Hekla Volcano

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Hekla, Iceland (Composite Volcano)

Eruption of February 2000

Location: 63.98 N, 19.70 W Elevation: 4,890 ft. (1,491 m)

February 26, 2000

On 26 February 2000, Iceland's most famous volcano, Mt. Hekla, began erupting at 1819 GMT. The seismic networks of the Science Institute, University of Iceland and the Iceland Meteorological Office recorded a short-term precursory earthquake activity. A seismograph near the summit of Hekla beginning at 1700 detected small earthquakes. The National Civil Defense of Iceland issued a warning, and the public was alerted. Thunder, lightening, and earth tremors accompanied the eruption. A 6-7km long fissure appeared and a steam column rose nearly 15 km (45,000 feet) into the sky. A discontinuous curtain of fire emanated from the entire fissure. The lava flows down the slopes of Hekla and covers a large part of the Hekla ridge. One lava stream flowed from the eruptive fissure towards the north. A more active lava stream emanates from three craters near the southern end of the eruptive fissure. On February 27, this lava stream was several kilometers long and was advancing at a rate of about a meter per minute. The Coast Guard reported that the new lava covers a stretch of about 3-4 km at its longest. The maximum thickness of the ash sector, 21 km north of the volcano, was 4-5 cm when measured 7 hours after the onset of the eruption. Most of the ash fell in uninhabited areas in the interior of Iceland. The eruption reached its peak intensity in the first hour of the activity. Presently, the lava flows and ash fall pose little danger to human settlement. Geologists said the activity could continue for about a month. Icelanders in the Middle Ages called the volcano the "Gateway to Hell." ( Figure 1. Hekla map of new lava flow as of March 6, 2000.)

Farmers are worried because several grazing animals were found dead in central and northern Iceland. Scientists believe that tephra containing high fluorine could be poisoning the livestock.

History of Hekla

Iceland's most famous volcano is built up on a WSW-ENE trending fissure by repeated fissure eruptions, forming a vaulted ridge about 5 km long and split lengthwise in major eruptions. The present height of the volcano is 1491 m (1447 before the 1947 eruption). Morphologically Hekla represents an intermediary stage between a crater row and a stratovolcano. Seen in the direction of the fissure it has the concave outline typical of a stratovolcano. Hekla erupts a magma type, which is unique for Iceland. It resembles the calcalkaline products of subduction volcanism. The postglacial products of Hekla can be described as two end members of a series, one high in silica, the other andesitic (icelandite). Intermediate magmas between these end members may result from magma mixing. After the 1980 eruption it was possible, by measurements of surface deformation, to determine the depth to the magma reservoir which is at about 8 km. Hekla has had a number of large postglacial eruptions, producing vast amounts of tephra which repeatedly covered up to two thirds of the country with light-colored tephra. ( Figure 2.)During historical time the first eruption (A.D. 1104) was a tremendous explosive eruption which destroyed the Þjórsárdalur valley. This eruption produced about 2.5 km3 of rhyodacitic tephra, which was carried towards NNW. The following eruptions in Hekla, producing both lava and tephra, occurred in 1158,1206, 1222, 1300, 1341, 1389, 1510, 1597, 1636, 1693, 1766, 1845, 1947, 1970, 1980 1991 and 2000. Some of these eruptions caused great damage, especially the eruptions in 1510, 1693 and 1766. The total volume of lava produced by Hekla in historical times is about 8 km3, and the total volume of tephra about 7 km3.

The three last eruptions of Hekla occurred in 1970, 1980 and 1991. The 1970 eruption started on 5 May. Fissures opened nearly simultaneously NE, S and SW of the Hekla ridge. During the tephra-producing phase, which lasted about 2 hours, about 30 million m3 of tephra were produced and carried towards NNW. The maximum thickness of the tephra layer in the vicinity of Hekla is 18 cm. About 170 km from Hekla it is 4 mm. The tephra sector within the 0.1 mm isopachyte covers nearly one-tenth of the country. The tephra was high in fluorine (exceeding 2000 ppm F in some places), which poisoned and killed grazing animals, especially in North Iceland.

Sources: (The Nordic Volcanological Institute, Iceland 2000)

(Central Kings Rural High School field trip Data : 2000)

Questions

1. What three volcanic materials were ejected from Hekla during the February 2000 eruption?

2 . Using Figure 1., estimate the total volume of new lava expelled by the February 2000 Hekla eruption in square kilometers. Use the Modified Kilometer Grid to estimate the area of new lava. For this map scale, 50 dots per square kilometer.

3. (a) List the historical eruptions of Hekla starting with the earliest to the latest.

(b) Place an X by the five largest eruptions.

(d) Would you expect the next eruption to be either a large eruption or a small eruption? Explain your reasoning.

4. (a) Using Figure 2., what are the two dominate wind directions during these historical eruptions? Remember! Give wind reading from origin.

(b) Using the Modified Kilometer Grid, estimate the total area effect by the tephra fallout from the 1104 eruption. Remember to include the land area only.

For this map scale, 36 dots = 50 kms)

5. Has the last four eruptions at Hekla had any impact on the people of Iceland?

Figure 1. Hekla Volcano: Eruption of 2000

Scale: