On July 4, 2009, about 1045 eastern daylight time, a Cessna 172M, N64178, collided with the ground during an emergency landing in Princeton, New Jersey. The student pilot was not injured and the airplane was substantially damaged. The solo instructional flight was operated under the provisions of 14 Code of Federal Regulations (CFR) Part 91, and no flight plan was filed. The flight had originated shortly before the accident and visual meteorological conditions prevailed. The student pilot stated that he prepared to remain in the traffic pattern for a touch-and-go landing. After a preflight inspection of the airplane, he taxied to the run-up area. He conducted all preflight checks and a run-up of the engine, and all engine indications were normal. He then taxied to the active runway and departed to stay in the closed traffic pattern. As the airplane climbed, he turned crosswind, downwind, and went through the landing checklist. He said that as he turned to base and reduced engine power, the engine went to idle. He immediately added full power with no response from the engine. Subsequently, he turned the airplane towards the runway and the power came back up briefly. The power decreased again and the pilot made an emergency landing in a field. The student pilot stated that prior to take off the carburetor heat check indicated a normal drop in rpm. He also reported that the carburetor heat was in use when the engine loss power. Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the airplane descended through trees into a field. The airframe and flight control system components revealed no evidence of preimpact mechanical malfunction. Examination of the fuel system revealed that the fuel tanks were full, and the fuel selector was in the "both" position. Further examination of the fuel system revealed no water or debris was found in the fuel system. A cursory examination of the engine and system components by the FAA inspector revealed no evidence of preimpact mechanical malfunction. The current ambient temperature was 75 degrees Fahrenheit, with a dew point of 55 degrees Fahrenheit. Review of the carburetor icing probability chart revealed that serious icing could occur at glide power.