Count the decay of radiocarbon in organic material that was associated with the eruption to gain an important insight to the dating. All organic material has carbon as a major part of its structure. A small portion of the carbon is naturally radioactive, and decays at a steady rate. If a layer of soil gives evidence of a volcanic eruption by the presence of ash or other debris, scientists can measure how old the organic debris in that part of the soil is by its radiocarbon reading. This, in turn, gives the approximate date of the volcano eruption.
Examine tree rings near the area of a volcano. Each year a tree adds a new ring of growth, which reflects its current environment. Heat, moisture, sunshine and chemicals emitted by volcanoes such as sulfur all affect the appearance of the tree ring. Volcanoes eruptions have a noticeable affect on the trees near it, leaving identifiable markers on the rings. Tree rings have been compiled dating back along a continuous path for almost 10,000 years. Studies on the trees near the major eruption of Therea in the Mediterranean Sea about 3,600 years ago are helping to define a more precise date for it.
Measure the amount of thorium-228 and radium-226 in basalt, which is the rock that cooled lava turns into. Uranium-238 gradually decays into thorium-228 and then radium-226, and by measuring the ratio of elements, a timeline for the creation of the basalt, or eruption of the lava, is established.