Grímsnes is a relatively small fissure or crater row volcanic system located in South Iceland, located south–east of Lake Thingvallavatn and east of the en echelon group of volcanic systems extending across the Reykjanes Peninsula, that erupted last in the Holocene.

Geography

The lava fields are spread out to the south-east of the older edifice of Búrfell being bounded to the east by the water body of Álftavatn, on the river Sog (Sogið) and reaches to the south the confluence of the Sog with the Hvítá. From these river boundaries at about 20 m the field reaches its highest point at the Seyðishólar cone of 214 m. Most of the lava fields are covered by birch and willow.

Geology

Tephrochronology approximates the volcano's last eruption as about 7000 years ago. There are at least ten vents and all have erupted olivine tholeiite basalt in effusive eruptions although one small tephra eruption is known. It has been classified as part of the Western Volcanic Zone, but is also part of the South Iceland seismic zone. The northern parts of the lava fields overlay lava fields erupted before the last ice age by the shield volcano Lyngdalsheiði to the north. This is known as the maar volcanic crater of Kerið towards the middle of the field has 7 – of the distinctive Lyngdalsheiði basalt exposed. The maar has a total depth of 55 m with 10 m being water filled.. In a crater row line extending 800 m to the south–east from Kerið are the Tjarnarhólar craters. These erupted what was historically called the Tjarnarhólahraun lavas, a name not currently used in this context, extending mainly to the south of about 12 km2, now covered by up to four more recent lava flows in part. This central area of the field also has the vent Rauðhólar to the south–east, that contributed some of the youngest lavas in the area, which is part of the many vent Rauðhólahraun flow, although true young Rauðhólar lava flows (Svönkuhraun ), that erupted more recently that 7050 BP, at a volume of 0.009 km3 are much smaller. To the east of the Rauðhólar vent are the also young Kolgrafarhóll and Borgarhóll vents and east of that the Selhóll I (Selhóll south) vent that is one of oldest vents and erupted 0.01 km3 more than 9500 years BP (cal). Kolgrafarhóll had a small eruption at 0.004 km3 more recent than 7050 years BP (cal) and so did Borgarhóll 0.001 km3. To the south of Selhóll I is the most southern vent of the field Álftarhóll (Álftarhólar ) that produced the larger 6.2 km2 (0.126 km3) Álftarhólshraun flow, that is less than 7050 years BP (cal). The most eastern vent and its Borgarhólar lava field (Borgarhólahraun ) is separate from the rest of the Grímsnes lava fields with a small eruptive volume of 0.03 km3 and dated to 8200 years BP (cal). The largest lava field to the north and east of Kerið is the 23.5 km2 Seyðishólar-Kerhólahraun field. Its vents are Seyðishólar that produced a volume of 0.26 km3 dated accurately by tephrochronology to 9500 years BP (cal) and Kerhóll that produced a volume of 0.36 km3 also dated well to 7050 years BP (cal). The Kálfshólar vent just to the east of Seyðishólar and Kerhóll produced the Kálfshólahraun field that flowed to the east to the Sog within area of 8.0 km2 (0.14 km3) and is more recent than 7050 years BP (cal).. The most northerly vent to the north of Kálfshólar is old at 9500 years BP (cal) or more and called Selhóll-north (Selhóll II) with a lava erupted volume of 0.04 km3. To the east of this is Rauðhóll (Kerlingarhóll ) lava field (Rauðhólshraun ) dated earlier than 7050 BP (cal). The total volume of lava produced in the lava flows of Grímsnes has been estimated at 1.2 km3.

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