Factors Affecting Icelandic Weathering
Implications of Icelandic Weathering
Figure 1. Image of the Grand Tetons
Chemical vs. Mechanical Weathering
Before discussing the differerent mechanisms that weather and erode rocks in Iceland, it is important to understand the difference between mechanical and chemical weathering. Mechanical weathering involves the physical breakup of parent rock into smaller pieces through the forces of water, ice, gravity, and wind (to name a few). The main take-away is that during mechanical weathering, a rock's chemical composition remains unaltered, and fragments are simply chipped away from a larger rock. On the other hand, chemical weathering involves reactions that remove or add elements to a parents rock, so the rock does becomes chemically altered (Feng, 9). Chemical weathering often involves the intereaction of acidic water with a rock surface. Precipitation contains carbonic acid due to the reaction of atmospheric carbon dioxide with water. This rainwater becomes incorportated into streams and groundwater, where it encounters rock. Various reactions result in the chemical alteration of rock minerals, which causes the erosion of parent rock and incorportation of dissolved constituents into water. Many of the minerals and ions that you hear about in drinking water can be traced to this process. As a visual example, limestone caves are a classic geologic feature that are formed through chemical weathering of carbonate rock (Figure 2).
Figure 2. The process of forming a limestone cave through chemical weathering.
As we have discussed, weathering and erosion play a huge role in shaping the earth surface. Glacial activity has formed beautiful U-shaped valleys, wind has created seas of sand dunes, and running water has cut canyons that can hardly be explored in a lifetime. Different areas of the world have defining characteristics that result in specific types of weathering that shape the landscape and alter water compositions. Some of these defining characteristics may include bedrock type, climate, topography, and tectonic activity. The country of Iceland contains a unique combination features that make its weathering processes and rates very different from almost anywhere else on Earth.
To begin, Iceland is an exposed mid-ocean ridge (discussed here), meaning that it is on a divergent plate boundary where a mantle plume supplies magma that is frequently erupted onto Iceland's surface (Gudmundsson, 2000) (Figure 3). As a result, about 90% of Iceland is dominated by Quaternary and Tertiary basalts (relatively new volcanic rocks) (Gislason et al., 1996). The chemical weathering of certain basalts happens at relatively fast rate, and the nearly constant supply of magma means that these basalts are very important for Icelandic weathering.
Figure 3. Schematic Drawing of the divergent plate boundary responsible for forming Iceland.
Another major characteristic of Iceland that significantly impacts weathering is the presence of glaciers, which happen to cover over 10% of Iceland's land area (Sigurosson, 2011). As explained in the Glaciers section of this website, glaciers are masses of ice that flow over the underlying bedrock. During the process, the glacier abrades, plucks, and grinds fragments of bedrock under its immense weight, and these fragments will eventually be carried by meltwater into streams (Figure 4). The characteristic milky-blue color of glacial streams is a result of the huge concentration of fine minerals and suspended sediment that have been ground by glaciers. Thus, glaciers are major contributers to weathering and erosion. Although glaciers currently occupy a significant portion of Iceland's land area, they are currently retreating, which will have a variety of implications for the future weathering in Iceland, which will be discussed later on this page (Compton, 2016).
Figure 4. Schematic drawing of glacial plucking and abrasion.
The final aspect of Iceland that may affect weathering is increaing precipitation over the past century. According to the 2013 OECD Studies on Water, precipitation increases as a function of temperature. From 1975 to 2008, Iceland has warmed at 0.35 degrees celsius per decade, which is much higher than the 0.17 degree per decade global average. Every degree celcius increase in temperature corresponds to a 2.5% increase in precipitation (OECD, 2013). Since precipitation is slightly acidic, we might expect the increase in precipitation to result in greater chemical weathering rates in the future. Yet, we must also consider other effects of increased temperature such as glacial melting. The water from glacial streams in Iceland is relatively alkaline (pH of 8-10) (Gislason et al., 1996), so glacial melt as caused by temperature increase may negate the increased weathering rates resulting from more rainwater. New studies are necessary to compare these two effects of increased temperature on chemical weathering rates.
High Chemical Weathering Rates
There are multiple factors that influence chemical weathering rates, but ultimately the most important are the actual rock type and chemical composition of the reacting fluid. The fastest chemical weathering rates in nature will result from highly acidic water and a rock type that reacts quickly such as limestone. Iceland contains an abundance of silicate rocks, which typically do not weather very quickly, but a variety of studies have shown that the weathering rate of Iceland's silicate rocks is surprisingly high (Gislason 2009, Louvat et al., 1999, Louvat et al., 2008).
As it turns out, the chemical weathering rate of Iceland's basalt is strongly correlated with the age of rock and presence of glassy basalt as opposed to crystalline basalt. Glassy basalts form during very rapid cooling processes, so volcanic eruptions under glaciers will often produce a glassy basalt, while an eruption on warmer glacier-free land will product more crystalline basalts. This is simply influenced by cooling time. The longer a rock takes to cool, the larger the mineral crystals are able to grow. The dissolution of glassy basalts is 10 times faster than crystalline basalts, so we find that the abundance of glassy basalts in Iceland results in high chemical weathering rates (Gislason et al., 1996). Additionally, there has been shown to be a strong inverse relationship between the age of rock and the corresponding chemical weathering rate (Figure 5).
Figure 5. Plot showing chemical ersion versus age of rocks.
(Louvat et al., 1999)
From the data collected in Iceland (Louvat et al., 1999), conclusions were made that the younger the rock, the faster the chemical weathering rate due to the abundance of unaltered and more mobile elements such as potassium and sodium. With age, the basaltic glass becomes heavily altered and less soluble, resulting in slower weathering rates. Since basalts form as magma is erupted and cools, we can assume that the youngest rocks are found near Iceland's spreading center, which contains the greatest concentration of active volcanoes on the island (Figure 6). If these young rocks undergo rapid cooling, they will be chemically weathered very quickly relative to other silicate rocks (Gislason 2008). Given the information from this section, Iceland has the perfect setting for highly weatherable silicates: an area with active volcanism and a cold climate that allow for the creation of young, glassy basalts.
Figure 6. Map showing the location of volcanic systems and rock ages.
High Mechanical Weathering Rates
The mechanical erosion rate of rock in Iceland is extraordinarily high due to the presence of glaciers, and this causes very high total suspended solid concentrations in glacial streams (up to 2300 mg/l) (Louvat et al., 1999). As previously explained, the flow of glaciers over bedrock causes the grinding of parent rock into smaller pieces, which forms glacial flour and rock fragments that become incorporated into streams. So, streams that contain the meltwater from large glaciers contains an abuncance of suspeded solids that are eventaully deposited on stream beds, lake bottoms, or the ocean floor (although some particles main remain suspended in the water).
The 1999 study conducted by Louvat and his colleagues reported that mechanical erosion rates for eleven rivers in Iceland were between 960 and 16500 t/km^2/yr, which is significantly higher than the 230 t/km^2/yr global average. Unsurprisingly, many of these streams drain glaciers, but another factor that is worth mentioning that may contribute to high suspended sediment concentrations is the lack of vegetation to prevent the erosion of soil. Thus, runoff water is able to easily erode minerals and rock fragments found in the soils and incorporate them into streams. Additionally, the steady increase in temperature may not contribute to overall greater chemical weathering rates (as discussed here), but it will certainly increase the amount of runoff fed by both glacial melting and precipitation. As explained by Gislason in his 2008 study, high runoff corresponds to higher mechanical weathering rates (which may seem obvious due to the energy supplied to the weathering process by increased flowing water).
Figure 7. Plot showing chemical erosion versus atmpspheric carbon dioxide consumption
(Louvat et al., 1999)
The average carbon dioxide consumption determined from the chemically weathered constituents of eleven rivers in Iceland was 0.74x10^6 mol/km^2/yr compared to the 0.3x10^6 mol/km^2/yr global average (Louvat et al., 2008). Considering that Iceland only releases 0.27x10^6 mol/km^2/yr of carbon dioxide into the atmosphere, we can consider Iceland a net sink for atmospheric carbon dioxide thanks to the abundance of young glassy basalts. Since carbon dioxide is a greenhouse gas, Iceland is responsible for mitigating global warming (although the impact is not significant enough to reverse global warming's effects).
The future of weathering in Iceland will be significantly impacted if temperatures continue to rise drastically over the next few centuries. If Icelandic glaciers disappear, mechanical weathering rates and suspended sediment concentrations in streams will greatly decrease. On the other hand, increased temperatures and the lack of more alkaline glacial meltwater may result in greater chemical weathering rates as precipitation increases and acidic rainwater dominates runoff and stream water.
Gislason, S. "Weathering in Iceland." Jokull Journal 58 (2008): 387. JokullJournal.Is. Web. 12 April 2017.
Gislason, S., Arnorsson, S., and Armannsson, H. "Chemical Weathering of Basalt in Southwest Iceland: Effects of Runoff, Age of Rocks and Vegetative/Glacial Cover." American Journal of Science 296.8 (1996): 837-907. AJSonline. Web. 12 April 2017.
Gudmundsson, A. "Dynamics of Volcanic Systems in Iceland: Example of Tectonism and Volcanism at Juxtaposed Hot Spot and Mid-Ocean Ridge Systems." Annual Reviews 28 (2000): 107-140. Annualreviews.org. Web. 12 April 2017.
Louvat, P., Gislason, S.R., Allegre, C.J., "Chemical and mechanical erosion rates in Iceland as deduced from river dissolved and solid material." American Journal of Science 308.5 (2008): 679-726. Print.
Louvat, P., Allegre, C.J., and Gislason, S.R. "Chemical and mechanical erosion of major Icelandic rivers: Geochemical budgets." Geochemistry of Earth's Surface 5 (1999): 111-114. Print.
OECD. "Iceland." Water and Climate Change Adaptation: Policies to Navigate Uncharted Waters, OECD Publishing, Paris. 2013.
Sigurosson, O. "Iceland Glaciers." Encyclopedia of Snow, Ice, and Glaciers. Micael P., Bishop. Springer, 2011. 630-636. Print.
By: Forrest Town