For the Gjalp fissure eruption see Bardarbunga.

In Norse mythology

Norse mythology

Norse mythology, a subset of Germanic mythology, is the overall term for the myths, legends and beliefs about supernatural beings of Norse pagans. It flourished prior to the Christianization of Scandinavia, during the Early Middle Ages, and passed into Nordic folklore, with some aspects surviving...

, Gjálp and Greip are two giantesses.

Thor's visit to Geirröðr

According to Skáldskaparmál

Skáldskaparmál

The second part of Snorri Sturluson's Prose Edda the Skáldskaparmál or "language of poetry" is effectively a dialogue between the Norse god of the sea, Ægir and Bragi, the god of poetry, in which both Norse mythology and discourse on the nature of poetry are intertwined...

 they were daughters of the giant Geirröðr. As Thor was wading across Vimur the river "waxed so greatly that it broke high upon his shoulders". He finds out why.

"Then Thor saw Gjálp, daughter of Geirröðr, standing in certain ravines, one leg in each , spanning the river, and she was causing the spate. Then Thor snatched up a great stone out of the river and cast it at her, saying these words: 'At its source should a river be stemmed.' Nor did he miss that at which he threw." Brodeur's translation

Once Thor reaches Geirröðr's dwelling he is offered a seat.

"Then he became aware that the chair moved under him up toward the roof: he thrust Grídr

Grid (Jotun)

In Norse mythology, Gríðr is female jötunn who, aware of Loki's plans to have Thor killed at the hands of the giant Geirröd, helped Thor by supplying him with a number of magical gifts which included a pair of iron gloves, and a staff known as Gríðarvölr. These items saved Thor's life...

's rod up against the rafters and pushed back hard against the chair. Then there was a great crash, and screaming followed. Under the chair had been Geirrödr's daughters, Gjálp and Greip; and he had broken both their backs." Brodeur's translation

The same myth is related in Þórsdrápa

Þórsdrápa

Þórsdrápa is a skaldic poem by Eilífr Goðrúnarson, a poet in the service of Jarl Hákon Sigurðarson. The poem is noted for its creative use of kennings and other metaphorical devices, as well as its labyrinthine complexity....

, though the giantesses are not named there.

"The peace-reluctant slayer of the reindeer of the Lister of the peak [Þórr] was put in a fix there, on the dire, grim hat of the giantess [chair].

They forced the high heaven of the flame of the brow-moon [Þórr's head] against the rafters of the (rock-)hall [cave], and were crushed against the rocks of the plain (of the rock-hall) [floor]. The hull-controller of the hovering chariot of the thunder-storm [Þórr] broke the ancient keel of the laughter-ship [backbone] of both cave-maidens [giantesses]." Eysteinn Björnsson's translation

Gesta Danorum

Gesta Danorum

Gesta Danorum is a patriotic work of Danish history, by the 12th century author Saxo Grammaticus . It is the most ambitious literary undertaking of medieval Denmark and is an essential source for the nation's early history...

 relates a similar story.

"[T]hree women, whose bodies were covered with tumours, and who seemed to have lost the strength of their back-bones, filled adjoining seats. Thorkill's companions were very curious; and he, who well knew the reason of the matter, told them that long ago the god Thor had been provoked by the insolence of the giants to drive red-hot irons through the vitals of Geirrod, who strove with him, and that the iron had slid further, torn up the mountain, and battered through its side; while the women had been stricken by the might of his thunderbolts, and had been punished (so he declared) for their attempt on the same deity, by having their bodies broken." Elton's translation

Other references

In Hyndluljóð

Hyndluljóð

Hyndluljóð or Lay of Hyndla is an Old Norse poem often considered a part of the Poetic Edda. It is preserved in its entirety only in Flateyjarbók but some stanzas are also quoted in the Prose Edda where they are said to come from Völuspá hin skamma.In the poem, the goddess Freyja meets the völva...

 Gjálp and Greip are mentioned as two of the mothers of Heimdall.

In Haustlöng

Haustlöng

Haustlöng is a skaldic poem composed around the beginning of the 10th century. The poem is preserved in the 13th century Prose Edda, which quotes two groups of stanzas from it, and is attributed to the Norwegian skald Þjóðólfr of Hvinir. The poem describes mythological scenes painted on a shield...

, Þjazi is called "the son of the suitor of Greip". Greip may be used there as a generic giantess name and the kenning

Kenning

A kenning is a type of literary trope, specifically circumlocution, in the form of a compound that employs figurative language in place of a more concrete single-word noun. Kennings are strongly associated with Old Norse and later Icelandic and Anglo-Saxon poetry...

 may mean simply "giant".

In a lausavísa

Lausavísa

In Old Norse poetry and later Icelandic poetry, a lausavísa is a single stanza composition, or a set of stanzas unconnected by narrative or thematic continuity....

 composed by Vetrliði Sumarliðason

Vetrliði Sumarliðason

Vetrliði Sumarliðason is a 10th century Icelandic skald.He was the great-grandson of Ketill hængr , one of the settlers of Iceland. He lived in Fljótshlíð, in the south of the island....

 and quoted in Skáldskaparmál, Gjálp is mentioned as being killed by Thor.

Leggi brauzt þú Leiknar, *lamðir Þrívalda, steypðir *Starkeði, stóttu of Gjálp dauða. Faulkes' edition

Thou didst break the leg of Leikn, Didst cause to stoop Starkad Starkad Starkad, Old Norse: Starkaðr or Störkuðr, Latin: Starcaterus, and during the late Middle Ages, also known as Starkodder, was a legendary hero in Norse mythology.... r, Didst bruise Thrívaldi Þrívaldi In Norse mythology, Þrívaldi , whose name means "thrice mighty", is a giant killed by Thor.This fact is mentioned by Snorri Sturluson in the Skáldskaparmál , according to which "killer of Þrívaldi" is a kenning for Thor... , Didst stand on lifeless Gjálp. Brodeur's translation