International Coal Group Inc. (NYSE: ICO) shares climbed 30.9% Monday to $14.44, after Arch Coal Inc. (NYSE: ACI) said it would buy the coal miner for $3.4 billion. Volume for International Coal stock was a towering 153.5 million shares, overwhelming a daily average of around 7.8 million.

The combined company will be the second largest U.S. metallurgical coal supplier and a top-five overall global coal producer and marketer. Arch will have a balanced metallurgical (“met”) and thermal coal portfolio with unparalleled operational diversification in every major U.S. supply basin and the No. 1 or No. 2 position in each of its three core operating regions.

“The acquisition of ICG is a significant strategic step that strengthens Arch’s position as a world-class, global coal franchise positioned for growth,” said Arch CEO Steven F. Leer.

“This transaction will greatly expand our participation in global met markets; provide a powerful platform for future organic met coal production growth; enhance and further diversify our met and thermal coal product slate; extend our operating portfolio into every major U.S. coal-producing basin; and solidify our position as one of the industry’s lowest cost producers.”

Ben Hatfield, president and CEO of International Coal Group, said, “ICG has assembled a high-quality portfolio of low-cost mining operations and reserves, and one of the industry’s most talented and productive workforces. By teaming up with Arch, we expect to realize tremendous value for the shareholders of both companies while ensuring that our operations achieve their full potential.”

ICG is a leading producer of coal in Northern and Central Appalachia and the Illinois Basin. The company has 13 active mining complexes, of which 12 are located in Northern and Central Appalachia and one in Central Illinois. ICG’s mining operations and reserves are strategically located to serve utility, metallurgical and industrial customers domestically and internationally.

Arch Coal is one of the world’s largest coal producers, with more than 160 million tons of coal sold in 2010. Arch’s national network of mines supplies cleaner-burning, low-sulfur coal to customers on four continents, including U.S. and international power producers and steel manufacturers. In 2010, Arch achieved record revenues of $3.2 billion.