By Anora Mahmudova and Barbara Kollmeyer, MarketWatch

NEW YORK (MarketWatch) -- The U.S. stock market ended Monday's volatile session sharply lower as selling intensified at the last hour.

Volatile trading came on the heels of last week's deep losses that had been triggered by global economic growth concerns.

Monday's trading marked the fifth consecutive day of 1% moves for the S&P 500 and triple-digit moves for the Dow Jones Industrial Average.

Key technical levels were in focus, especially as there was no data on the economic calendar and bond markets were closed for Columbus Day holiday.

The S&P 500 (SPX) closed down 31.39 points, or 1.6%, to 1,874.74. The benchmark index has pulled back more than 6% from its peak reached on Sept. 18.

All of the main benchmarks are trading below their 200-day moving averages. Falling below the 200-day moving average level, and in the case of the S&P 500, below the 1,900 level, is considered significant as many analysts see the breach as a sign of further declines. Vote here: Does this stock slump have further to go?

The Dow Jones Industrial Average (DJI) dropped 223 points, or 1.3%, to 16,321.07. The blue-chip index turned negative for the year on Friday and is now down 1.5% year-to-date.

The Nasdaq Composite (RIXF) dropped 62.58 points, or 1.5%, to 4,213.66. The Russell 2000 (RUT) gave up solid gains of early morning and closed down 4.3 points, or 0.4%, at 1,049.17.

Karyn Cavanaugh, senior market strategist with Voya Investment Management, is not surprised by increased volatility.

"Normal markets are volatile and the 6% pullback on the S&P 500 seems like a big drop while in the middle of it, but it's not," Cavanaugh said.

Cavanaugh pointed out that companies that were profitable two months ago may see their share prices run up again after earnings are released soon. She argues that volatility only made stocks cheaper and more attractive.

On Tuesday, earnings season will get under way in earnest with major banks such as J.P. Morgan Chase & Co.(JPM) and Citigroup Inc. (C) among those due to report.

Stocks to watch:

CSX Corp. (CSX) jumped 5.9%. Citing sources, The Wall Street Journal reported that Canadian Pacific Railway Ltd. (CP) approached CSX about a tie-up. CSX rebuffed the overture, made in the past week, those sources said. It's unclear if Canadian Pacific will pursue it.

Shares of Tekmira Pharmaceuticals Corp. (TKMR) , one of the pharmaceutical companies whose drug TKM-Ebola has been used in the fight against the deadly virus, was up 3.9%.

Hazmat-suit maker Lakeland Industries Inc. (LAKE) surged 48% after a 126% jump last week on fears related to the Ebola virus. In addition, shares of face-mask maker Alpha Pro Tech (APT) soared 35%. Also read: Ebola stock trading volumes should raise red flags

J.C. Penney Co. (JCP) shares jumped as much as 6% before the bell, but pared those gains and closed 0.4% lower. The retailer on Monday said Home Depot executive Marvin Ellison will take over as a chief executive next year, succeeding current CEO Myron Ullman. The move comes days after J.C. Penney cut its sales forecast for the current quarter. Read about more notable stock moves in Movers and Shakers

Other markets: European stocks ended mostly flat, while Hong Kong stocks ( closed higher after strong Chinese trade data.

Gold (GCZ4) futures rose 1%, while crude oil (CLX4) slid.

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