Dotted lines indicate continued, but diminished fragmentation. The textures used to construct the model are described

Figure 7.1 Intrusion of Basalt into Frozen Sediments and Generation of Coherent-Margined

Volcaniclastic Dikes (CMVDs). ... 233

Figure 7.2 Images of examples of CMVDs. Common features are the consistent glassy margins

and complex volcaniclastic interiors. A) Example from main area of CMVD distribution, the

segments on the scale are 10 cm. Note the lack of mechanical or thermal alteration of the host

sediment by the dike. B) Example of CMVD from western massif of Askja (17 km from main

cluster), backpack for scale. ... 235

Figure 7.3 Dike margins in hand sample and thin section. Field scale increments are 1 cm. A)

CMVD chill margin hand sample and sketch showing textures. B) Petrographic image of the

outer margin of the dike. Note the relict clast, a product of shear. C) Petrographic image of

interior of dike margin. D) Hand sample of dike margin revealing multiple chills. E) Hand

sample of interior portion of margin displaying complex ropey structures. F) Hand sample of

CMVD margin in cross-section the reveals the variations in vesicularity within the margin. ... 237

Figure 7.4 Dike interiors in outcrop and thin section. Yellow lines indicate dike margins; white

lines indicate pillows. A) CMVD with pillows and vitric ash interior. B) CMVD with pillow and

pillow breccia interior. C) Ash-rich CMVD with deformation of ash banding (parallel to strike)

around larger clasts. D) Detailed sketch of the interior of a pillow-dominated CMVD with

strike-parallel bands of ash-sized particles between the pillows and the chill margin, as detailed in the

inset. ... 240

Figure 7.5 C MVD with ash and lapilli dominated interior. Sketch indicates zones of clast

concentration with white lines to indicate the weak structure present within the domains. The

sedimentary structure of the host sediment is not disrupted by the dike. Complex margins

indicate multiple magma pulses that resulted in the trapping of intact host sediment between an

earlier chill margin and a later, final margin which truncates it. ... 241

Figure 7.6 Petrographic image of CMVD ash interior matrix with variable grain shapes and

intact vesicular clasts mixed with blocky grain shapes. B) Petrographic image of typical CMVD

ash tuff host matrix. Grains are well-sorted and shapes reflect fewer vesicles than CMVD

interiors. Free crystals of pyroxene and feldspar, similar in size to the average grain size, are

visible in cross-polarized light (xpl) in dike host sediments only. ... 242

Figure 7.7 Example of a CMVD transition from coherent basaltic to volcaniclastic dike interior

with coherent glassy margins (double lines). Radial cooling cracks are found within the dense

dike below the transition. The correlation of a stratigraphic boundary between a massive lapilli

tuff and ash tuff with the transition (horizontal line) is limited to this CMVD. ... 243

Figure 7.8 Field image and B) sketch of the Rosa structure, a cylindrical void (outlined) in a 90

cm wide tabular coherent basaltic dike that transitions to a CMVD up dip. View is perpendicular

to strike. The void is symmetrical with thick (3 cm) glassy margins and does not disrupt the dike

width or the host. This is interpreted as the result of a cryolith (ice-block) that fell into a drained

dike after margin formation. Subsequent dike pulses chilled around and thermally eroded the

cryolith and associated vapor, creating a round void. ... 244

Figure 7.9 Model of CMVD formation in ice-cemented host (ash / lapilli). A-1) Chill margins

form along a rising basaltic dike. The ice-cemented host and overlying ice fracture. The gas

driven pulses of magma depressurize near the host / ice / meltwater interface. A-2) Dike drainage

creates space, allowing downward flooding of the dike. A-3) Meltwater and magma interact

non-explosively, forming a slurry. A-4) Later pulses interact with the slurry; mingling continues. B)

Motion of magmatic gas, steam and clasts develop near-vertical flow banding. A final pulse is

chilled against the interior, resulting in radial cooling cracks. Evidence of the preceding steps

may be preserved in the CMVD (labels). C) Formation of a very thin peperite between chill

margin and ice-cemented host. D) Dike behavior when cryolith becomes trapped in the drained

dike, forming a Rosa structure. ... 252

Figure 8.1 Oceanographic climactic influences on Iceland, adapted from Knudsen and Eriksson,

2002. ... 258

Figure 8.2 Modeled extent of the Last Glacial Maximum ice sheet in Iceland adapted from

Hubbard et al. (2006). Numbers indicate feature ages in thousands of years, and references are

noted. The location of offshore end-moraines is illustrated. The position of Askja was fairly

central to this large ice sheet. ... 261

Figure 8.3 Modeled ice flow models for the LGM ice sheet from Bourgeois et al. (2000). ... 265

Figure 8.4 Location of paleoenvironmental indicators overlain on c hemostratigraphy map of

Austurfjöll. The largest paleo-water level is at 1290 m a sl. Arrows indicate the orientation of

drainage channels and proposed direction of flow. ... 271

Figure 8.5 Example of ice confined lava in Unit 6 at an elevation of 850 m asl. The orientation of

the radial cooling cracks can be used to estimate a cooling surface that likely reflects the position

of ice around the flow. The cooling surface may be a thin water film, or direct ice contact. ... 274

Figure 8.6 Exposure of subaerial lavas buried by subaqueous pillow lavas at 800 m elevation in

eruptive Unit 2. White lines indicate outlines of pillows in the outcrop. The angle of the image

does not show the thin (20 cm) of bedded ash between the two deposits. ... 276

Figure 8.7 Coated lapilli in subaerial lapilli tuff (Lt3). Divisions on black and white scale are 1

cm. ... 277

Figure 8.8 Cartoon of ice thickness and relative position at Austurfjöll massif during construction

for each eruptive unit. Elements of greatest speculation are denoted with a question mark in the

color or arrow appropriate to the features in question. Lake size is intended to suggest order of

magnitude scale only. ... 279

Figure 8.9 Location of samples for volatile analysis. Samples either occur in Unit 2 or Unit 5.