What are two ways a primary magma can be generated? What will ...

[lunatika]

What are two ways a primary magma can be generated?
 
  What will be an ideal response?

Question 2

In July 1863, Union and Confederate forces fought in a three-day battle at Gettysburg, Pennsylvania. The Confederate position was on Seminary Ridge, an igneous dike. The Union position was on Cemetery Ridge, an igneous sill.
 
  Using what you know about the history of the battle, interpret how the geology of the battlefield aided the Union victory.
  What will be an ideal response?

[akudia]

Answer to Question 1

Answer: Primary magmas can be generated through decompression melting at mid-ocean ridges or when water dehydrated from a subducting plate promotes partial melting.

Answer to Question 2

Answer: Cemetery Ridge is composed of a dense, resistant basaltic material, so resistant that even though it is geologically older than the dike at Seminary Ridge, it stands higher and steeper in relief. This made it very, very difficult for Confederates to storm, particularly under fire and weighed down with gear. The steepness also made firing uphill accurately difficult for the Confederates. Finally, the basaltic material was so resistant to weathering that some units (particularly those on Little Round Top) built breastworks out of smaller chunks of the material. Reports from soldiers indicate that the lead bullets bounced off the dense rock of the breastworks.