Short answers, pulled from the story.
Land art is an art movement of the 1960s and 1970s that emerged in 1968 when artists deliberately walked away from the white walls of the museum to create works in the mud, the desert, and the water. The movement was a calculated political act against the commercialization of art and a reaction to the heightened political activism of the year and the emerging environmental and women's liberation movements.
Robert Smithson created the famous land art work Spiral Jetty which was completed in 1970. The work is a 1500-foot spiral-shape jetty protruding into Great Salt Lake in northern Utah and is composed of rock, earth, and algae.
Alan Sonfist used an alternative approach to working with nature and culture by bringing historical nature and sustainable art back into New York City. His most inspirational work is Time Landscape, an indigenous forest he planted in New York City at the intersection of LaGuardia and Houston Streets which was designed to grow and change over decades.
The land art movement faded out due to the sudden economic downturn of the mid-1970s which caused funds from wealthy patrons and private foundations to largely stop. The death of Robert Smithson in a plane crash in 1973 marked a turning point for the movement as it lost one of its most important figureheads.
The most prominent non-American land artists are the British Chris Drury, Andy Goldsworthy, Richard Long and the Australian Andrew Rogers. Rogers' Rhythms of Life project is the largest contemporary land-art undertaking in the world forming a chain of stone sculptures or geoglyphs around the globe with twelve sites located in disparate exotic locations.