And how can we know how much radioactive decay occurred in their mantle source before these lavas erupted?If the youngest lavas inherited all of their supposed radioactive decay age, then so could the “ancient” lavas.That is, their rubidium-strontium composition has nothing to do with their age, but everything to do with their source!However, the molten rock that produced the “ancient” Cardenas Basalt lavas also came from the same region of the earth’s mantle beneath the Grand Canyon.
Worse still, their rubidium-strontium isochron age is 1,143±220 million years!
These were once molten lavas that were successively erupted onto the earth’s surface through volcanic vents and fissures and flowed over already deposited layers of siltstones.
They quickly hardened into the dense, black rock called basalt .
However, what ages do these same methods yield when applied to rocks for whose formation there are independent cross-checks? There are up to 160 volcanic cones on the plateau to the north of the Canyon rim out of which these basalt lavas flowed.
The eruptions were so recent, occurring after the Grand Canyon formed, that some of these basalt lava flows cascaded like molten waterfalls over the Canyon rim, down the Canyon walls and into the Canyon, where they formed dams that temporarily blocked the flow of the Colorado River .