What spurred american trade with northern mexico?

The answer is the revolution that declared Mexico as an independent country. Both countries knew that they can benefit from it in terms of free trade and migration which promotes economic development. Spain was not happy of such negotiation. They believe that the trade will jeopardize the safety of Mexico. Mexico paid no heed and continued with the agreement with the United States.

meaning the clothes cost alot of money

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.