Early methods relied on uranium and thorium minerals, but potassium—argon, rubidium—strontium, samarium—neodymium, and carbon—carbon are now of considerable importance. Uranium decays to lead with a half-life of 4. It is important that the radioactive isotope be contained within the sample being dated. Carbon is contained within plant material, but potassium, argon, and uranium are contained satisfactorily only within crystals. Igneous rocks are the most suitable for dating.
Radiometric Dating and the Geological Time Scale
The cliff area is about meters long almost 1 mile. We know that ancestors of Native Americans were the first people to inhabit Montana, and we have a fair understanding of which tribes lived here, and how they lived over the past years. White settlers, explorers, soldiers, and journalists wrote about this era, and oral accounts were passed down over generations among Indian people. However, to learn about the heritage of Montana's tribes centuries farther back in time is much more challenging because experts have to rely on types of evidence that are harder to find and more difficult to interpret. Mother Lodes. Stories, artifacts, trails, tipi rings, and rock art all offer glimpses into the lives of pre-historic Native American cultures. Another tremendous source of evidence are the numerous bison kill sites scattered throughout the region.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4. Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free. These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.