Independent Project at the Department of Earth Sciences
Självständigt arbete vid Institutionen för geovetenskaper
2019: 8
Interaction Between Different Magma Types in the Reyðarártindur Magma Chamber, SE Iceland
Interaktion mellan olika magmatyper i magma- kammaren Reyðarártindur, sydöstra Island
Sabine Rousku
DEPARTMENT OF EARTH SCIENCES
I N S T I T U T I O N E N F Ö R G E O V E T E N S K A P E R
Independent Project at the Department of Earth Sciences
Självständigt arbete vid Institutionen för geovetenskaper
2019: 8
Interaction Between Different Magma Types in the Reyðarártindur Magma Chamber, SE Iceland
Interaktion mellan olika magmatyper i magma- kammaren Reyðarártindur, sydöstra Island
Sabine Rousku
Title page: Figure made in ImageJ by Rousku, 2019
Copyright © Sabine Rousku
Published at Department of Earth Sciences, Uppsala University (www.geo.uu.se), Uppsala, 2019
Abstract
Interaction Between Different Magma Types in the Reyðarártindur Magma Chamber, SE Iceland
Sabine Rousku
Southeast Iceland exhibits a granophyre pluton called Reyðarártindur, which has never been described in detail before. The Reyðarártindur magma chamber formed 7.30 ± 0.06 Ma ago (Padilla, 2015). Glacial and coastal erosion expose the pluton, and a river cuts through the pluton roof and walls, revealing interaction between different blob-like structures of magma. The formation of magma chambers can take a very long time, it is therefore likely for several different magmas to interact.
Incremental formation of different magma batches give rise to mixing and mingling in magma chambers. To understand when the magma mixing was initiated and the mechanisms controlling it, descriptive analysis were made to obtain textural
properties of collected rock samples from the field. The purpose for this thesis study was to examine if there is a frequency size and shape distribution of the magma blobs and if the different magma blobs are systematically distributed across the river.
Previous studies have inferred conduit locations and magma mixing processes through similar methods. Extensive field studies have provided all samples for this thesis.
Five distinct, magma types were described and found to be interacting. There was one ‘host magma’ which the other four different magma types are exposed as ‘blobs’
within. The statistical analysis involved mapping the blob-like structures from photos taken with an Unmanned Aerial System (URA; drone), using the software Inkscape.
The data and measurements for the blobs was collected and summarized in ImageJ.
The data was then statistically analyzed in Excel, illustrating the frequency of the magma blob’s size and shape distribution in selected parts of the river. The results of the statistical analysis of the magma blobs showed that ~80 % of the blobs existed in a size interval between 0 – 0.1 m2. This thesis provides a discussion about the
implications of the blob distributions for magma chamber recharge and processes within this section of the magma chamber. The shape distribution analysis showed an indication for all the blobs to be more rounded and equant. This suggest that the magma mixing event probably happened at the same time, during a liquid phase.
Key words: magma chamber, magma mixing, granophyre pluton, Reyðarártindur, igneous petrology
Independent Project in Earth Science, 1GV029, 15 credits, 2019 Supervisors: Emma Rhodes and Steffi Burchardt
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala (www.geo.uu.se)
The full publication is available at www.diva-portal.org
Sammanfattning
Interaktion mellan olika magmatyper i magmakammaren Reyðarártindur, sydöstra Island
Sabine Rousku
På sydöstra Island återfinns en granofyrisk pluton kallad Reyðarártindur. Det är en magmakammare som aldrig tidigare blivit beskriven i detalj. Magmakammaren bildades för 7,30 ± 0,06 Ma sedan (Padilla, 2015). Plutonen har blivit exponerad genom glacial- och fluvial erosion samt att en flod skär igenom plutonens tak och väggar. Den eroderande floden exponerar olika fläckliknande strukturer av
magmainteraktioner. Ett gradvist bildande av olika magmasatser har över lång tid gett upphov till en blandning i magmakammaren. För att förstå när
magmablandningen initierades och mekanismerna bakom fenomenet, har en beskrivande analys gjorts för att ta reda på texturella egenskaper av insamlade bergartsprover från fält. Syftet med denna studie var att undersöka om det fanns en storleks- och formdistribuering av magmafläckarna samt om de olika
magmafläckarna är systematiskt distribuerade i flodbädden. Tidigare studier har antytt var undersökta magmakanalsystemen kan finnas samt hur
blandningsprocesser för magma går till med hjälp av liknande metoder. Redan genomförda fältstudier har samlat in allt råmaterial som ligger till grund för denna studie.
Fem olika magmatyper har beskrivits och påvisades interagera. Det fanns en
”värdmagma” som de andra fyra magmatyperna var exponerade som fläckar i. De statistiska analyserna inkluderade kartering av de fläckliknande magmaformerna baserat på foton tagna med hjälp av en drönare. Tre olika mjukvaror användes för att samla in, mäta och analysera data; Inkscape, Image J och Excel. Resultaten från den statistiska analysen visade att ungefär 80 % av alla fläckar existerade inom ett
areaintervall mellan 0 – 0,1 m2. Denna studie innehåller en diskussion om implikationerna av magmafläckarnas distribution med avseende på
magmaomladdning och -processer inuti denna sektion av magmakammaren. Den generella formdistributionen visade en indikation för att alla fläckar tenderar att vara mer rundade och kvadratiska. Detta antyder att magmans blandningsförlopp troligtvis inträffade vid ungefär samma tidpunkt, under en flytande fas.
Nyckelord: magmakammare, magmablandning, granofyr pluton, Reyðarártindur, magmatisk petrologi
Självständigt arbete i geovetenskap, 1GV029, 15 hp, 2019 Handledare: Emma Rhodes och Steffi Burchardt
Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, 752 36 Uppsala (www.geo.uu.se)
Hela publikationen finns tillgänglig på www.diva-portal.org
Table of contents
1. Introduction ... 1
2. Geological background ... 2
2.1. Magma chamber formation ... 3
2.2. Southeast Iceland ... 3
3. Method ... 4
3.1. Descriptive analysis of Reyðarártindur rock samples ... 4
3.2. Mapping and statistical analysis ... 4
4. Results ... 6
4.1. Descriptive analysis of Reyðarártindur rock samples ... 6
4.1.1. Host magma ... 6
4.1.2. Enclave A ... 9
4.1.3. Enclave B ... 10
4.1.4. Enclave C ... 11
4.1.5. Enclave D ... 12
4.1.6. Examples of mixed magma ... 13
4.2. Maps of analyzed river outcrops ... 14
4.3. Summary table ... 16
4.4. Size distribution ... 16
4.5. Shape distribution ... 18
4.6. Form factor ... 20
5. Discussion ... 21
5.1. Rock types ... 21
5.1.1. Summary and interpretation ... 21
5.1.2. Limitations ... 22
5.2. Maps and statistical analysis ... 22
5.2.1. Summary and interpretation ... 22
5.2.2. Limitations ... 23
5.3. Outlook and recommendations for future studies ... 23
6. Conclusions ... 24
7. Acknowledgements ... 24
8. References ... 24
9. Appendix ... 26
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1. Introduction
On the southeast coast of Iceland there is a granophyre pluton called Reyðarártindur (figure 1). This pluton has never been described in detail before and is therefore of interest for this study. The Reyðarártindur pluton roof has been exposed by glaciers eroding the landscape, and a river cutting the magma chamber, revealing an
interaction between five different magma types. These magma types are revealed as blob-like structures in the river bed. Four images of the river were selected for
analysis, based on clarity of blobs. For this thesis the enclaves of magma are
referred to as blobs, as they are unidentified objects with unknown composition. The blob-objects has been mapped from aerial footage, which reduce the possibility for individual blob compositional description. Complementary textural data of collected rock samples from the Reyðarártindur pluton has been added, to provide a general idea of the magma mixing dynamics in the area.
Figure 1. Map of studiedlocation, coordinates in Universal Transverse Mercator (UTM). The small red square marks position of studied area in SE Iceland. The yellow contour (50 m wide) marks the Reyðarártindur magma chamber, SE Iceland. The large red square shows the location of figure 3 and where the four mapped outcrops is located (figure provided by Emma Rhodes, 2019).
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The importance of studying old magma chambers lies in the fact that we cannot study magma chambers directly in active volcanic systems. An old magma chamber, as Reyðarártindur provide information about the pluton-volcano connection together with the formation and evolution of silicic magma on Iceland (Wiebe, 1994;
Bachmann et al., 2007; Padilla, 2015). If we know more about the pluton structure, we can ideally help to predict if an eruption is about to happen and prevent potentially dangerous situations.
The aim for this thesis is to characterize the different magma types. In particular, examine if there is a frequency size and shape distribution of the magma blobs and if the different magma blobs are systematically distributed across the river, with
software such as Inkscape, ImageJ and Excel. Previous studies of nearby intrusions have observed similar features and magma mixing processes through comparable methods. The results of the magma blobs size and shape distribution could then provide information about how the magma was mixing, where in the magma chamber new magma from deeper below was entering, and how different magmatic processes formed the magma chamber.
2. Geological background
Iceland is located on the Mid-Atlantic Ocean Ridge, where the diverging plates continuously accumulates magma to produce new oceanic lithosphere (Burchardt,
2009; Gudmundsson, 2013;
Padilla, 2015; Wyk de Vries &
Wyk de Vries, 2018). Iceland is also argued to be situated on top of a large mantle plume, The Iceland Plume, on the
southeastern side of the island.
The mantle plume and the rift zone generate a variety of volcanic and igneous plumbing systems in the mantle and crust (Burchardt, 2009; Gudmundsson, 2013; Padilla, 2015). The
magmatic plumbing system are constantly evolving and growing through intrusions, fractures, movements and faults
(Gudmundsson, 2013; Wyk de Vries & Wyk de Vries, 2018). The active volcanic region of Iceland, the rift zone and the two
intraplate volcanic belts, create up to 30 volcanic systems
(Gudmundsson, 2013). These regions are active due to fissures Figure 2. A schematic picture of how magma can
be generated and transported from the mantle into the crust through different magmatic mechanisms and processes (Burchardt, 2009).
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and faults, transporting the magma through the upper crust, which can result in volcanic eruptions (Gudmundsson, 2013).
2.1. Magma chamber formation
The magma rising starts when partial melting occurs in the mantle, producing segregation of different compositional molten rocks (figure 2) (Petford et al., 2000;
Van der Pluijm & Marshak, 2004; Burchardt, 2009; Wyk de Vries & Wyk de Vries, 2018). The partially melted magma ascends into the upper crust as a result of pressure and temperature gradients (Blatt et al., 2006). When magma forms by partial melting, the composition of the original rock has changed since some of the minerals were melted, and the partially melted magma becomes less dense than surrounding rock in the mantle or crust (Petford et al., 2000; Van der Pluijm &
Marshak, 2004; Burchardt, 2009). The difference in density between the magma and surrounding rock causes the magma to ascend into the crust by buoyancy forces.
Ascent of magma occurs by creeping along grain surfaces or through a vein network of dykes and sills. The magma rises until it reaches neutral buoyancy, where the surrounding rock and the magma obtain the same lithostatic pressure. Magma emplacement forms pools of molten rock in the upper crust through addition of many small batches of magma, and crystallization eventuates in a solidified pluton (Petford et al. 2000; Van der Pluijm & Marshak 2004; Burchardt 2009). Magma emplacement into the upper crust evolves slowly through many heat pulses, accumulating diverse types of magma batches, resulting in replenished magma mixing (Morgan, 2018).
Magma mixing is a complex process occurring when two or more magma differentiate and produce a new solitary magma (Blatt et al., 2006).
2.2. Southeast Iceland
On the southeast coast of Iceland, shallow crustal magmatism has generated a complex of four intrusions found within 30 km north to south from each other, named Austurhorn, Reyðarártindur, Slaufrudalur and Vesturhorn (Cargill et al., 1928; Furman et al., 1992; Burchardt et al., 2010, 2012; Padilla, 2015). All the intrusions are of silicic origin (Cargill et al., 1928) and belongs to the Neogene period 17 – 3.3 Ma (Padilla, 2015). Reyðarártindur is a granophyre pluton, which has been exposed due to glacial erosion, forming a valley called Reyðará. The glacial erosion has gone ~2 km down into the crust, exposing the roof and walls of the Reyðarártindur pluton (Furman et al., 1992). Its composition consists of granophyre rocks, related gabbros and mafic-silicic composite zone magma, that provide indication for mixing and mingling of magma, creating hybrid rocks (Padilla, 2015). Padilla (2015) was the first to have reported an age for Reyðarártindur, using high-precision zircon U-Pb
geochronology, and he found the Reyðarártindur intrusion to be 7.30 ± 0.06 Ma.
Padillas (2015) results suggest that the Reyðarártindur intrusion is the oldest of the four silicic intrusions, mentioned above. Incremental formation from different magma batches, allowing the new magma to mix in where the previous magma is not solid yet.
Weidendorfer et al. (2014) examined the Austurhorn intrusion and its magma mixing properties, and their findings suggest increasing complexity of magma mixing towards the central area of the intrusion. Weidendorfer et al. (2014) describes the margins and rims of the pillows and enclaves (blobs) of magma as being chilled,
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diffused, non-diffused or non-chilled. Thus, the rock composition varies between mixing of mafic, hybrid and felsic magma in Austurhorn causing different types of dynamics for magma interaction. Brittle deformation dominates the outer sections of the magma chamber, producing angular clasts and blobs of magma, due to a
shattering process comparable to hydro-fracturing. In the intermediate section, pillows and enclaves were described as having chilled margins, varying in being rounded, angular and elongated in shape. In the inner section of the magma
chamber, the enclaves displayed non-chilled edges, probably generated by reduced temperature gradients (Weidendorfer et al., 2014).
To compare previous studies of the felsic intrusions on Southeast Iceland, this thesis examines if the different silicic magmas in the Reyðarártindur magma chamber could be related, and if the frequency size and shape distribution occur systematically in the outcrop river area.
3. Method
The photos of the river outcrop were taken in the field by Emma Rhodes with an Unmanned Aerial System (URA; drone), DJI Phantom 4 Pro. Four photos were chosen from the river: upstream, middle and downstream (figure 3). Images were selected based on clarity of blobs in order to generate the best results. Resolution of photos was 4864x3648 pixels. The photos were edited before analyzed to change the color contrasts, making the blobs more visible with Windows 10 photo editing tool.
3.1. Descriptive analysis of Reyðarártindur rock samples
To get a complete picture of the Reyðarártindur magma chamber and its magma distribution, my supervisors and their colleagues collected rock samples from the river outcrop. The rock samples were cut by Emma Rhodes, to obtain a fresh
surface. The collected samples consisted of five distinct, magma types. A descriptive analysis of the Reyðarártindur rock samples was made, macroscopically, with a hand lens, water, light and a ruler. Photos were taken of all the rock samples to complete the description.
3.2. Mapping and statistical analysis
The images were imported into Inkscape, to map the different blobs of magma. Blobs were colored green with a thin black border. The black border was necessary for blob differentiation at the analysis stage. The area of the four river images was divided into three different color sections: outlined blobs (green), outlined area of host magma (white) and outlined area for excluded parts of the river (black) (see figures 17, 18, 19, 20). A scale bar was added to each excluded section of the river images, measured from the length of a high visibility vest shown in figure 20. The vest was measured 68 cm. The finished images were exported as high resolution png.
To get the size distribution data from the Inkscape images, the images were imported into ImageJ. The line tool was used to draw a line over the scale bar, to set the scale. The selected colors in Inkscape was required to adjust the threshold of the analyzed image in ImageJ to get the area of the blobs. The image was then
despeckled to reduce pixels that would otherwise be analyzed as a blob. Analysis of the particles generated all the size data, and an outline of every object was made to
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check that the blobs were measured properly. The ImageJ method for size calculations of blobs was repeated for all excluded areas. To get the shape
distribution data, a plugin filter was added to ImageJ called ij-shape-filter plugin. The shape filter was used to extract the aspect ratio and the form factor called Thinnes ratio in ij-shape-filter plugin.
The aspect ratio was calculated and shows the relationship between short and long axis of the best fitted bounding rectangle, and gives a value between 0 – 1, where 1 is a square and 0 is infinitely elongated:
𝐴𝑠𝑝𝑒𝑐𝑡 𝑟𝑎𝑡𝑖𝑜
= 𝑆ℎ𝑜𝑟𝑡 𝑎𝑥𝑖𝑠 𝐿𝑜𝑛𝑔 𝑎𝑥𝑖𝑠The Thinnes ratio produces a value between 0 – 1 for each mapped blob of magma and shows how rounded or angular blobs are. The Thinnes ratio was calculated by the following equation:
𝑇ℎ𝑖𝑛𝑛𝑒𝑠 𝑟𝑎𝑡𝑖𝑜
=4𝜋𝐴 𝑃2A = Area
P = Perimeter of an object
The office software Excel was used to plot different diagrams and a table to present the data:
• Summary table of data
• Size distribution for all four river images - % of blobs/Surface area
• % of blobs/Aspect ratio
• Surface area/Aspect ratio
• % of blobs/Thinnes ratio
• Surface area/Thinnes ratio
The Y-axis for surface area for both the aspect ratio and Thinnes ratio diagram have been changed to a logarithmic scale to make the results more readable.
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4. Results
The results from this study are divided into six sections for qualitative and quantitative analysis:
1. Descriptive analysis of Reyðarártindur rock samples 2. Maps of analyzed river outcrops
3. Summary table 4. Size distribution 5. Shape distribution 6. Form factor
Figure 3. Map of GPS location where the analyzed photos of the river were taken (figure Emma Rhodes, 2019).
4.1. Descriptive analysis of Reyðarártindur rock samples
The Reyðarártindur rock samples collected from the river consisted of five different magma types. The first magma type was the ‘host magma’, in which the other four different magmas were exposed as ‘blobs’.
4.1.1. Host magma
The host magma is a granophyre rock. Light purplish grey/beige color of the ground mass. Coarse to medium grained white sub-rounded to angular grains of quartz and feldspar, 1 – 7 mm long (figure 4). The grains vary from elongated to equant. Finer matrix in between the larger phenocrysts of quartz and feldspar grains, suggesting a porphyritic texture. The four rock samples of host magma varied, were some of the rock samples had larger phenocrysts of felsic minerals, making the color appear
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white to light grey (figure 5, 6, 7). The size and amount of mafic crystals also seems to vary from, ~5 to 10 %.
Figure 4. Granophyre rock with porphyritic texture.
Figure 5. Example of large phenocrysts of felsic minerals.
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Figure 6. Example of light purplish grey color, of host magma.
Figure 7. Higher amount of mafic minerals than in figure 4.
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4.1.2. Enclave A
Enclave A is a felsic rock. Light grey fine-grained ground mass mixed with 0.2 – 1.7 cm long mafic needles (figure 8). The mafic needles constitute ~30 % of the rock sample. Medium sized subangular grey quartz and feldspar grains in between the mafic needles.
Figure 8. Enclave A, exhibit mafic needles between 0.2 – 1.7 cm.
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4.1.3. Enclave B
Enclave B has a felsic fine-grained ground mass. Grey to light grey medium-grained rock with white areas in the ground mass (figure 9). Hard to distinguish the crystals that constitutes the white areas, probably fine-grained quartz and feldspar. Consists of 0.1 – 3 mm mafic needles (figure 10). Larger grains of other mafic mineral, that is more rounded in grain shape among the mafic needles. Enclave B is more mafic than enclave A, ~50 % mafic mineral composition.
Figure 9. Enclave B, white areas of ground mass mixed into the medium-grained rock of mafic minerals.
Figure 10. Enclave B with 0.1 – 3 mm mafic needles.
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4.1.4. Enclave C
Enclave C is a felsic rock. Enclave C looks like it is mixed in with the host magma, it is finer grained compared to type A and B, lacking the porphyritic texture (figure 11, 12). Light grey to light purplish grey. Medium to fine grains of quartz and feldspar with some mafic needles, 0.1 – 3 mm long. The quartz-rich and feldsparic content is higher than in enclave A, B and D making the rock lighter than the other magma types. Some larger coarser subangular/angular white quartz and feldspar crystals in different parts of the rock samples indicate that the host magma probably is mixed in with magma type C. Other samples of enclave type C look like type B, because of more mafic minerals, mafic needles and white grains of finer felsic material.
Figure 11. Enclave C, fine-grained matrix, mixed with host magma.
Figure 12. Enclave C, fine-grained matrix, mixed with host magma.
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4.1.5. Enclave D
Enclave D has a white to light grey ground mass with 0.1 – 4 mm long mafic needles (figure 13, 14). Resembles enclave B, but has higher and coarser feldspar and quartz content. Medium-grained mix of mafic and felsic minerals. Coarser mafic minerals than enclave B. ~40 % mafic mineral composition.
Figure 13. Enclave D, intra granular rock with coarse feldspar, quartz and mafic minerals.
Figure 14. Example of coarser mafic minerals than in enclave B, see figure 9.
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4.1.6. Examples of mixed magma
Examples of described enclaves with mixed magma in them (figure 15, 16).
Figure 15. Enclave with host magma mixed in with enclaves of types B and D. Type B displays an angular shape and non-diffuse rims. Type D appears to have more rounded egdes than type B, and non-diffuse margins.
Figure 16. A small blob of magma type C in host magma. Example of enclave C as a rounded blob with diffuse rims.
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4.2. Maps of analyzed river outcrops
Four pictures of analyzed areas containing blobs, taken from north to south, photo (left) and map (right), with a scale bar of 68 cm (figure 17, 18, 19, 20).
River outcrop 1:
Figure 17. Photo and map of outcrop river 1. 1465 blobs are mapped in this outcrop in a total area of 195 m2.
River outcrop 2:
Figure 18. Photo and map of outcrop river 2. 429 blobs are mapped in this outcrop in a total area of 199 m2.
15 River outcrop 3:
Figure 19. Photo and map of outcrop river 3. 867 blobs are mapped in this outcrop in a total area of 224 m2.
River outcrop 4:
Figure 20. Photo and map of outcrop river 4. 81 blobs are mapped in this outcrop in a total area of 135 m2.
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4.3. Summary table
A summary table of the four river outcrops and their different results from the statistical analysis in Excel (table 1). The minimum mapped blob area was 0.0000063 m2 and the maximum mapped blob area was 50.4 m2.
Table 1. A summary table of a selective number of results from statistical analysis of the four mapped river outcrops.
River 1 River 2 River 3 River 4
Number of blobs 1465 429 867 81
Total mapped area [m2] 195 199 224 135
Comparative outcrop area [%] 145 148 167 100
Blobs per m2 13.8 4.8 5.4 0.7
% of blobs 54 45 71 82
Blob size max [m2] 4.5 8.2 50.4 40.0
Mean blob size [m2] 0.07 0.21 0.18 1.4
Median blob size [m2] 0.01 0.03 0.03 0.03
4.4. Size distribution
The statistical analysis of blob size distribution, for each outcrop image of the river, the bar graphs show that approximately 80 % of all the blobs have a surface area of 0 – 0.1 m2 (figure 21, 22, 23, 24).
Figure 21. The frequency size distribution of all the 1465 mapped blobs in outcrop river 1.
84,2
7,2 2,9 1,8 1,0 0,8 0,8 0,2 0,3 0,1 0,1 0,1 0,1 0,1 0,2 0,1 0,1 0,1 0
10 20 30 40 50 60 70 80 90 100
% of blobs
Surface area [m2]
River 1 - Size distribution
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Figure 22. The frequency size distribution of all the 429 mapped blobs in outcrop river 2.
Figure 23. The frequency size distribution of all the 867 mapped blobs in outcrop river 3.
77,4
9,3
3,3 1,9 1,9 0,2 0,5 0,2 0,5 0,7 0,5 0,2 0,5 0,7 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0,2 0
10 20 30 40 50 60 70 80 90 100
0-0,1 0,1-0,2 0,2-0,3 0,3-0,4 0,4-0,5 0,5-0,6 0,6-0,7 0,8-0,9 1-1,1 1,1-1,2 1,3-1,4 1,5-1,6 1,7-1,8 1,9-2 2-2,1 2,5-2,6 2,9-3 3,3-3,4 3,9-4 4,2-4,3 4,4-4,5 4,9-5 5,8-5,9 8,1-8,2
% of blobs
Surface area [m2]
River 2 - Size distribution
81,3
9,6
2,8 2,0 1,2 0,1 0,3 0,6 0,3 0,5 0,2 0,1 0,2 0,1 0,1 0,1 0,1 0,1 0,1 0,1 0
10 20 30 40 50 60 70 80 90 100
0-0,1 0,1-0,2 0,2-0,3 0,3-0,4 0,4-0,5 0,5-0,6 0,6-0,7 0,7-0,8 0,8-0,9 0,9-1 1-1,1 1,6-1,7 1,7-1,8 2,5-2,6 3,5-3,6 4,4-4,5 8,9-9 9-9,1 14-14,1 50,3-50,4
% of blobs
Surface area [m2]
River 3 - Size distribution
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Figure 24. The frequency size distribution of all the 81 mapped blobs in outcrop river 4.
4.5. Shape distribution
Most of the blobs seem to have an aspect ratio between approximately 0.3 – 1 (figure 25, 26, 27). This indicates that the blobs shape is more equant than elongated. The trend for river 1 indicates that smaller magma blobs tend to be equant. Thus, for river 2 and river 3 the trend line shows a slight significance for smaller magma blobs to be more equant-shaped, but as the trend line is almost horizontal, it can indicate that the magma blobs aspect ratio does not depend on size.
Figure 25. A histogram (left) showing the mapped blobs aspect ratio in outcrop river 1 and a scatter plot (right) showing each individual mapped blob size and its aspect ratio in outcrop river 1.
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5 1 2 2 1 1 1 1 1 1 1 1 1 1 1
0 10 20 30 40 50 60 70 80 90 100
% of blobs
Surface area [m2]
River 4 - Size distribution
0 5 10 15 20 25
0-0,1 0,1-0,2 0,2-0,3 0,3-0,4 0,4-0,5 0,5-0,6 0,6-0,7 0,7-0,8 0,8-0,9 0,9-1
% of blobs
Aspect ratio
River 1
y = 0,0322e-1,385x R² = 0,0213
0,00001 0,0001 0,001 0,01 0,1 1 10
0 0,2 0,4 0,6 0,8 1
Blob Size [m2]
Aspect ratio
River 1
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Figure 26. A histogram (left) showing the mapped blobs aspect ratio in outcrop river 2 and a scatter plot (right) showing each individual mapped blob size and its aspect ratio in outcrop river 2.
Figure 27. A histogram (left) showing the mapped blobs aspect ratio in outcrop river 3 and a scatter plot (right) showing each individual mapped blob size and its aspect ratio in outcrop river 3.
0 5 10 15 20
0-0,1 0,1-0,2 0,2-0,3 0,3-0,4 0,4-0,5 0,5-0,6 0,6-0,7 0,7-0,8 0,8-0,9 0,9-1
% of blobs
Aspect ratio
River 2 y = 0,0369e-0,177x
R² = 0,0004
0,0001 0,001 0,01 0,1 1 10
0,0 0,2 0,4 0,6 0,8 1,0
Blob Size [m2]
Aspect ratio
River 2
0 5 10 15 20
0-0,1 0,1-0,2 0,2-0,3 0,3-0,4 0,4-0,5 0,5-0,6 0,6-0,7 0,7-0,8 0,8-0,9 0,9-1
% of blobs
Aspect ratio
River 3 y = 0,0312e-0,137x
R² = 0,0003
0,00001 0,0001 0,001 0,01 0,1 1 10 100
0 0,2 0,4 0,6 0,8 1
Blob Size [m2]
Aspect ratio
River 3
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4.6. Form factor
The Thinnes ratio results show that the blobs tend to be more rounded than angular (figure 28, 29, 30). Most of the data demonstrates to have a Thinnes ratio value between 0.4 – 1, the overall trends indicate that smaller magma blobs are more rounded.
Figure 28. A histogram (left) showing the mapped blobs Thinnes ratio in outcrop river 1 and a scatter plot (right) showing each individual mapped blob size and its Thinnes ratio in outcrop river 1.
Figure 29. A histogram (left) showing the mapped blobs Thinnes ratio in outcrop river 2 and a scatter plot (right) showing each individual mapped blob size and its Thinnes ratio in outcrop river 2.
0 5 10 15 20 25
% of blobs
Thinnes ratio
River 1
y = 0,3803e-4,809x R² = 0,2081
0,00001 0,0001 0,001 0,01 0,1 1 10
0 0,2 0,4 0,6 0,8 1
Blob Size [m2]
Thinnes ratio
River 1
0 5 10 15 20 25
% of blobs
Thinnes ratio
River 2 y = 0,3864e-3,451x
R² = 0,1347
0,0001 0,001 0,01 0,1 1 10
0 0,2 0,4 0,6 0,8 1
Blob Size [m2]
Thinnes ratio
River 2
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Figure 30. A histogram (left) showing the mapped blobs Thinnes ratio in outcrop river 3 and a scatter plot (right) showing each individual mapped blob size and its Thinnes ratio in outcrop river 3.
5. Discussion
Southeast Iceland demonstrates a variety of felsic intrusions, in particular the
Reyðarátindur granophyre pluton with diverse magma interaction, in the form of more equant and rounded blobs of magma. The roundness and equant-shape may provide evidence for a mixing event happening through incremental formation from different magma batches, allowing new magma to be mixed in where the previous magma has not solidified yet. The discussion for this study has been divided into several sections, resembling the layout of the results.
5.1. Rock types
The five rock types described differ in the amount of mafic minerals and textures, causing a variation in rock color. The feldspar and quartz crystals varied in size between the rock types. The host magma obtained the largest phenocrysts of feldspar and quartz, producing a porphyritic texture and enclave C displayed the most fine-grained composition of quartz and feldspar. Enclave B and D both had a similar amount of mafic minerals and were equigranular, what separated the
enclaves were the size of the mafic mineral populations. Enclave A exhibit the largest mafic needles and were also equigranular.
5.1.1. Summary and interpretation
The host magma displays a spectrum of textural and color configuration, with a fine- grained matrix ranging from being white to light purplish grey. The large feldsparic and quartzitic phenocrysts is typical for granophyre rocks. Enclave C are mostly found within or together with the host magma as rounded blobs with chilled or diffuse rims (figure 11, 12, 16). This might indicate that magma type C was the first type of the four magma types, to have been mixed in with the host magma. The rounded shape and diffuse margins of type C decipher that the mixing happened while the host magma still was in its liquid phase.
Magma chambers form slowly, during long periods of time which allows for many opportunities of incremental heat pulses, initiating ascent of several different types of magma batches. This can explain why these five types of magma are found within
0 5 10 15 20 25
% of blobs
Thinnes ratio
River 3
y = 0,3439e-3,651x R² = 0,1554
0,00001 0,0001 0,001 0,01 0,1 1 10 100
0 0,2 0,4 0,6 0,8 1
Blob Size [m2]
Thinnes ratio
River 3
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the same pluton. The diffuse rims of some blobs, the similar composition, the mixing of the enclaves and the more rounded shape of the enclaves indicates that the rock types most likely have crystallized at approximately the same time. Enclave A, B and D all show the same mineral types and textures, with slightly different distribution of felsic and mafic mineral and different grain sizes. It is therefore likely for enclave A, B and D to be related. Further analysis via geochemistry and thin section is required to determine how and if the different blob types are related.
The mafic needles are believed to may have been transported from the lower parts of the magma chamber, as they crystallize first at a lower depth than the felsic
minerals, but supplementary examination on a geochemical level needs to be done to confirm this speculation.
5.1.2. Limitations
The enclaves chosen for descriptive analysis were limited to 1 – 4 examples for each rock type. This limitation was based on the most suitable enclaves for analysis and the quantity rock samples from the studied location.
5.2. Maps and statistical analysis
The selected areas for mapping blobs, were based on clarity of blobs in the photos.
The time frame for this thesis narrowed down the quantity of mapped photos.
5.2.1. Summary and interpretation
The blob size distribution for all four analyzed river images shows a significant abundance of 80 % in the size interval ranging from 0 – 0.1 m2. This might indicate that the mixing of magma mostly occurs through small batches and heat pulses of magma, or that the magmatic composition is limited to give rise to small blobs due to specific pressure and temperature gradients. The shape distribution points to the blobs being more equant and round, this further validates the event of magma mixing to have happened during a liquid stage. The percentage of blobs increased towards the central part of the pluton (figure 1 & 3), covering 54 % of the area in outcrop river 1 and 82 % of the area in outcrop river 4. Worth noting is that the smallest blob also is found in outcrop river 1 and the largest blob is found in outcrop river 3. This can be correlated to the percentage of blobs increasing towards the center of the pluton.
The appearance of more blobs in the center, can be compared to Weidendorfer et al.
(2014) description of more reduced temperature gradients in the inner section of the Austurhorn intrusion. A more reduced temperature gradient would allow for more stable temperature conditions which might have produced more magma mixing and mingling. This would result in a warmer center and a cooler outer section. To further compare with Weidendorfer et al. (2014) describing more elongated enclaves with rims alternating between being chilled and non-chilled in the Austurhorn intermediate section of the intrusion, and outcrop river 1, which displays these features as well.
This comparison can propose that outcrop river 1 is located in the intermediate section of the Reyðarártindur intrusion. Since outcrop river 1 has a high number of small blobs, it will affect the shape distribution result as seen in figure 28, where small blobs tend to be slightly more equant and round, out-ruling the larger blobs being elongated (figure 17).
Textural and shape-wise; some of the more elongated blobs in figure 17, 18, 19, 20, appear to have been disconnected from each other, probably by being fractured
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and separated by the host magma dispersing it. Another point to mention is the fact that these blobs only are found in the river, which is our bottommost exposed area of the magma chamber. The Reyðarártindur pluton is a shallow felsic magma chamber, and the bottom exposed area of the pluton can hopefully provide information about how the magma was entering and evidence on the magma mixing properties.
Geochemical analysis on the river bed samples is therefore required to be able to give a more certain explanation on the magmatic event. The blobs may be
systematically distributed across the river, since the analyzed data from the four river outcrops differ, and the percentage of blobs increase downstream, towards the central section of the Reyðarátindur magma chamber
5.2.2. Limitations
River 4 was excluded from the shape distribution results, as one of the largest blobs in the image got cut off by the camera. This would have made the shape distribution comparisons wrong between the four images, as the blobs true shape would not have been analyzed. Another problem with the shape distribution data is that some of the blobs near the edges of the images were cut off by the camera taking the photos.
A reason for not excluding all the pictures but river 4 is the quantity of blobs. River 4 got only 81 measured blobs whereas River 1, 2 and 3 got 1465, 429 and 867 blobs respectively available for analysis. We chose to keep all the blobs for size distribution analysis as some of the blobs who were being cut by the edges had a large area which would have disappeared if removed. This is also a reason for not including river 4 in the shape distribution analysis as one of the blobs being cut by the edge constituted approximately a third of the analyzed area.
For future studies a recommendation is to exclude all the blobs touching the edges while working in Inkscape. That would give an opportunity to mark the excluded blobs as an excluded area from the mapped river area. We could have removed the blobs that touched the edges in ImageJ but then we would not have the true host magma area, as the excluded blob would be added to the host magma area, which is not a true observation of the picture taken. It is therefore better to mark it in Inkscape as an excluded area, because then the area will not be calculated as mapped host magma area in ImageJ. For further development of these results, more photos should be taken into account when analyzing the blob shape. More photos would provide additional data for the shapes of the blobs being cut by the edges of the photos.
There is a larger number of blobs in outcrop river 1 (1465 blobs), with a lot of small blobs, which affect the median and mean blob size results. This also have to be taken into account when analyzing the magmatic formation.
5.3. Outlook and recommendations for future studies
The methods used for this thesis study have operated well and with minor changes on how they were used, these methods might efficiently provide extensive results for further studies. A recommendation is to change the scalebar to meter directly in ImageJ instead of in excel, it will save time. Additional development can be made by analyzing the adjacent photos of the four selected river photos, to generate more accurate data.
To continue the study, geochemistry on the rock samples would be necessary.
The geochemistry analysis would give a more precise result on how the mixing
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events and dynamics occurred and if the rock samples are related. Textural analysis can only provide information about how the rocks look and if they display the same features. To say something about the composition and what minerals occur in the rock samples, thin sections of the rock samples and geochemical data are needed.
Textural analysis indicated that enclave C formed while the host magma was crystallizing or that they crystallized in some parts at the same time. Geochemical examination, could further confirm or disconfirm these conclusions.
6. Conclusions
The results for this study point to a more rounded and equant shape of the magma blobs and are therefore argued to have been mixed in with each other at a liquid stage. The host magma and enclave C have probably formed at approximately the same time. Enclave A, B and D appears to be of same origin as they exhibit the same mineral composition, where the mineral distribution and grain-size vary between the three. No relationship between the size and shape distribution of the magma blobs was distinguished. To elaborate the conclusion of this thesis, both geochemical analysis on the rocks’ composition and statistical analysis of several photos would preferably have been made to get a better estimation on the frequency size and shape distribution of the blobs in the Reyðarártindur river area.
7. Acknowledgements
I would like to thank my supervisors – Ph.D. student Emma Rhodes and Doc. Steffi Burchardt for all the guidance and help I have gotten during this thesis study.
8. References
Bachmann, O., Miller, C.F. & de Silva, S.L. (2007). The volcanic–plutonic connection as a stage for understanding crustal magmatism. Journal of Volcanology and Geothermal Research, vol. 167 (1), pp. 1–23 (Large Silicic Magma Systems).
Blatt, H., Tracy, R.J. & Owens, B.E. (2006). Petrology: igneous, sedimentary and metamorphic. 3. New York: Freeman.
Burchardt, S. (2009) Mechanisms of magma emplacement in the upper crust.
Göttingen, Univ., Diss., 2009 Online version available at:
http://webdoc.sub.gwdg.de/diss/2009/burchardt/burchardt.pdf
Burchardt, S., Tanner, D.C. & Krumbholz, M. (2010). Mode of emplacement of the Slaufrudalur Pluton, Southeast Iceland inferred from three-dimensional GPS mapping and model building. Tectonophysics, vol. 480 (1), pp. 232–240.
Burchardt, S., Tanner, D. & Krumbholz, M. (2012). The Slaufrudalur pluton, southeast Iceland—An example of shallow magma emplacement by coupled cauldron
subsidence and magmatic stoping. GSA Bulletin, vol. 124 (1–2), pp. 213–227.
Cargill, H.K., Hawkes, L. & Ledeboer, J.A. (1928). The Major Intrusions of South- Eastern Iceland. Quarterly Journal of the Geological Society, vol. 84 (1–4), pp.
505–535.
Furman, T., Meyer, P.S. & Frey, F. (1992). Evolution of Icelandic central volcanoes:
evidence from the Austurhorn intrusion, southeastern Iceland. Bulletin of Volcanology, vol. 55 (1), pp. 45–62.
Gudmundsson, A.T. (2013). Living Earth – Outline of the Geology of Iceland. 1 ed.
Reykjavík: Mál og menning.
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Morgan, S. (2018). Pascal’s Principle, a Simple Model to Explain the Emplacement of Laccoliths and Some Mid-Crustal Plutons. I: Burchardt, S., Volcanic and Igneous Plumbing Systems – Understanding Magma Transport, Storage, and Evolution in the Earth´s Crust. Amsterdam: Elsevier, pp. 139 – 161.
Padilla, A. (2015). Elemental and isotopic geochemistry of crystal-melt systems:
Elucidating the construction and evolution of silicic magmas in the shallow crust, using examples from southeast Iceland and southwest USA [PhD Dissertation:
Vanderbilt University].
Petford, N., Cruden, A.R., McCaffrey, K.J.W. & Vigneresse, J.-L. (2000). Granite magma formation, transport and emplacement in the Earth’s crust. Nature, vol.
408 (6813), pp. 669–673.
Van der Pluijm, B.A. & Marshak, S. (2004). Earth structure: an introduction to structural geology and tectonics. 2nd ed. New York: W.W. Norton.
Weidendorfer, D., Mattsson, H.B. & Ulmer, P. (2014). Dynamics of Magma Mixing in Partially Crystallized Magma Chambers: Textural and Petrological Constraints from the Basal Complex of the Austurhorn Intrusion (SE Iceland). Journal of Petrology, vol. 55 (9), pp. 1865–1903.
Wiebe, R.A. (1994). Silicic Magma Chambers as Traps for Basaltic Magmas: The Cadillac Mountain Intrusive Complex, Mount Desert Island, Maine. The Journal of Geology, vol. 102 (4), pp. 423–437.
Wyk de Vries, B., Wyk de Vries, M. (2018). Tectonics and Volcanic and Igneous Plumbing Systems. I: Burchardt, S., Volcanic and Igneous Plumbing Systems – Understanding Magma Transport, Storage, and Evolution in the Earth´s Crust.
Amsterdam: Elsevier, pp. 191 – 221.
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9. Appendix
Appendix 1. The mixing of different magmas during the formation of magma chambers
Sabine Rousku
Popular Science Summary of Independent Project in Earth Science 2019 Department of Earth Sciences, Uppsala University
Iceland is an island created through both tectonic and volcanic geological activity in the north Atlantic Ocean. The island’s geology is therefore of high interest to
scientists. Iceland lies on the Mid-Atlantic Ocean Ridge, which is created as a result of two diverging tectonic plates, the North American plate and the Eurasian plate.
Divergence of the two plates and a mantle plume under southeast-central Iceland cause spreading of the ocean floor and the rise of molten rock material from the mantle to penetrate through the oceanic crust. The molten rock is called magma. A mantle plume is a pool of partially molten rock beneath the Earth’s crust,
accumulating magma and pressure, which can result in ascent of magma, and conclusively end with a volcanic eruption. Imagine a rising oil-blob in water.
The volcanic activity that results from the presence of magma in Iceland has shaped Iceland’s geology and transformed the landscape.
Why is it important to study magma chambers?
Magmatism generates different types of magma bodies or chambers in the Earth´s crust. Magma chambers grow through addition of many small batches of magma and can function as a magma source for volcanic eruptions. It is therefore not unusual for different magmas to be mixed in a magma chamber. When the magma chamber cools down, it starts to crystallize. Solidified magma bodies are usually called plutons.
When a pluton is fully crystallized it can become visible at the Earth´s surface through erosion.
It is important to study plutons since we cannot study magma chambers directly in active volcanoes. We cannot say how volcanoes form and work without studying magma chambers. If we know more about the pluton structure, we can ideally help to predict if an eruption is about to happen and prevent potentially dangerous situations.
In Southeast Iceland, Reydarátindur is a magma chamber that has never been described in detail before. The magma chamber has been eroded by glaciers and a river, exposing the interaction between different magma types. These magma types are revealed as blob-like structures in the river bed.
With this study we want to characterize the different magma batches. In particular, we want to find out if there is a frequency size and shape distribution of the magma blobs and if the different magma blobs are systematically distributed in selected parts of the river. The results of the magma blobs distribution could then tell us how the magma was mixing, where in the magma chamber new magma from deeper below was entering, and how magma chamber processes formed the magma chamber.
