Multi-element composition map of andesite from Colorado.  Each color indicates a high concentration of a different element. 
Purple = magnesium, green = calcium and aluminum, turquoise = potassium and aluminum (1 Mb full resolution image)

Andesite is a type of volcanic rock.  This sample of andesite erupted from volcanoes in southern Colorado during the Oligocene epoch, roughly 30 million years ago.  Volcanic rocks form by crystallization of molten liquid rock (magma) when it erupts out of the ground.  The Earth’s surface is a cold environment compared to deep underground, so the liquid magma freezes to form crystals (minerals) just like water in puddles freezes on cold winter nights.  Crystallization usually starts underground before the magma erupts to the surface.  Initially, slow cooling stars slowly forming larger, well-shaped crystals (phenocrysts).  When the magma erupts onto the Earth's surface, the remaining liquid magma freezes quickly to form much smaller crystals, usually too small to see without a microscope.

These images are chemical maps of a sample of andesite that I cut with a diamond-edged rock saw, then polished, and then coated with a thin layer of carbon.  The image labeled BSD is the backscatter image that highlights the difference in compositions of different crystals.  In BSD images, brighter gray and white indicate things made of higher atomic number elements.

Each of the single-color maps shows where the EDS detected a specific element (Al = aluminum, Ca = calcium, K = potassium, Fe = iron, Mg = magnesium, Si = silicon, Na = sodium, and Ti = titanium).  More intense brightness indicates higher concentration.  The “multi” map is a composite map with these layers all combined into one image.

These images tell us that magnesium-bearing minerals formed first (the three biggest crystals near the corners of the image.  Calcium-sodium-aluminum bearing silicon minerals formed next (the smaller, blocky crystals).  Finally, the potassium-bearing minerals formed last, filling in the space between the older, blocky crystals.  We can see a lot more information in these data, but this is a website for the general public.