Why do the Greeks need to have colonies?
What is the terrain like in Greece that would
cause people to live elsewhere?

There was a lack of food in Greece, so the Greeks sent (forced) people to settle colonies across the sea at fertile locations. Those colonies supplied food they had in excess to the Greek mainland. Those colonies reached as far as Spain, and spread Greek culture around the Mediterrenean.

b toward the volcano.landward. the landward side of something is the side nearest to the land or facing the land, rather than the sea.

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