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European Journal of Applied Sciences – Vol. 12, No. 3
Publication Date: June 25, 2024
DOI:10.14738/aivp.123.17110.
Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The
Faroe Islands. European Journal of Applied Sciences, Vol - 12(3). 570-579.
Services for Science and Education – United Kingdom
Sill Propagation and Climbing in Layered Crystalline Host-Rocks:
Examples from Saucer-Shaped Sills of The Faroe Islands
Jógvan Hansen
HansenReSearch, Undir Heygnum 8, FO 100 Tórshavn, Faroe Islands
ABSTRACT
Mafic sills, which commonly occur in layered sedimentary and crystalline settings
worldwide, may occur as sub-lateral sheets or as saucer-shaped bodies. Values
and distributions of Young’s Modulus within their ambient host-rocks determine
their mode of emplacement. Current models on development of saucer-shaped
sills depict either melt propagation from single sources along sub-lateral
relatively weak layers, from which they abruptly climb/transgress through
stronger layers at intervals, or they may evolve by radial melt
propagation/intrusion from one or more sources, while gradually/continuously
ascending/climbing through strong and weak layers alike. The first model invokes
involvement of sill overburdens and overlying free surfaces, while the latter
envisages closed igneous systems, where host-rocks both above and below the
advancing magmas are affected without involvement of overlying free surfaces.
Margins of saucer-shaped sills at various stages of developments, cropping out in
the FaroeIslands, offer some new insights into sill evolvement in layered
crystalline host-rocks. This study suggests that the slightly upward-curving
geometries of Faroese sills stem from initial radial propagations/intrusions of thin
magma fronts, where systematic depth-dependent variations of Young’s Modulus
in the Earth’s crust governed gradual and continuous climbing of these. Some of
their melts likely propagated initially as lobes or thin magma-fingers in a mole- like fashion before coalescing, without noticeably affecting the overlying free
surfaces prior to the main inflation phases.
Keywords: Faroe Islands, flood basalts, sill intrusion, sill propagation, sill climbing
INTRODUCTION
Basaltic sills, displaying a variety of geometries and sizes, have been reported in numerous
extension/rifting related settings worldwide, the most common of which being sedimentary
basins and basaltic lava piles (1,2). Sills may occur individually, but do not un-commonly
occur in droves, where individual intrusions can be interconnected and are thus often
thought to have acted as reservoirs and conduits during magma transport to the Earth’s
surface (1,2,3,4,5,6). As sill intrusions reside in both offshore and onshore environments,
investigations of these are in general based on seismic images/profiles for the former and on
direct visual observations and measurements for the latter versions. Although widespread sill
networks can be highly visible in seismic images from offshore sedimentary basins (7,8),
direct investigation of sill networks and/or individual sills, cropping out onshore, may better
disclose details in their developing history.
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Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The Faroe Islands.
European Journal of Applied Sciences, Vol - 12(3). 570-579.
URL: http://dx.doi.org/10.14738/aivp.123.17110
A mechanism with sub-lateral melt propagation along weak layers with intermittent
transgressions/climbing through stronger stratum, i.e. melt advancement through layers with
varying values of Young’s Modulus, has frequently been invoked for development of saucer
shaped sills in stratified sedimentary media (9). Interaction between advancing melts and
overlying free surfaces during sill formation is commonly envisaged for sill emplacement in
relatively shallow crustal environments, where certain sill lengths/diameters versus their
depths of intrusion are thought to determine their angles of transgression/climbing and
hence their ultimate sizes (6,9). An alternative theory, applicable for sill intrusion into
layered crystalline media, such as basaltic lava successions of various thicknesses, which are
occasionally separated by thinner sedimentary/volcaniclastic sequences, proposes
propagation of thin magma fronts, where uneven displacement of strata above and below
advancing sill tips determine their angles of gradual and continuous transgression/climbing
(1,4,10). These authors did not envisage any noticeable interaction between the advancing
melts and overlying free surfaces prior to any main inflation phases. This latter model relies
on the fact that systematic depth dependent variations in values of Young’s Modulus have
been reported for the Earth’s crust previously (11).
In this study, we focus on well-exposed margins of a few Faroese sills of various thicknesses.
Field evidences suggest that at least some of their initial magmas progressed in a radial
fashion in part as lobes or as broad fingers prior to ultimately coalescing.
GEOLOGY OF THE FAROE ISLANDS
The basaltic lava pile, making up the archipelago of the Faroe Islands, which are situated at
the NW European margin hosts a number of saucer-shaped sills of various sizes, which are
‘frozen in time’ in various stages of developments (1,4,10). In general, most of the eight
formations making up these basaltic successions are intruded by a host of sub-vertical dykes
and numerous irregular intrusions, while the sills of this study crop out in the three
uppermost formations (Fig. 1).
The sills of the Faroe Islands were emplaced in an extensional/rifting related geological
environment during a relatively wide time span from ~55.5 Ma to ~50.5 Ma (12). These sills
are typified in that they decreased in sizes and volumes with time, i.e. the largest sills are
~55.5 Ma old with younger ones decreasing in size until intrusion of the least voluminous at
~50.5 Ma (12). The oldest Faroese sills and sill segments decrease in thicknesses from the SE
towards the NW, which is also consistent with intrusion at increasingly greater
stratigraphically depths from the SE towards the NW (1,4,10,12).
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Figure 1. Brief description of the general area and locations utilised in this study (Modified
from: Hansen and Ganerød, 2023). a, Geographical location of the Faroe Islands, based on
Google Earth. b, Geological map of the Faroe Islands, based on topographic data from Munin fo,
displays local geological formations and geo-locations of the local saucer-shaped sills of this
study. c, Close-up view of the three sills included in this contribution, based on topographic
data from Munin fo, where small black circles indicate exact localities.
FIELD EVIDENCES AND MEASUREMENTS
The sill margins being focused on in this study display thicknesses of 0.35 to 0.60 metres for
the Kvívík Sill, 1.5 metres for the Vestmanna Sill and 15 metres for the Streymoy Sill. If it
is assumed that the uppermost Faroese Enni Formation (Fig. 1) had an original average
thickness of ~1 to ~1 1⁄2 km (13), then the melts that gave rise to the fringes of the sills, which
are utilised in this contribution, originally crystallised at stratigraphical depths of ~1 to ~1 1⁄2
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Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The Faroe Islands.
European Journal of Applied Sciences, Vol - 12(3). 570-579.
URL: http://dx.doi.org/10.14738/aivp.123.17110
Figure 2. Images of a very thin margin of the Kvívík Sill in the valley of Gassádal. a, slightly
inclined thin sill lobes ( 3-4 dip towards ESE), being 0.5 m thick, arranged in en-echelon
fashion, which are more or less connected by a moderately inclined ‘dykelets’. Yellow dashed
lines indicate sill outlines. b, ‘Dykelets’ become thinner with decreasing depths. c, Closer view
of b, with inset showing close-up view of dykelets.
km as regards the ~55.5 Ma old Streymoy Sill and ~1.4 to ~1.9 km as regard the ~55.5 Ma
Kvívík Sill and the ~50.5 Ma Vestmanna Sill. Hence, the studied thin margins of the Kvívík and
Vestmanna sills crystallised at 400 to 500 metres greater stratigraphical depths when
compared to the much thicker studied margin of the Streymoy Sill. The characteristics
displayed by these three sill margins/fringes are described below.
The actual locality of the Kvívík Sill is characterised by discontinuous slightly inclined thin sill
lobes, which are more or less arranged in en-echelon fashions (Fig. 2a). Here, the front of one
exemplified section to the right (SE) is connected to the tail of the next one at slightly lower
elevation to the left (NW) by a number of very thin dykes/ ‘dykelets’ (Fig. 2b). It is
noteworthy that thicknesses and density of ‘dykelets’ decrease slightly downwards from the
SE towards the NW (Fig. 2c).
The studied margin of the Vestmanna Sill represents a cross section of two relatively thin
sub-horizontal magmatic lobes exposed at slightly different elevations, with the higher to the
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left (NW) and the lower to the right (SE) (Fig. 3a). These two lobes are connected ‘end to end’
by a much thinner sub-vertical ‘connection’ (Fig. 3b).
Figure 3: Images of a thin margin of the Vestmanna Sill on the mountain of Hægstafjall. a, Cross- section of thin ( 1.5 m thick) roughly sub-horizontal sill lobes arranged at slightly different
altitudes. Yellow dashed lines indicate lobe outlines. b, the sill lobes are connected by an even
thinner ( 0.35 m thick) short sub-vertical section.
The studied margin of the Streymoy Sill is characterised by a thick and continuous margin to
the left (SE), with a somewhat thinner and lower-lying lens-shaped margin/section to the
right (NW). These are separated by a belt of host-rocks measuring around 15 to 20 metres
(Fig. 4a). These two margin sections are more or less connected by a multi-dike, emanating
sub-vertically upwards from the SE tip of the lens-shaped lower margin and sub-vertically
downwards from the bottom of the upper margin. The uppermost parts of this multi-dyke,
consisting of 4 to 5 thin individual dykes/’dykelets’, extend downwards for ~1/4 of the total
distance from the upper margin, while ~12 closely spaced individual thin dykes/’dykelets’,
with individual thicknesses from a few centimetres to a maximum of ~12 cm with a
combined maximum width of between 1.6 to 1.8 metres for the entire multi-dyke, extend
upwards for ~3/4 of the total distance from the lower margin (Fig. 4b, c).
Strike and dip measurements suggest that initial magma flows progressed more or less from
right to left for the Kvívík Sill (Fig. 2), while initial melts broadly progressed towards the
reader at slight upwards angles in the other two sill margins (Fig. 3; Fig. 4).
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Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The Faroe Islands.
European Journal of Applied Sciences, Vol - 12(3). 570-579.
URL: http://dx.doi.org/10.14738/aivp.123.17110
Figure 4: Images of the thick margin of the Streymoy Sill in the valley of Hundsarabotn. a,
Cross-section of a more than ~20 m thick sill margin, moderately inclined ( ~5° dip.) towards
the SW arranged as a lens-shaped body to the right and a continuous section to the left at
slightly higher altitude being separated by a belt of hostrocks. The margin/sections are more
or less connected by a sub-vertical multi-dyke. Yellow dashed lines indicate outlines of sill
margin/sections. b. A few ‘dykelets’ of the upper1/4 of the multi-dyke. c, Numerous ‘dykelets’
of the lower 3/4 of the multi-dyke.
MODES OF SILL INTRUSION
Based on some of the noticeable characteristics displayed by the investigated sill
margins/rims of this contribution, a few inferences can be put forward.
The actual margin of the Streymoy Sill is thick and this intrusion was probably fully
developed, while the studied margins of the basal sections of the Vestmanna and Kvívík sills
are thin and were quite likely only in their infancy once they crystallised at deeper
stratigraphical levels. It is noticeable that the upper rims of e.g. the Kvívík Sill a few
kilometres towards the ENE (Fig. 1, not shown) are much thicker (£ 15 m), thus giving this sill
an overall wedge-shaped appearance, a feature also displayed by the Faroese Morskranes and
Sundini sills (1,10) (Fig. 1b).
The multi-dykes/’dykelets’, associated with the margins of the Kvívík and Streymoy sills of
this study reveal details on initial principal stress axes in host-rocks around their advancing
tips and on either side of the evolving intrusions. During sill intrusion prior to any breach of
overlaying free surfaces two sets of principal stress axes are in existence within their
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Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The Faroe Islands.
European Journal of Applied Sciences, Vol - 12(3). 570-579.
URL: http://dx.doi.org/10.14738/aivp.123.17110
provide an ample proof of the compression of host-rocks below thin advancing sub- horizontal magma tips/fronts, and not only above them. The restricted thicknesses of the
actual lobes of the Kvívík Sill in particular and their relatively great stratigraphical depths
strongly suggest that their initial melts advanced and climbed as slightly inclined lobes or
fingers in a molelike fashion (prior to ultimately joining together) without interacting with
the Earth’s free surface. Here, general depth-dependent variations in Young’s Modulus in the
Earth’s crust (11), where host-rocks atop thin advancing magma fronts are displaced slightly
more than those below these due to uneven values of Young’s Modulus, most probably caused
the magma lobes/fingers to advance and climb at slight angles, as envisaged in previous
studies (1,4,10) (Fig. 5a, b).
When it comes to the actual margin of the Vestmanna Sill, it could represent a slight evolution
from its thinner counterpart in the Kvívík Sill, where the thin connecting section either
represents a single ‘dykelet’ that has inflated to ~0.35 metres or it may represent several
‘dykelets’ that ultimately coalesced into one following displacement of initial slivers of host- rocks in between (Fig. 3; Fig. 5c).
Figure 6: Simplified drawings depicting sill evolution in basaltic lava successions (Modified
from: Hansen, 2011, Hansen et al., 2011). a, Fully developed sill geometry from propagation of
thin magma fronts in a closed system. b, Sill overburden is breached atop its highest point and
inflation has commenced. The Faroese Kvívík, Sundini and Morskranes sills display similar
wedge-shaped geometries. Small open circle denoted KS points to studied margin of the Kvivik
Sill while that being denoted VS points to studied margin of the Vestmanna Sill. c, Sill
overburden is breached atop either ends and intrusion is fully inflated. The Faroese Streymoy,
Eysturoy and Svínoy-Fugloy sills largely display similar geometries. Small open circle denoted
SS points to the studied margin of the Streymoy Sill.
It is overwhelmingly likely that the actual margin of the Streymoy Sill, situated at a relatively
high stratigraphic level, displaced and breached its overburden and the Earth’s free surface
during inflation. This scenario, where tensile strengths in host-rocks on top of the inflating sill
no longer played a vital role, most likely caused a reorganisation of local principal stress axes,
which is reflected by the orientation and overall asymmetrical nature of the actual subvertical
multi-dyke (Fig. 4; Fig. 5d). Here, the greater density and lengths of ‘dykelets’ emanating
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upwards from the lower-laying lens-shaped sill margin, as compared to those stretching
downwards from the higher sill margin, point to interaction with the overlaying free surface
during sill inflation. Accordingly, the probable breaching of the overlaying free surface likely
interfered with the local largest principal stress axis 1 between the two inflating sill sections,
so as to reduce compression below both of these during the final phases of their
evolvement/inflation. Therefore, host-rocks above these margins were relatively more
compressed by propagation/inflation compared to those below them (Fig. 4; Fig. 5d).
CONCLUDING REMARKS
In conclusion, the following statements may be put forward: Sill margins of this study
crystallised at various stratigraphical levels and at various stages of development. The
characteristics displayed by these and associated multi-dykes may disclose general facts on
evolution patterns of basaltic sills intruded into volcanic lava successions. Here, the
characteristics of the thinner sill margins point to development by slight climbing of thin
magma fronts across strong and weaker layers alike in response to general depthdependent
variations in values of Young’s Modulus in the Earth’s upper crust. Accordingly, the greater
parts of sill geometries in such settings may form prior to any involvement or breaching of
overburdens and overlaying free surfaces. Once sill overburdens and overlaying free surfaces
are indeed breached, general sill inflation will take place (Fig. 6a-c).
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Hansen, J. (2024). Sill Propagation and Climbing in Layered Crystalline Host-Rocks: Examples from Saucer-Shaped Sills of The Faroe Islands.
European Journal of Applied Sciences, Vol - 12(3). 570-579.
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