1. The property of a structure whereby it has a closing contour. Vertical closure is the vertical distance from the apex to the lowest closing contour; areal closure is the area contained within the lowest closing contour. Compare trap. 2. The cumulative error around a survey loop; mis-tie (q.v.). 3. The condition where radioactive daughters are created and preserved, or lost at a known rate, so that counting them can be used for age determination. Closure occurs at different temperatures for different minerals.