Explanation: When DC voltage is applied across the plane surfaces of a quartz crystal, the crystal experiences a physical distortion. This phenomenon is known as the **piezoelectric effect**, where mechanical stress is produced in response to an applied electric field. The internal structure of quartz causes it to expand or contract depending on the voltage polarity and magnitude. If the applied DC voltage is excessive, the resulting physical distortion can exceed the mechanical limits of the quartz material. This severe strain can cause the crystal to crack or fracture, leading to permanent damage. Therefore, both the initial physical distortion and the potential for cracking under high voltage are correct consequences.