A study by glaciologists of two small glaciers in South America reveals another retreat. More than 80% of all glacial ice in the northern Andes is concentrated on the highest peaks in small plains of approximately in size. A 1992 to 1998 observation of the Chacaltaya Glacier in Bolivia and Antizana Glacier in Ecuador indicate that between and of ice was lost per year on each glacier. Figures for Chacaltaya show a loss of 67% of its volume and 40% of its thickness over the same period. Chacaltaya Glacier has lost 90% of its mass since 1940 and was expected to disappear altogether sometime between 2010 and 2015. Antizana is also reported to have lost 40% of its surface area between 1979 and 2007. Research also indicates that since the mid-1980s, the rate of retreat for both of these glaciers has been increasing. In Colombia, the glaciers atop Nevado del Ruiz have lost more than half their area in the last 40 years.