Berkshire Hathaway's class B shares (BRK.B) are going nuts today, with volume surpassing 13 million. Shares are up even as other market indexes are down, reports TheStreet.com.

Traders are heavy on the shares after the announcement that Berkshire will become a member of the S&P 100 and S&P 500 indexes, replacing Burlington Northern Santa Fe (BNI).

Warren Buffett's Berkshire is buying the railroad for $100 a share in cash and stock. It split its class B shares 50-to-1 to make it easier to issue stock to Burlington Northern shareholders, according to Dow Jones.

Now Berkshire is one of the 10 largest U.S. companies, with a market cap of nearly $170 billion, reports the Bespoke Investment Group.

Expect Berkshire's entry to cause some short-term havoc, Bespoke reports. That's because it will likely weight about 1.1% in the S&P 500 when it is added. That means index managers will have to make room for Berkshire by selling about 1.1% of their holdings in the index's remaining 498 stocks.