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How do I comprehend the lines after dashes?

For example:

The ice shelf cores were long enough to penetrate through glacial ice—which is formed from the compaction of snow and contains air bubbles—and to continue into the clear, bubble-free ice formed from seawater that freezes onto the bottom of the glacial ice.

After the first dash, it can be understood that the author is referring to glacial ice (as "which" stands for glacial ice). What about the sentence following the second dash?

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The two dashes belong together and delimit a single parenthetic statement. "The em dash [...] often demarcates a break of thought or some similar interpolation stronger than the interpolation demarcated by parentheses[.]" (Wikipedia). –  RegDwigнt Jun 27 '12 at 10:17
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closed as general reference by RegDwigнt Jun 27 '12 at 10:17

This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information.If this question can be reworded to fit the rules in the help center, please edit the question.

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