Passage IV

The Mediterranean Sea is home to many volcanic islands. Studying the volcanic ash that has settled on the islands and the ocean floor around the islands can tell scientists about the sequence of eruptions, the ash source, and the magma source, or sources, that feed the volcanoes.

Study 1

Scientists took 10 ash samples from a volcano on each of 5 islands. The samples were analyzed for their percent silica (SiO₂) and potassium oxide(K₂O) contents. Also measured were the zirconium (Zr), yttrium (Y) ,and rubidium (Rb) contents, in parts per million (ppm). The averaged sample analyses arc shown in Table 1.

Study 2

A 6 m long hollow tube was dropped to the ocean floor at 5 different sites around the volcanic islands to collect cores of undisturbed ocean floor sediments. Each core was sampled every 25 cm, and the samples were analyzed as in Study 1. Those analyses were compared to the results of Study 1 in order to identify the volcano that had produced a given ash layer present in the cores. The sediment and ash layers identified in each core are shown in Figure 1.

Study 3

To determine whether one or more magma sources feed volcanoes in the study area, scientists plotted the SiO₂ content of ash samples from 8 of the volcanoes versus the respective K₂O content of those samples. The SiO₂ and K₂O contents of the ash derived from a given magma source will fall within a narrow range of values. The 8 ash samples included samples from the 5 volcanoes in Study 1. The results are shown in Figure 2.

Table 1 and Figure 2 adapted from Darlene Richardson and Dragoslav Ninkovich, "Use of K₂O, Rb, Zr, and Y versus SiO₂, in Volcanic Ash Layers of the Eastern Mediterranean to Trace Their Source." @1976 by the Geological Society of America.