Weaving is a method of textile production in which two distinct sets of yarns or threads are interlaced at right angles to form a fabric or cloth. Other methods are knitting , crocheting , felting , and braiding or plaiting. The longitudinal threads are called the warp and the lateral threads are the weft , woof, or filling. Weft is an old English word meaning "that which is woven"; compare leave and left. A fabric band which meets this definition of cloth warp threads with a weft thread winding between can also be made using other methods, including tablet weaving , back strap loom , or other techniques without looms.
Frequently Asked Questions - National Fossil Day (U.S. National Park Service)
Dating techniques are procedures used by scientists to determine the age of rocks, fossils, or artifacts. Relative dating methods tell only if one sample is older or younger than another; absolute dating methods provide an approximate date in years. The latter have generally been available only since Many absolute dating techniques take advantage of radioactive decay , whereby a radioactive form of an element decays into a non-radioactive product at a regular rate. Others, such as amino acid racimization and cation-ratio dating, are based on chemical changes in the organic or inorganic composition of a sample. In recent years, a few of these methods have come under close scrutiny as scientists strive to develop the most accurate dating techniques possible.
A number of methods are used today to date fossils. Most of the methods are indirect meaning that the age of the soil or rock in which the fossils are found are dated, not the fossils themselves. The most common way to ascertain the age of a fossil is by determining where it is found in rock layers.
The potassium-argon K-Ar isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale. Potassium occurs in two stable isotopes 41 K and 39 K and one radioactive isotope 40 K. Potassium decays with a half-life of million years, meaning that half of the 40 K atoms are gone after that span of time.