Intrusive and Extrusive Igneous Rocks

At first, many people are puzzled when they learn about magma and lava. They are both exactly the same stuff! The only difference is that it is called magma while it is underground, and lava when it is on the surface. Why do we need two different words for it? As you will see, the rocks formed underground by magma are very different from the rocks that form on the surface from lava. As you see how different those rocks are, you will get a better understanding of why we use "magma" and "lava".

Identifying Igneous Rocks

When we identified minerals, we had very precise tests to perform and specific properties to measure. Identifying igneous rocks is not that simple. They are classified by what minerals they contain, and the size of those mineral pieces, which are called grains. This creates a system that is not incredibly precise. For example, to be classified as granite, a rock must have at least 10% quartz. But what if it only has 9.99% quartz? For that matter, how can you measure exactly what percentage of a rock is quartz?

Geology in the City

What if you live in a big city, with no road cuts or rock outcrops nearby? Are you out of luck for finding rocks to study? No! Many of the buildings in most cities have decorative stonework covering their outer walls and their lobbies. You won't be able to collect specimens, but you can collect some marvelous photographs.

Foliated and Unfoliated Rocks

One of the first steps towards identifying metamorphic rocks is to determine if it has foliation or not. Luckily, this is usually easy to do. Foliation means banding or alignment of the minerals resulting from pressure. A good example of that is the banding present in a metamorphic rock called gneiss, pronounced "nice".

This piece of gneiss formed from granite similar to the rock sample seen on the left hand side of the photo.

Sorting Sediments

One of the main things that we use for identifying clastic sedimentary rocks is grain size, the size of the pieces. Rocks that have been deposited by water or wind tend to be very well sorted, with all of the pieces being pretty much the same size. How does that happen?

Clastic Rocks

Clastic rocks are sedimentary rocks that are made up of pieces of other rocks that have been deposited by wind, water, ice, or gravity. They are identified by the size of those pieces, and whether the pieces are loose or cemented together.


In spite of my lingering vertigo, Nancy and I joined my sister and my niece for a quick trip to the Grand Canyon. The Grand Canyon cuts through many layers of rock, but the Coconino Sandstone is very different from the other layers. What makes it so different?

Understanding "Solid"

At first, the word "solid" may seem simple, but when it comes to rocks and minerals, it can be tricky.

To explore this, we will use the same materials that we used to explore "naturally occurring." You will need as many of the items from the following list as you can find: