displacement rate (R) is equal (see Figures 6.12, 6.15A, 6.16A, 6.18, and 6.21 below). The history of vertical crustal displacements at a coastal locality is derived graphically by plotting the displacement (D) of each independently dated strandline as a function of its age (A) (Figure 6.5). The resultant locus of points is the apparent sea-level history, which, as stated previously, is the inverse of the displacement history. If this locus of points defines a straight line, the displacement rate (R), which is the inverse of the slope of the line, was constant. If it does not define a straight line, the displacement rate was variable (see Figures 6.9A and 6.10B below). Commonly, only one strandline in a sequence can be dated independently, and, therefore, only an average displacement rate can be derived from a relative sea-level record.
A plot of strandline elevation (E) as a function of age (A) yields the relative sea-level history, which is the inverse of the approximate displacement history where vertical displacements are large compared to sea-level changes. Frequently, uplift histories based on highly emergent Holocene strandlines (see Figures 6.25, 6.26, and 6.27 below) or very old Pleistocene strandlines (Bender et al., 1979) are approximated by relative sea-level curves. In other words, present sea level can be used as an approximate datum.
The presence of emergent marine strandlines along most active-tectonic coastlines suggests that crustal uplift is more common than subsidence. However, subsidence is probably underestimated merely because most geologic evidence for downward crustal movements is buried in sedimentary basins (Atwater et al., 1977) or is submerged offshore (Lewis, 1974). Many studies of long-term crustal movements in coastal areas focus on uplift mainly because the emergent strandline record is better exposed and easier to interpret than the submergent record.
Most long-term crustal movements recorded by marine strandlines reflect sustained tectonic deformation along active plate boundaries, but the most rapid known rates of vertical crustal displacement reflect transitory volcanic tumescence and glacio-isostatic rebound. Both processes are noteworthy primarily for comparative purposes, but also because they are, themselves, related to tectonic processes in various ways.
Volcanic Displacements The highest known rates of sustained vertical ground displacement exceed 100 mm/ yr and are recorded by marine strandlines on the flanks of active insular and coastal volcanoes. For example, on the island of Iwo Jima, the tip of a large volcano in the western Pacific Ocean, radiometrically and historically dated emergent strandlines yield an average uplift rate
of 200 mm/yr over the past 800 yr (Figure 6.8) (Kaizuka et al., 1983). However, tide-gauge data suggest that uplift rates on the island probably fluctuated between 100 and 800 mm/yr over this period of time (Figure 6.8B), during which only minor phreatic eruptions are known to have occurred (Corwin and Foster, 1959).
An even longer record of vertical ground displacements related to volcanic processes is found in the Phlegraean Fields caldera on the Mediterranean coast near Naples, Italy, where historically dated strandlines on man-made structures document alternating subsidence and uplift that averaged 12 mm/yr over the past 2 ka (Figure 6.9) (Berrino et al., in press). However, uplift and subsidence exceeded 150 mm/yr for a few decades before and after a minor eruption in A.D. 1538. Tide-