By then, "greater understanding of the ocean floor and the discoveries of features like mid-oceanic ridges, geomagnetic anomalies parallel to the mid-oceanic ridges, and the association of island arcs and ocean trenches occurring together and near the continental margins," suggested convection might be at work (Plate pp). .

             These discoveries and more led Harry Hess and R. Deitz to publish similar hypotheses based on mantle convection currents, known as "sea floor spreading," and although the idea was basically the same as that proposed by Holmes over three decades earlier, now there was much more evidence to further develop and support the idea (Plate pp). .

             Today, scientists have a fairly good understanding of how the plates move and how such movements relate to earthquake activity (Understanding pp). The majority of movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident (Understanding pp). The four type of plate boundaries are:.

             Divergent boundaries - where new crust is generated .

             as the plates pull away from each other. .

             Convergent boundaries - where crust is destroyed as .

             one plate dives under another. .

             Transform boundaries - where crust is neither produced .

             nor destroyed as the plates slide horizontally past each .

             other. .

             Plate boundary zones - broad belts in which boundaries .

             are not well defined and the effects of plate interaction .

             are unclear.

             (Understanding pp).

             The consequences of plate movement are easy to recognize around Krafla Volcano, in the northeastern part of Iceland, where existing ground cracks have widened and new one appear every few months (Understanding pp). From 1975 to 1984, numerous episodes of surface cracking, called rifting, took place along the Krafla fissure zone, some accompanied by volcanic activity in which the ground would gradually rise 1-2 m before abruptly dropping, signaling an impending eruption (Understanding pp).