Final answer:
Different Strontium 87 concentrations on either side of a line in the Pacific Northwest is attributed to the radioactive decay of Rubidium 87 into Strontium 87 over time, resulting in older rocks with higher Sr-87 concentrations on the east side of the line.
Step-by-step explanation:
The phenomenon described in the question where a line through northeastern Washington to the border of Oregon and Idaho separates two areas with different values of Strontium 87 concentration is due to the radioactive decay process of Rubidium 87 (Rb-87) into Strontium 87 (Sr-87).
The exposed rocks contain a radioactive isotope of rubidium, 87Rb, which decays into a stable isotope of strontium, 87Sr.
This process can be understood with the aid of nuclear chemistry, where 87Rb, which has 37 protons and 50 neutrons, experiences a decay over time during which a proton is converted into a neutron yielding 87Sr, now with 36 protons and 51 neutrons.
The number of these isotopes changes over time, and this change can be used to measure the age of rocks, as shown by the example in which a rock contains certain amounts of Rb-87 and Sr-87 and its age is calculated using the half-life of the decay of Rb-87, therefore, learning that the rock is approximately 1.7 billion years old.
Rocks East of the line with a higher concentration of Sr-87 suggests they are older, as they have had more time for Rb-87 to decay into Sr-87 as compared to rocks on the West.