Aluminium or aluminum (₁₃Al) has 22 known isotopes from ²²Al to ⁴³Al and 4 known isomers. Only ²⁷Al (stable isotope) and ²⁶Al (radioactive isotope, t_1/2 = 7.2×10⁵ y) occur naturally, however ²⁷Al comprises nearly all natural aluminium. Other than ²⁶Al, all radioisotopes have half-lives under 7 minutes, most under a second. The standard atomic weight is 26.9815385(7). ²⁶Al is produced from argon in the atmosphere by spallation caused by cosmic-ray protons. Aluminium isotopes have found practical application in dating marine sediments, manganese nodules, glacial ice, quartz in rock exposures, and meteorites. The ratio of ²⁶Al to ¹⁰Be has been used to study the role of sediment transport, deposition, and storage, as well as burial times, and erosion, on 10⁵ to 10⁶ year time scales. ²⁶Al has also played a significant role in the study of meteorites.