Earth Materials There are three main categories of Earth materials considered in rock mechanics. Rocks Banded Rocks http://www.sscadda.com/2016/05/study-notes-on-types-of-rock-and-their.html Foremost, rocks are aggregated mineral grains. The three types of rocks are igneous, metamorphic, and sedimentary. One aspect of rock strength is determined by how well the grains are interlocked. Igneous, metamorphic, and chemical sedimentary rocks are generally the most compact. The other type of sedimentary rock, clastic, are made up of smaller rocks that were then cemented together. It is also important to note that large rock masses are not always completely solid, but have fractures and planes of weakness, which in turn affects overall rock strength. Soils Different Types of Soils https://botanicseye.com/plant/outdoor-growing/soil-types/ Soil can be comprised of different types of rock particles and minerals derived from rocks, as shown in the picture above. A key distinguishing feature between soils and rocks are grain size and compaction. It is less difficult to excavate soils compared to rock, since they are much more loosely compacted. Fluids Magma Chamber http://www.livescience.com/33330-yellowstone-caldera-supervolcano-eruption.html This category is comprised of several types of fluids - water, magma, and oil. Gas is also considered as a fluid here. Presence of fluids impacts rock strength.