Method

(Landsat 2 MSS), 1985 (Landsat 5 Thermal Mapper (TM)), 1991 (Système Probatoire pour l'Observation de la Terre 2 (SPOT 2)), and 1999, 2002 and 2005 (Landsat 7 Enhanced TM Plus (ETM+)). The 1964 aerial photographs and the 1991 SPOT image did not cover the northern part of the island. Fieldwork in 2001, 2002, 2003 and 2005 contributed with supplementary information on glaciological features and proglacial landforms.

It has become standard practice to classify surge-type glaciers according to the strength of evidence for surge behaviour, often by using a surge index (e.g., Clarke et al., 1986). Hence, glaciers with observed surge events or one or more distinct surge-related characteristics are assigned as surge-type glaciers. As many characteristics are not unique to glacier surging, this quantitative approach was combined with a qualitative assessment of individual features. However, there will always be uncertainties in the identification process, and many surge-type glaciers may remain unidentified because no surge features appear on aerial photographs and satellite imagery during the quiescent phase. For instance, the small glacier 1HD06009 surged in 1953, but no glaciomorphological surge features appeared on later images. Even fieldwork may not reveal unequivocal evidence of past glacier surging.

Surge-type glaciers were diagnosed using glaciomorphological features (elements) such as surge fronts (Kamb et al., 1985), heavily crevassed surface (Post, 1969), looped medial moraines (Post, 1969; Jiskoot et al., 2000; Post and LaChapelle, 2000), and circular depressions termed potholes or lacunas (Sturm, 1987; Sturm and Cosgrove, 1990; Post and LaChapelle, 2000). The first two features occur during the active surge phase, whereas the latter two features may remain for one or more surge cycles. Other features, such as the occurrence of glacier-derived naled (Yde and Knudsen, 2005b), thrust-block naled (Yde et al., 2005a), stranded ice along valley walls in the ice reservoir area (Post and LaChapelle, 2000; Copland et al., 2003), and large circular depressions termed chasms (Yde and Knudsen, 2005a) are also related to glacier surging, but may also occur on normal glaciers. It can be difficult to distinguish between potholes and chasms as they may have the same geometric dimensions. Potholes are formed as remnants of supraglacial lakes between transverse crevasse ridges immediately after surge events and therefore appear as assemblages in the central parts of surge-type glaciers, whereas chasms are supraglacial subsidence depressions formed as a consequence of roof collapse in subglacial drainage cavities. Chasms generally occur along lateral margins during periods of enhanced melt rates and high subglacial water discharge such as during the initial quiescent phase.

Proglacial landforms such as thrust-block moraines, eskers, flutes, drumlins and crevasse-squeezed ridges are associated with fast glacier flow and, thus, may indicate previous surge activity (Evans and England, 1991; Evans and Rea, 1999, 2003). Many glaciers on Disko Island have extensive proglacial areas with debris-covered dead ice and complex moraines, which indicate that these glaciers have experienced significant advances and recession in historical time, which may relate to past surge events. Rapid recession rates some years after the termination of a surge event is common as surge-type glaciers may advance down-valley where relatively high temperatures prevail.

Table 1. List of surge-type glaciers on Disko Island.

No. Glacier code Diagnostic surge features and comments

1 1HB07004 Potholes, undulated surface

2 1HB10008 Potholes

3 1HB10013 Active surge in 1953, crevassed surface

4 1HB10027 Active surge between 1985-1999, looped medial moraine, potholes

5 1HB10035 Potholes

Active surge in 1964. Potholes, looped medial moraine, stranded ice,

6 1HB10036 ii-

extensive dead ice area

7 1HB11009 Potholes

8 1HB11013 Potholes, complex proglacial moraines

9 1HB11014 Potholes, complex proglacial moraines

10 1HB11015 Potholes, complex proglacial moraines

11 1HB11029 Potholes, looped medial moraines

12 1HB11033 Extensive dead ice area

13 1HB15005 Potholes, undulated surface, extensive dead ice area

14 1HB15008 Same glacier as 1HB11034. Active surge between 1931-1953 (?), which erased the end moraine of glacier 1HB11033. Proglacial surge landforms

15 1HB15009 Potholes, undulated surface

16 1HB15012 Active surge in 1953, crevassed surface, potholes, proglacial naled

17 1HB15013a Potholes, chasms, looped medial moraine

18 1HB15013c Potholes, looped medial moraine, extensive dead ice area

19 1HB15016 Potholes

Active surge in 1995-1998, crevassed surface, potholes, looped medial

20 1HB15017 moraine, chasms, undulated surface, proglacial naled, thrust-block naled, hydrofractures

21 1HB15029 Extensive dead ice area, looped medial moraine, chasms

Table 1. (Continued)

No. Glacier code Diagnostic surge features and comments

22 1HB15031 Extensive dead ice area, chasms

23 1HB16010 Undulated surface, extensive dead ice area

24 1HB16017 Potholes, extensive dead ice area, undulated and crevassed surface

Extensive dead ice area, undulated surface, proglacial naled, potholes, looped

25 1HB16020 , medial moraine

26 1HB16047 Undulated and crevassed surface

27 1HC03010 Crevassed terminus in 1985, but no frontal advance

28 1HC03011 Looped medial moraine

29 1HC03013 Looped medial moraine

30 1HC04019 Potholes

31 1HC05004 Potholes

32 1HC05006 Potholes

33 1HC05007 Extensive dead ice area

34 1HD02004 Very steep terminus in 2002 and 2005

35 1HD02005 Looped medial moraine, extensive dead ice area

36 1HD02011 Looped medial moraine, extensive dead ice area

37 1HD04007 Tributary glacier cut off, proglacial naled, chasms, looped medial moraine?

38 1HD06005 Crevassed surface, potholes, looped medial moraine, extensive dead ice area

39 1HD06009 Active surge in 1953, crevassed surface

40 1HD06012 Potholes

41 1HD06020 Potholes

42 1HD06030 Active surge between 1953-1964, steep terminus in 1953, potholes

43 1HD06035 Chasms, looped medial moraine, extensive dead ice area

44 1HD06041 Looped medial moraine, extensive dead ice area, potholes?

No.

Glacier code

Diagnostic surge features and comments

45

1HD06042

Potholes, tributary glacier cut off, looped medial moraine, complex moraine

46

1HD06046

Potholes, proglacial naled

47

1HD06048

Potholes, looped medial moraine

48

1HD06051

Potholes, looped medial moraine

49

1HD06053

Extensive dead ice area, looped medial moraine?

50

1HD06055

Potholes, looped medial moraine

51

1HD07013

Potholes, crevassed surface

52

1HD07018

Active surge between 1931-1953? Potholes, looped medial moraine

53

1HD07028

Looped medial moraine, tributary glacier cut off, proglacial naled, extensive dead ice area, two proglacial moraines, crevassed surface

54

1HD07040

Potholes, undulated surface, looped medial moraines, complex of three glaciers where at least one is categorized as surge-type

55

1HE01024

Looped medial moraine, extensive dead ice area, chasms

56

1HE01036

Active surge between 1931-1953? Extensive dead ice area, looped medial moraine, proglacial naled

57

1HE01038

Active surge between 1953-1964. Potholes, chasms

58

1HE06001

Tributary glaciers cut off, extensive dead ice area

59

1HE08010

Looped medial moraine

60

1HE09002

Looped medial moraine, extensive dead ice area, proglacial naled

61

1HE09014

Chasms, extensive dead ice area

62

1HE09042

Extensive dead ice area superimposed upon a lateral moraine, which is formed during a presumed surge event of glacier 1HE09044

63

1HE09044

Potholes, extensive dead ice area, tributary glaciers cut off, proglacial naled

64

1HE09050

Crevassed surface, looped medial moraine?

65

1HE09051

Potholes, chasms, tributary glacier cut off, rapid recession

Table 1.(Continued)

No.

Glacier code

Diagnostic surge features and comments

66

1HE09062

Looped medial moraine, proglacial naled, extensive dead ice area

67

1HE09083

Looped medial moraine, potholes, proglacial naled, chasms, looped end

moraine

68

1HE09089a

Looped medial moraine by glacier 1HE09090, crevassed surface, proglacial

naled, potholes?

69

1HE09090

Potholes, looped medial moraine, chasms, proglacial naled

70

1HE09094

Potholes, looped medial moraine by glacier 1HE09095, surge could be

triggered by glacier 1HE09095?

71

1HE09095

Potholes, proglacial naled, extensive dead ice area, looped medial moraine

72

1HE09105

Extensive dead ice area

73

1HE10005

Looped medial moraine

74

1HE10007

Looped medial moraine by glacier 1HE10005, surge could be triggered by

glacier 1HE10005?

75

1HE11013

Potholes, chasms, proglacial naled

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