Jökull


Jökull - 01.11.1998, Blaðsíða 6

Jökull - 01.11.1998, Blaðsíða 6
naissance field observations and measurements from a single soil profile, containing historic and pre-historic tephra layers. This paper presents new tephro- chronological and radiometric age data that show un- equivocally the age of the Leiðólfsfell cone group and thereby its origin. A full account on the geology of the cone group will be presented elsewhere. LEIÐÓLFSFELL CONE GROUP AND LAKI LAVA FLOW The Laki lava flow almost completely covers the floor of the river valley to the north and west of Mt. Leiðólfsfell. It is composed of two distinct morpholog- ical lava types; the ‘a‘a-like main lava stream which branches into an association of relatively small slab pahoehoe flow units (for definition see Wentworth and Macdonald, 1953). The boundary between the ‘a‘a and pahoehoe is a 10-20 m high barrier, delineated by the largest scoria cones and ramparts of the Leiðólfsfell cone group, now forming a 300-400 m wide stretch of discontinuous outcrops along the edge of the main lava stream. These outcrops extend from the NW foot of Leiðólfsfell towards Hólahaft in the SW, where they turn lo the NNW across the valley in the direction of Sæmundarsker (Fig. 1). The outcrops reveal erratic as- sociations of partly overlapping scoria cones and ram- parts (50-300 m in basal diameter) and numerous small explosion craters (5 to 30 m in diameter), which are the surface manifestation of a larger cone field, which now is largely buried by the lava. The main lava stream is piled up against the up-valley flank of the cone field with a main outflow channel in the north. Elsewhere at points of low elevation it barely extends over the crest of the cone field. The slab páhoehoe flow units, howev- er, formed as multiple breakouts of lava from the edge of the main lava stream. These almost completely cover the down-flow side of the cone field with the ex- ception of several kipukas (i.e., óbrynnishólmi) which are mantled by the scoria fall deposit produced by the explosive activity of the Leiðólfsfell cones (Fig. 1). Outcrops along the edges of these kipukas, and at a re- cent quarry, show that the Laki lava rests directly on the scoria (Fig. 2). Nowhere did we find any evidence of soil, other sediments, or remains of vegetation be- tween the two. THE HISTORIC TEPHRA SEQUENCE IN SOIL PROFILES AROUND LEIÐÓLFSFELL The Leiðólfsfell scoria deposit and other historic tephra layers were examined at outcrops within the kipukas and in soil banks in the area to the south of the Laki lava flow field. Fifteen profiles were mea- sured, documenting thicknesses and other distin- guishing features of the tephra deposits (Fig. 1). With- in the largest kipuka in the center of the flow field (Location 1 on Fig. 1), the Leiðólfsfell scoria fall de- posit is 1-3 m thick and rests unconformably on an eroded soil horizon containing prehistoric tephra lay- ers (see Fig. 3 in Jónsson, 1985). However, further south at Hólahaft (Location 2 on Fig. 1), the scoria deposit is 1.1-1.7 m thick, and resting directly on soil contaning numerous tephra layers and is covered by several cm of topsoil containing the 1918 Katla tephra layer. The soil below the Leiðólfsfell scoria contains both historic and pre-historic tephra layers, including the Settlement layer (AD -870; Grönvold et al., 1995) which marks the boundary between the two time peri- ods (Fig. 3). At Location 2 the Leiðólfsfell scoria deposit con- sists of eight 10-35 cm thick fall units of dark gray to black tephra, although the top two units are reddish- brown due to high contents of oxidized clasts. Each unit consists of poorly sorted, fine to coarse scoria lapilli and are inversely graded (Fig. 4). Scattered 6- 30 cm poorly vesicular scoria and spatter bombs occur near the top of each unit and these bombs often have breadcrust surfaces. The clast population is pre- dominantly comprised of dense (<30 vol.% void space) juvenile lapilli (85%) and hypocrystalline lava crust fragments (15%). Armored bombs are common, featuring a core of vesicular páhoehoe crust frag- ments and an armor of poorly vesicular glassy lava. The major element concentrations of the core and armor are identical (Table 1) and the composition of the Leiðólfsfell scoria is cognate to that of the Laki lava (Þórðarson et al., 1996). The Leiðólfsfell scoria rests on a 0.3-0.5 cm thick strombolian ash layer con- taining Pele's hair and glassy clasts with fluidal outer surfaces and highly vesicular interior (60-90 vol.% voids). Chemical analyses by electron microprobe 4 JOKULL, No. 46, 1998
Blaðsíða 1
Blaðsíða 2
Blaðsíða 3
Blaðsíða 4
Blaðsíða 5
Blaðsíða 6
Blaðsíða 7
Blaðsíða 8
Blaðsíða 9
Blaðsíða 10
Blaðsíða 11
Blaðsíða 12
Blaðsíða 13
Blaðsíða 14
Blaðsíða 15
Blaðsíða 16
Blaðsíða 17
Blaðsíða 18
Blaðsíða 19
Blaðsíða 20
Blaðsíða 21
Blaðsíða 22
Blaðsíða 23
Blaðsíða 24
Blaðsíða 25
Blaðsíða 26
Blaðsíða 27
Blaðsíða 28
Blaðsíða 29
Blaðsíða 30
Blaðsíða 31
Blaðsíða 32
Blaðsíða 33
Blaðsíða 34
Blaðsíða 35
Blaðsíða 36
Blaðsíða 37
Blaðsíða 38
Blaðsíða 39
Blaðsíða 40
Blaðsíða 41
Blaðsíða 42
Blaðsíða 43
Blaðsíða 44
Blaðsíða 45
Blaðsíða 46
Blaðsíða 47
Blaðsíða 48
Blaðsíða 49
Blaðsíða 50
Blaðsíða 51
Blaðsíða 52
Blaðsíða 53
Blaðsíða 54
Blaðsíða 55
Blaðsíða 56
Blaðsíða 57
Blaðsíða 58
Blaðsíða 59
Blaðsíða 60
Blaðsíða 61
Blaðsíða 62
Blaðsíða 63
Blaðsíða 64
Blaðsíða 65
Blaðsíða 66
Blaðsíða 67
Blaðsíða 68
Blaðsíða 69
Blaðsíða 70
Blaðsíða 71
Blaðsíða 72
Blaðsíða 73
Blaðsíða 74
Blaðsíða 75
Blaðsíða 76
Blaðsíða 77
Blaðsíða 78
Blaðsíða 79
Blaðsíða 80
Blaðsíða 81
Blaðsíða 82
Blaðsíða 83
Blaðsíða 84

x

Jökull

Beinir tenglar

Ef þú vilt tengja á þennan titil, vinsamlegast notaðu þessa tengla:

Tengja á þennan titil: Jökull
https://timarit.is/publication/1155

Tengja á þetta tölublað:

Tengja á þessa síðu:

Tengja á þessa grein:

Vinsamlegast ekki tengja beint á myndir eða PDF skjöl á Tímarit.is þar sem slíkar slóðir geta breyst án fyrirvara. Notið slóðirnar hér fyrir ofan til að tengja á vefinn.