In geology, the term "tension" refers to a stress which stretches rocks in two opposite directions. The rocks become longer in a lateral direction and thinner in a vertical direction. One important result of tensile stress is jointing in rocks. However, tensile stress is rare because most subsurface stress is compressive, due to the weight of the overburden.
Tensile stress forms joints in rocks. A joint is a fracture that forms within a rock, whose movement to open the fracture is greater than the lateral movement that takes place. Joints are formed in the direction perpendicular to the least principal stress, meaning that they are formed perpendicular to the tensile stress. One way in particular that joints can be formed is due to fluid pressure, as well as at the crest of folds in rocks. This occurs at the peak of the fold or due to the fluid pressure because a localized tensile stress forms, eventually leading to jointing. Another way in which joints form is due to the change in the weight of the overburden. Since rocks lay under a great deal of overburden, they undergo high temperatures and high pressures. Over time, the rocks are eroded and the weight of the overburden is lifted, so the rocks cool and are under less pressure, which causes the rock to change shape, often forming breaks. As the compression is lifted from the rocks, they are able to react to the tension on them by forming these breaks, or joints.
A tribe is viewed, historically or developmentally, as a social group existing before the development of, or outside, states. A tribe is a distinct people, dependent on their land for their livelihood, who are largely self-sufficient, and not integrated into the national society. It is perhaps the term most readily understood and used by the general public. Stephen Corry, director of Survival International, the world's only organisation dedicated to indigenous rights, has defined tribal people as "those which have followed ways of life for many generations that are largely self-sufficient, and are clearly different from the mainstream and dominant society". This definition, however, would not apply in countries in the Middle East such as Iraq, where the entire population is a member of one tribe or another and therefore tribalism itself is dominant and mainstream.
There are an estimated one hundred and fifty million tribal individuals worldwide, constituting around forty percent of indigenous individuals. However, although nearly all tribal people are also indigenous, there are some who are not indigenous to the areas where they live now.
In each series, Parry visits a number of remote tribes in such locales as the Himalayas, Ethiopia, West Papua, Gabon and Mongolia, spending a month living and interacting with each society. While there, Parry adopts the methods and practices of his hosts, participating in their rituals and exploring their cultural norms. This often enables him to form personal bonds with the members of each tribe.
Parry tries to learn the basics of the tribe's language but is also accompanied by a translator.
The series is co-produced by BBC Wales and the Discovery Channel. A second series aired in July 2006 and the third began on 21 August 2007 on BBC Two, and ended on 25 September 2007. No further series have been made, though Parry's 2008 series, Amazon has a similar synopsis.
Parry was awarded the BAFTA Cymru "Best On-Screen Presenter" award in 2008 for his work on the 'Penan' Episode. A BAFTA Cymru "Best Camera: Not Drama" award was also awarded for Gavin Searle's work in the same episode.