Emmons Glacier in Mount Rainier National Park has the largest surface area of any glacier in the lower 48 states at 4.3 square miles. Its terminus is almost hidden beneath rocks and debris, making it difficult to spot. In 1963, a rockfall from Little Tahoma Peak covered the lower glacier with debris, which continues to insulate the ice from melting. Because melting decreased, the glacier advanced rapidly in the mid-1960s and repeatedly in the following decades. A glacier advances when more snow accumulates than is lost. Snow compresses into ice, and debris collects on the glacier. Where the glacier ends, melting erodes the sides and leaves behind piles of rubble called terminal and lateral moraines. Climate affects glacier health, and periods of cooling or increased precipitation can lead to glacier advance.