The answer depends on the terms of the mortgage.

Most adjustable rate mortgages adjust the interest rate and the payment amount at specific intervals to ensure that the entire principal balance is paid off at the specified term (usually 15 or 30 years).

In your situation, the bank will adjust the loan at 10 years with a new interest rate and calculate the payment based on another 20 years of amortization of the principal.

There are mortgages that do not necessarily amortize in the stated term and the borrower can owe a "balloon" payment to satisfy the mortgage at the end of the term. They are not common, however.