Geology rocks!

Happy Earth Science Week 2018!

Earth Science Week is an annual week-long celebration of everything earth science.  This celebration is sponsored the American Geosciences Institute.

Of the four earth science disciplines (geology, meteorology, oceanography, and astronomy), geology is my favorite.  Geology is the study of the earth itself, the substances that make up the earth, the processes that shape the earth, and how those processes and substances have changed over time (and continue to change today). 

Everywhere we go we see geologic processes and materials, especially rocks in their various states.  It is easy to overlook them because they are so commonplace, but when you understand the processes that shape them and the history behind them rocks are much easier to appreciate. 

It all starts with knowing how rocks form.

Rocks are divided into three main types based on how they form: igneous, sedimentary, or metamorphic.

Igneous rocks form when molten magma or lava cool down and become solid.  Magma and lava are both the same thing – rock that is so hot it has melted.  The only difference between magma and lava is their location.  Magma is found below the surface of the earth.  Lava is found above the surface.

When magma and lava cool down, the molten minerals that are contained within crystallize.  We can tell where an igneous rock formed by looking at the size of the crystals.  Igneous rocks that form below the surface cool slowly and form large crystals.  These rocks are known as intrusive igneous rocks or plutonic rocks.  Igneous rocks that form above the surface cool down quickly and the crystals are smaller because they have less time to form.  These are known as extrusive igneous rocks.

Pegmatite (an intrusive igneous rock) has very large visible crystals.

Devil's Tower is made of intrusive igneous rock - the softer rock surrounding it has eroded away over time.

Sedimentary rocks are formed from smaller particles of rocks and/or minerals that have been fused together by pressure and the crystallization of dissolved minerals.  Large rocks of all types break down over time.  Natural factors such as wind, water, and temperature change cause the surface of rocks to weaken.  This process is known as weathering.  If the surface of a rock breaks down enough, wind and water may remove this weathered material from the surface of the parent rock and deposit it in another location.  This process of removal and deposition is known as erosion.  The particles that are carried away and deposited are known as sediment.

Given enough time these sediments can build up in thick layers.  The weight of upper layers of sediment can cause particles in the lower layers to fuse together.  Sometimes dissolved minerals in the lower layers will crystallize as water is forced out by the pressure of the upper layers.  These minerals act like a cement or glue, further fusing the sediments together.  The layers of sediment can often be seen in sedimentary rocks.

 

Visible layers in sandstone show how this sediment was deposited over time.

 

The angled layers in this sandstone indicate that a current was present when the sediment was deposited.

Metamorphic rocks are rocks that have been changed by heat, pressure, or chemical processes.  The word metamorphic actually means “changed in form”.  The intense heat and pressures that are found deep within the earth can cause changes to preexisting rocks.  The minerals within a rock may partially melt and become glassy in appearance.  The crystals may rearrange themselves in reaction to external pressures.  This often results in bands or “foliations” forming within the metamorphic rock.  Metamorphic rocks can also form through a change in the chemical composition of the minerals within a preexisting rock.

Metamorphic rocks can form from any type of rock.  Igneous rocks, sedimentary rocks, and even metamorphic rocks can all change form.

 

Fracturing and folding in a metamorphic rock indicate that this rock faced pressure coming from the sides