Review the three photographs from the image collection. What does each image add to your knowledge of life in New York City tenements? How do these three images connect to the ideas and messages in the other three types of documents?

It means at a diverging boundary when magma spreads to both sides it is almost identical in both the side.

Explanation:

At a diverging boundary when magma spreads to both sides it is almost identical in both the side.

If you take a picture of one side of the boundry and the spreading of sea floor and place it before a mirror you can see the image is identical to the picture of other side.

Therefore, the meaning of saying, each side of the sea floor away from the mid ocean ridge is a mirror image of the other side is explained above.