How is the tragic outcome of the marriage between Romeo and Juliet foreshadowed in Act 2, Scene 6?

The tragic outcome of this hasty marriage is foreshadowed by Friar Lawrence's statement to Romeo that

These violent delights have violent endsAnd in their triumph, die, like fire and powderWhich, as they kiss, consume. (2.6.9-11)

In other words, the friar says, intense and volatile joys have intense and volatile endings.  When these joys triumph, such an end is inevitable, and the end will be swift and destructive, like when gunpowder is ignited by...

The tragic outcome of this hasty marriage is foreshadowed by Friar Lawrence's statement to Romeo that

These violent delights have violent ends
And in their triumph, die, like fire and powder
Which, as they kiss, consume. (2.6.9-11)

In other words, the friar says, intense and volatile joys have intense and volatile endings.  When these joys triumph, such an end is inevitable, and the end will be swift and destructive, like when gunpowder is ignited by fire.  All they must do is touch for the briefest of moments, and then they consume one another.  This statement seems to foreshadow that for Romeo and Juliet, because their love for one another came on them so suddenly and intensely, it will necessarily end as passionately as it began, and not in a good way.  They may "kiss" but that metaphorical kiss will lead to their ultimate and swift destruction.  At this point, Juliet rushes in, and the friar marries them in secret.