Jörð is simply the Old Norse word for "earth," walking as a giantess-goddess. Snorri counts her among the Æsir's goddesses and among the jötnar at once — the earth belongs to both sides, which is rather the point. By Odin she is the mother of Thor: sky-father on earth-mother, the oldest equation in Indo-European religion, and the reason Thor's other name is "son of Jörð" in skaldic verse.
Her genealogy is pure poetry: her mother is Nótt, the Night, making the earth the daughter of darkness. When Thor wades rivers and giants throw mountains, the myths never quite forget that his mother is the terrain.
Daughter of Night
Snorri lays out Jörð's family tree in Gylfaginning, in the passage that explains how day and night came to circle the sky. Nótt — Night herself, "black and dark as her ancestry," daughter of the giant Narfi — was married three times. By her second husband, a figure called Annarr ("Another" or "the Second"; manuscripts also spell him Ónarr), her daughter was Jörð, the Earth. By her third, the shining Dellingr of the Æsir's kin, she bore Dagr, the Day. The Earth is thus Night's daughter and Day's half-sister — a genealogy that is really a cosmology: the dark came first, and out of it were born the ground beneath and the light above.
Snorri's classifications are cheerfully contradictory, and the contradiction is the lesson. In Gylfaginning he says Odin's wife was "Jörð, and from her he had his first son, that is Ása-Þórr." In Skáldskaparmál's lists she appears both among the Ásynjur, the goddesses, and among names for giantesses — and Snorri says outright that Jörð is "reckoned among the Ásynjur" while being the daughter of a giant line. No other figure is filed so explicitly on both sides of the great divide, and none needs to be: the earth is the thing the gods defend and the stuff the giants are made of, the battlefield and the mother of the defender at once.
Who is Thor's mother?
Ask the poems and you get three names, and the puzzle is instructive. In Völuspá, at Ragnarök, Thor is mögr Hlóðynjar — "son of Hlóðyn" — and two lines later Fjörgynjar burr, "Fjörgyn's child," as he walks his nine last steps from the serpent. In Þrymskviða and elsewhere he is Jarðar burr, "son of Jörð." Snorri harmonises: Hlóðyn and Fjörgyn are simply other names of Jörð, and the earth-goddess had several. Most scholars accept this while noting what the multiple names imply — behind the tidy Eddic Jörð stand older strata of earth-worship. Hlóðyn has been connected (speculatively but persistently) with the goddess Hludana, attested in Roman-era votive inscriptions from the Rhineland; Fjörgyn has a masculine counterpart, Fjörgynn, named as Frigg's father, and the pair may descend from a very old thunder-and-earth complex — the name is cognate with words connected to the Baltic thunder-god Perkūnas. The Viking Age poet, calling Thor "son of Earth" as the god strides out to die killing the World Serpent, was drawing on imagery already ancient: the thunderer as child of the very ground he defends.
That is the real force of the genealogy. Thor's mother is not a character in his adventures — she never speaks, never acts, never appears at his side. She is present the way the land is present. When Thor wades the rivers to Geirröðr's hall, when he hallows the fields, when the last battle is fought on the plain of Vígríðr, his mother is underfoot. The kenning "son of Jörð" makes every Thor-story quietly ecological: the sky's strength, born of the earth, spending itself in the earth's defence.
The earth in the poets' mouths
Jörð's richest attestation is in the working language of skaldic poetry. Skáldskaparmál asks: how shall the earth be referred to? — and answers with her portrait: call it "Ymir's flesh" (for the world was built from the primeval giant's body), "mother of Thor," "daughter of Nótt," "sister of Auðr and Dagr," "wife of Odin," "rival of Rindr and Gunnlöð" — this last a sly one, ranking the Earth among Odin's loves as though the Allfather's famous wanderings included his marriage to the ground itself. The tenth-century skald Eyvindr skáldaspillir could praise a jarl's rule as his embrace of "Odin's bride"; holding land and holding the goddess collapse into one image. Poets of the conversion era pushed it further still, and the earth as body — flesh of Ymir, bride of the sky, mother of thunder — remained one of the tradition's deepest veins of metaphor.
In the primary sources
Jörð appears in the Prose Edda (Gylfaginning: her parentage from Nótt and Annarr, her marriage to Odin, her motherhood of Thor; Skáldskaparmál: earth-kennings, her place in both goddess- and giantess-lists) and throughout the Poetic Edda and skaldic verse in kennings — Thor as "son of Jörð" in Þrymskviða and Lokasenna, "son of Hlóðyn" and "Fjörgyn's child" in Völuspá. She has no narrative myth of her own: no journey, no speech, no scene. Place-name and cult evidence for an earth-goddess under this name is uncertain, though the first-century Roman historian Tacitus famously describes the North-Sea Germanic tribes worshipping Nerthus, "Terra Mater" — Mother Earth — a figure often compared, cautiously, with the later Norse earth-personifications.
Legacy
Jörð's name never died, because it never stopped being a word: jörð is still simply "earth" in Icelandic, jord in the mainland Scandinavian languages — every deed of land and gardening manual in the Nordic countries speaks her name. Her mythological role survives wherever Thor's story is told, since "son of Earth" is half of what Thor means. And in the modern environmental imagination, the old equation she embodies — the earth as a living mother, older than the gods who quarrel over her — has found, without much need of translation, a new congregation.
Frequently asked questions
Who is Jörð in Norse mythology?
Jörð is the personification of the earth — her name is simply the Old Norse word for "earth." She is counted both as a goddess and as a giantess, and she is the mother of the thunder-god Thor by Odin.
Is Jörð a goddess or a giantess?
Both, according to Snorri's Prose Edda, which lists her among the Ásynjur while giving her giant ancestry — her mother is Nótt (Night), of giant kin. The double status is fitting: the earth belongs to gods and giants alike.
Who are Jörð's parents?
Gylfaginning makes her the daughter of Nótt, the personified Night, and Nótt's second husband Annarr (also spelled Ónarr). The earth is thus the daughter of darkness and half-sister of Dagr, the Day.
Is Jörð the same as Fjörgyn and Hlóðyn?
The poems call Thor's mother by all three names — Jörð, Fjörgyn (Völuspá), and Hlóðyn (Völuspá) — and Snorri treats them as one figure with several names. Most scholars accept this, while seeing in the multiple names traces of older layers of Germanic earth-goddess worship.
Why is Thor called "son of Jörð"?
Because in skaldic and Eddic poetry a god is routinely named by his parentage, and Thor's defining pedigree is sky-father Odin on earth-mother Jörð — the ancient Indo-European pairing of sky and earth. The kenning appears in Þrymskviða, Lokasenna, and elsewhere.