An earth-boring tool , comprising: a bit body ; and a cutting element secured to the bit body , the cutting element exhibiting a contact back rake angle with respect to a surface of a formation to be cut by the bit body , wherein the contact back rake angle is an angle formed by the intersection of a centerline of the cutting element and the surface of the formation , the cutting element comprising a polycrystalline superabrasive material comprising: a first volume including catalyst material in interstitial spaces among interbonded grains of the polycrystalline superabrasive material; a second volume at least substantially free of catalyst material in the interstitial spaces among the interbonded grains of the polycrystalline superabrasive material; and a continuous boundary between the first volume and the second volume of the polycrystalline superabrasive material, wherein the boundary is non-linear in a cross-sectional plane that includes the centerline of the cutting element and an anticipated point of contact of the cutting element with the surface of the formation to be cut by the earth-boring tool , and wherein each line tangential to the boundary in the cross-sectional plane forms an angle with the centerline of the cutting element greater than the contact back rake angle of the cutting element . a bit body ; and a cutting element secured to the bit body , the cutting element exhibiting a contact back rake angle with respect to a surface of a formation to be cut by the bit body , wherein the contact back rake angle is an angle formed by the intersection of a centerline of the cutting element and the surface of the formation , the cutting element comprising a polycrystalline superabrasive material comprising: a first volume including catalyst material in interstitial spaces among interbonded grains of the polycrystalline superabrasive material; a second volume at least substantially free of catalyst material in the interstitial spaces among the interbonded grains of the polycrystalline superabrasive material; and a continuous boundary between the first volume and the second volume of the polycrystalline superabrasive material, wherein the boundary is non-linear in a cross-sectional plane that includes the centerline of the cutting element and an anticipated point of contact of the cutting element with the surface of the formation to be cut by the earth-boring tool , and wherein each line tangential to the boundary in the cross-sectional plane forms an angle with the centerline of the cutting element greater than the contact back rake angle of the cutting element . a first volume including catalyst material in interstitial spaces among interbonded grains of the polycrystalline superabrasive material; a second volume at least substantially free of catalyst material in the interstitial spaces among the interbonded grains of the polycrystalline superabrasive material; and a continuous boundary between the first volume and the second volume of the polycrystalline superabrasive material, wherein the boundary is non-linear in a cross-sectional plane that includes the centerline of the cutting element and an anticipated point of contact of the cutting element with the surface of the formation to be cut by the earth-boring tool , and wherein each line tangential to the boundary in the cross-sectional plane forms an angle with the centerline of the cutting element greater than the contact back rake angle of the cutting element . <EOS>