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Geologists, understandably, have accepted the Rb-Sr "Isochron date" for the Cardenas Lavas and attribute the younger K-Ar "model dates" to be errors caused by uniform argon loss by these rocks after the lava cooled.
The Western Grand Canyon lava flows are among the youngest formations of the Grand Canyon. Mitchell, "Name and Age of the Upper Precambrian Basalts in the Eastern Grand Canyon," Geological Society of America Bulletin, 83 (Jan.
Do analyses of the radioactive isotopes of rocks give reliable estimates of their ages?
That is a good question, which ordinarily requires a lengthy and technical answer.
These flows came from volcanoes on the Ulnkaret Plateau north of the Colorado River.
Some of the lava flowed over the north rim, cascading into Grand Canyon, forming spectacular "frozen" lava falls.
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.
The correlation of these six points to the 1500-million-year isochron is almost as good as the six points forming the 1090-million-year isochron for the Cardenas Lavas (Figure 1), which overturned five K-Ar "model dates." Geologists, however, must reject the alleged "isochron" for the Western Grand Canyon lava flows (Figure 2) because these most recent flows would otherwise appear to be the oldest rocks of the Grand Canyon.
No geologist could accept the erosional form of the Grand Canyon as enduring for 1.5 billion years!
Popular Dating Methods Both the K-Ar and Rb-Sr methods make use of radioactive decay of a parent isotope to a stable daughter isotope. Leeman, "Late Cenozoic Alkali-Rich Basalt from the Western Grand Canyon Area, Utah and Arizona: Isotopic Composition of Strontium," Geological Society of America Bulletin, 85 (Nov. Six analyses of Uinkaret Plateau basalts (U-33, U-35, U-59, U-24, U-43, U-58) were used.
Potassium-40 (Rb has occurred since the rock cooled from the lava flow.
Such ages defined by these fines are called "isochrons." An "isochron date," which relies on multiple rock samples, is considered to be much more reliable than a "model date" determined from a single rock sample.