As student loan debt piles up, defaults rates are accelerating and posing a threat to future economic growth, according to Federal Reserve experts.

The explosion of student loan debt over more than a decade, and particularly since the Great Recession, has been a worry spot for economists as the trend has accelerated. A paper this month from Don E. Schlagenhauf and Lowell R. Ricketts at the St. Louis Fed's Center for Household Financial Stability crystallizes the problem.

As the financial crisis and accompanying recession hit, consumers retreated on debt. Where household levels grew at an annualized rate of close to 10 percent before the recession, they've been rising at less than 1 percent since, according to Fed data.

And since growth rates in debt began to accelerate around 2012, some 90 percent of new debt has come in auto and student loans, according to Schlagenhauf and Ricketts. However, while auto debt balances have merely returned to around where they were in 2003, student loan debt has accelerated dramatically — up about 58 percent during the same period to an average of $25,000 per borrower. That's more than double the average auto balance of $12,200.

As the two researchers summarized: