What makes the soil rich along the pacific coast but also makes it dangerous to live there?

Volcanoes.

Volcanoes are dangerous because of all destruction they cause, but theyr're also great for the soil.

When a volcano erupts, the result is ash dispersed all around the eruption site. The ash can contain different soil nutrients depending on the chemistry of the magma.

Eruptions also release bit of rocks, some are rich in magnesium, iron and potassium.

That's why regions with deposit of volcanoes soil are fertile.