Which layer is responsible for earth's magnetic field? Why?

Give a geographical reason for each of the following:

(i) Nife is so called.

(ii) The core of the earth is the most inaccessible part of the earth.

(iii) The earth's crust undergoes many changes.

The earth's crust is only one percent of earth's mass yet it is a large reservoir of resources for mankind. Justify this statement with examples.

What would happen if the earth's core no longer remain hot and the magma cools and hardens inside the earth? Give reasons to support your answer.