Google's parent company hit a major financial milestone on Monday, reaching a $4tn valuation for the first time and surpassing Apple to become the second-most valuable company in the world. Alphabet is the fourth company to hit the $4tn milestone after Nvidia, which later hit $5tn, Microsoft and Apple . The spike in share price comes after Apple announced it had chosen Google's Gemini AI model to power a major overhaul of the iPhone maker's digital assistant Siri, which comes installed in every iPhone. Neither company disclosed how much the deal was worth. "After careful evaluation, we determined that Google's technology provides the most capable foundation for Apple Foundation Models," Apple said in a statement to CNBC . As tech stocks continue a years-long meteoric rise, fears of a bubble in the stock market persist; however, Wall Street's excitement for new avenues of investment in AI does as well.

FILE - This Oct. 20, 2015, file photo, shows a sign outside Google headquarters in Mountain View, Calif. Authorities have arrested a Northern California man suspected of attacking the headquarters of Internet search giant Google with Molotov cocktails and a gun. Mountain View police arrested Raul Diaz on the company's campus shortly after midnight Thursday, June 30, 2016. The 30-year-old is charged with one count of arson in connection with an attempted firebombing of a Google vehicle used to map streets. Authorities are investigating whether the 30-year-old is connected to two other attacks, including the torching a company self-driving car and the shooting out of office windows.