formation. Figure 4 shows the apparent eastern movement of the hotspot, now located directly under Iceland.
The buoyant magma produced from the oceanic crust, pushed Iceland into a dome shape (9). This magma was release in more explosive and viscous (more resistant to flow) volcanic events, forming intermediate rocks like rhyolite and diorite. Eventually, the hotspot and the diverging plate met and together formed the complex mixture of geology that continues to build Iceland today (Figure 5).
As stated above, the hotspot contributes large amounts of magma to Iceland, forming subaerial rocks. But, if magma is continually flowing to the surface, how is the island habitable?
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Volcanism is one of the major attractions in Iceland, whether tourists know they are experiencing the result of volcanism or not. You will learn more about this in the Tourism and Geothermal Activity sections of the site. When we think of volcanism, we think of large catastrophic eruptions. Well, while they may not always be catastrophic, Iceland experiences a volcanic eruption about once every five years. Based on the type of exposed lava, we can draw conclusions about which magma chamber that lava came from, the one at the spreading center or the one forming the hotspot. Above, we discussed how magma at the spreading center and some from the hotspot is usually very mafic and less viscous. The result of this is the basalt that makes up most of the island (~85%). More felsic magma sourced from the mantle plume makes up most of the rest of the rock on the island (1). Very recent geological features have been shaped by these volcanic processes as well as glacial processes.
There are three main recent stratigraphic formations:
1. Tertiary Basalt (aged 16-3.3 Ma)
This basalt makes up ½ country (either side of rift zones) and was sourced from 40 extinct volcanic systems and dike swarms.
2. Plio-Pleistocene Formation (aged 3.3-0.7 Ma) “autumn”
This formation is mostly clastic and volcaniclastic sediment, making up ¼ of the country. This formation is referred to as the “autumn” because it marks the period before significant glacial and interglacial cycles. It was defined by cooling, which lead to glaciation and table mountains and ridges. There have been 9 glacial and interglacial cycles since this time. The specific dynamics of glaciers and the surrounding land are better described in the Glaciers section of this website, but Figure 7 shows a broad overview of a glacial cycle and ice accumulation.
3. Upper Pleistocene Formation (aged 0.7ma-present)
This more surficial formation makes up prominent topographic features and is defined by lava shields and flood lavas from around 8-9.5 Ka. It is the result of unevenly distributed volcanism and further glacial and interglacial cycles (6).
Figures 6 and 7 show the distribution of volcanism and an example of glaciation over time in Iceland.
The recent volcanic history of Iceland proves how dynamic the island is, showing interactions between hotspots and mantle plumes forming magma at the surface and then surface processes that affect this rock when it cools. As the only place on Earth where a mid ocean ridge is exposed subaerially, Iceland is a truly unique geological country. We can learn more and more about recent historical plate tectonics from Iceland, but we can also learn a lot about current processes affected by climate change and anthropogenic forcings on the island. To continue learning about this truly dynamic wonder, read on to the section about Iceland's geothermal activity.
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Iceland is a young island, with rocks no older than ~25 million years, a blip in the timescale of the Earth. Iceland was formed due to volcanic process in two different settings, hot spot and mid ocean ridge, which converged to form a complex story of unique geology. Rifting along the Mid Atlantic Ridge caused Iceland to accumulate magma from the spreading center, eventually accumulating enough to expose subaerial volcanic rock. The Iceland mantle plume, or hotspot, formed after this spreading center and eventually the two worked together to shape the landscape in this subsection of the Mid Atlantic Ridge. Complex interactions between the divergent boundary and hotspot create an interesting mixture of mafic and intermediate rock forming the still-dynamic island that is Iceland. These processes expose the surface volcanism. We next look at how the formation of Iceland facilitated changes in the cryosphere, hydrosphere, and biosphere.
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Tectonic Plates - 20 distinct pieces of the relatively rigid lithosphere (See Figure 8).
Rifting - What occurs at a spreading zone (see Divergent Plate Boundary)
Divergent Plate Boundary - A boundary at which two lithosphere plates move apart from each other; they are marked by mid-ocean ridges. (Figures 1 and 2)
Dikes and Sills - A tabular (wall-shaped) intrusion of rock that cuts across the layering of country rock.
Mafic - A term used in reference to magmas or igneous rocks that are relatviely poor in silica and rich in iron and magnesium.
Lithosphere - The relatively rigid, nonflowable, outer 100- to 150-km-thick layer of the Earth, constituting the crust and the top part of the mantle. (Figure 1)
Decompression Melting - The kind of melting that occurs when hot mantle rock rises to shallower depths in the Earth so that pressure dereases while the temperature remains unchanged. See Figure 9 for more detail. (Figure 1)
Mantle - The thick layer of rock below the Earth's crust and above the core.
Mantle Plume - A column of very hot rock rising up through teh mantle.
Hotspot - A location at the base of the lithosphere, at the top of hte mantle plume, where temperatures can cause melting.
Viscous - The resistance of material to flow.
Intermediate - In between Mafic and Felsic
Felsic - An adjective used in reference to igneous rocks that are rick n elements forming feldspar and quartz.
Fissure Swarm - A conduit oin a magma champer int eh shape of a long crack through which magma rises and erupts at the surface.
*Quoted from Stephen Marshak, Essentials of Geology, 4th Edition 2004
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By: Cara Piske