This does not make sense.
Answer: Cosmic rays in the upper atmosphere are constantly converting the isotope nitrogen-14 (N-14) into carbon-14 (C-14 or radiocarbon). Living organisms are constantly incorporating this C-14 into their bodies along with other carbon isotopes. When the organisms die, they stop incorporating new C-14, and the old C-14 starts to decay back into N-14 by emitting beta particles. The older an organism's remains are, the less beta radiation it emits because its C-14 is steadily dwindling at a predictable rate. So, if we measure the rate of beta decay in an organic sample, we can calculate how old the sample is. C-14 decays with a half-life of 5,730 years.
So if the cosmic rays are not getting to this lava flow. how is it decaying?
can you answer these?
Question, how can you measure how old LAVA is? The most familiar method is radiocarbon dating, also known as Carbon-14. Charcoal is produced when plant material is buried by lava flows. Living plants and animals take in carbon from the air, most of which is the stable isotope C-12 and a tiny amount of the unstable isotope C-14.
so if this is true. How old were the plants in the lava fields produced by Mt St Helens? which from reports I have seen date them to millions of years?
we also have this
Carbon-14 decay cannot be used to directly date lava because lava itself does not contain organic material necessary for carbon-14 dating; instead, scientists use carbon-14 dating on trapped organic material like charcoal within the lava flow to estimate the age of a relatively young lava flow, as carbon-14 is only effective for dating materials up to around 50,000 years old.