Two weeks ago, Apple Computer had its day in the sun, briefly surpassing Exxon Mobil in market capitalization to become the largest corporation in the world. Now it’s gold’s turn.

When the markets opened Monday, according to Robert Goldsborough, an ETF analyst at Morningstar, the total amount of money invested in the SPDR GLD ETF, the first and largest ETF for gold and backed by physical bullion held by the issuer, State Street Bank, was $76.67 billion.

Its total value easily outstripped the $74.38 billion tied up in the SPDR SPY ETF, which is the oldest and largest ETF and tracks the S&P 500 Index.

By the close of trading Tuesday, the assets of the SPDR GLD ETF had fallen slightly to $75.95 billion, while the SPDR EPY’s asset value increased to $79.62 billion, putting it back up on top. In this prevailing period of market volatility and concerns about the direction of the U.S. and global economies, it is likely that these two giant ETFs will continue to battle for the top spot in terms of total ETF asset value.

Goldsborough said the rise in assets in the GLD ETF is the result of both increased inflows of investor money -- as investors look for a safe haven from volatile equities at a time when interest rates are near zero -- and a product of the spike in the underlying price of gold, which crested above $1,800 an ounce last week. 

Likewise, some of the decline in the asset value of the SPY ETF is the result of investors pulling out of equities in favor of what they hope are less risky assets as well as a decline in the S&P 500 Index itself.

In essence, writes Goldsborough, what is going on “can be captured in one word: fear.”  He explains, “Even with relatively attractive valuations of U.S. stocks, investors right now are afraid to invest in large-cap U.S. equities,” and they are looking for a safe haven.

He said that with money market funds offering a 0% return, and with yields on 10-year Treasuries at just over 2%, “it’s easy to envision investors shunning a negative real return in money market funds when they can potentially see better returns from holding gold.”

Goldsborough told On Wall Street that when all the ETFs that track the S&P 500 are added up together, their combined asset value at Tuesday's close was about $110 billion, while the asset value of all gold ETFs combined was about $105 billion. He predicts the two groups of ETFs will be trading places in the assets ranking for some time, just like the SPDR GLD and SPDR SPY ETFs.

Of course, there is much more money invested in equities than in gold, but Goldsborough still said it is significant that the current investor search for a financial safe haven has allowed a gold ETF to become as big as the oldest and biggest ETF on the market, the SPDR SPY.