The analytical equipment used to determine the abundances of isotopes is more accurate in determining ratios of isotopes than their absolute abundances. Therefore, Sr as the y-axis would allow the isotope ratios of all rocks in a lava flow to be plotted. Damon, "K-Ar Age of Lava Dam in the Grand Canyon," Geological Society of America Bulletin, 79 (Jan.
The y-intercept of the isochron is the initial Sr value and the correspondence to a straight line measures the degree to which the lava has remained a "closed system." "Model dates" have no check for internal consistency.
Grand Canyon Lava Flows Two lava flow formations occur in the Grand Canyon, the Cardenas Lavas and the Western Grand Canyon lava flows. The Cardenas Lavas are believed to be among the oldest strata of the Grand Canyon, occurring underneath 10,000 feet of strata in the eastern Grand Canyon.
Several lava flows even reached the river, forming lava dams.
Although no Indian legends attribute them to recent eruptions, one early geologist remarked about the extraordinary freshness of lava: "It looks as fresh as any coulee of Vesuvius ejected twenty or thirty years ago." indicate a line appearing to form a good isochron in Figure 2, the slope determining an "age" of 1500-million-years.
Fictitious Isochron Ages The isochron method assumes that the datable lava flow cooled from molten rock having different Rb/Sr ratios but having uniform mixing isotopically (all samples having the same Sr graph with the slope of the line having no identifiable time significance! Noble, "Age of the Cardenas Lavas, Grand Canyon, Arizona," Geological Society of America Bulletin, 87 (Aug.