When you ask a student what information is studied in geology class, the first word out of their mouth is “Rocks!” My response to this declaration is “Yes, geologists study rocks; among other topics.” Learning to identify rocks is a skill that takes significant effort and practice. This blog is the first in a series dedicated to learning how to identify rocks.
Igneous Rock Identification
Igneous rocks are arguably the most difficult type of rock to identify as they can form in many ways and therefore have many appearances. Brand new on my TpT store (MidnightStar) is a resource on identifying igneous rocks. The information is designed for use with high school or early college level students. Included with this product is a slide presentation with descriptions of primary and secondary igneous rock textures and photographs of each to help students accurately identify igneous rocks. An easy-to-read identification chart, as well as a log sheet for students to fill out during the identification process are also included. And for the teacher…. an answer key for common igneous rocks. This will be in an easy to print format so that students can fill in information as they are identifying the unknown igneous rock samples.
Students struggle most with the secondary texture, porphyritic.
A porphyritic texture is classified as fine grained, as the majority of the sample is of fine-grained material. However, a porphyritic rock has some scattered, larger crystals. They will be seen as rectangles, squares and other geometric shapes within the rock and students will erroneously conclude that the sample is coarse grained. I have discovered that using the phrase “Porphyritic is polka dotted” works well to help students differentiate between these two categories of rock. Clarification that the “polka dots” are not circles, but scattered crystals large enough to be seen with the unaided eye is recommended. After significant practice, students are able to accurately discern the difference between coarse grained and porphyritic rock samples.
Another struggle with igneous rock identification is differentiating fragmental rocks made with large fragments of rocks, from coarse-grained samples. The best description of their differences is that coarse-grained rocks will have vitreous, identifiable minerals, whereas volcanic breccia contains rock fragments that are NOT vitreous.
If you are looking for a stream-lined lesson to use for teaching people how to identify igneous rocks, look no further. The material provided here is a tested and trusted resource that has been used and tweaked over many years of teaching.
Note: the picture included here is of my box of igneous rock samples that I have used to teach students.