Squamous cell carcinoma is the second most common type of skin cancer. It forms when squamous cells begin to grow uncontrollably in the top layers of the skin, called the epidermis. In most cases, it is caused by repeated exposure to UV rays over time. Squamous cell carcinoma usually first appears as a red, scaly plaque of skin or as a hard domed bump. Both varieties typically feel rough and crusty and can bleed when scraped.