What Characteristics Do Geologists Look For When Observing A Rock Sample

Driveway cement is actually naturally occurring rock that has been pulverized.

Geologists have many fields of study depending upon the work they do. Some geologist work for environmental firms, some collect and study rocks and minerals for science labs and some work for the EPA. Many geologists are employed by major oil and gas-producing companies. If a geologist works in the field, he most likely collects rock and mineral samples for evaluation and study. Geologists look for basic characteristics in rocks that identify their structural and mineral makeup, age and origin.


Geologists can make instant determinations upon viewing the color or hue of a rock, since rocks contain chemicals and minerals. Some rocks have obvious colors, like sandstone, which can range from nearly white to dark brown. Magma generally appears very dark, while obsidian appears jet black — this clue indicates a volcanic origin. Sulfur-bearing rock can appear light to dark yellow. Rock containing copper can appear green. Large amounts of surface rust that looks reddish-brown indicates an iron-bearing rock, such as hematite.

Grain Shape

Geologists look at the grain shape to determine the rock’s weathering history. By using a magnifying glass, they can determine if the rock has broken recently by a sharp angular fracture — angular grains indicate the rock has been relatively static. The scientists classify grains according to roundness, which means if they have smooth, rounded edges, the rocks have been around for a long time, and have undergone transport by wind, glacier and water influences. A geologist knows a rounded grain rock might have traveled a great distance, and undergone very abrasive forces. Geologists classify grain size as angular, medium or rounded.

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Grain Size

Grain size can be either large or small, and tells the geologist about the force or energy that was required to move it. The larger the grain size, the more difficult it was to move it from the action of a stream or flood. The geologist knows that heavy, rounded rocks and smooth boulders have been acted upon by gravity and water forces, whereas small grain size and angular rocks have not moved or been eroded significantly.

Igneous Rocks

Geologists know that igneous rocks crystallize when subjected to great heat in volcanoes. The melting takes place near the plate boundaries and hot spots. Intrusive igneous rock results when large streams of molten rock rise to the surface, but are trapped underground where the rock cools slowly for thousands or millions of years. Intrusive igneous rocks have very course and large grain sizes. Extrusive igneous rocks form when magma leaves a volcanic chamber and breaks the surface. It then cools very rapidly on the earth’s surface as lava. A geologist knows that extrusive rocks have smaller and smoother grains.


Geologists look for sedimentary characteristics in rock, which is one of the most common traits. Sedimentary rocks consist of pieces or “clasts” that contain other rocks. Sedimentary rocks move via natural forces to depressions or basins where they become trapped, allowing layers of sediment to form over them. With enough pressure, different rocks fuse and become cemented together. Small clastic sedimentary grains begin as the smallest sizes with clay, then silt and then sand. Geologists can see the different compositions in the rock by viewing an edge, informing them it is sedimentary in structure and origin.

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Metamorphic rocks can contain a combination of sedimentary or original igneous structures, and sometimes an earlier metamorphic version of themselves. Geologists know these rocks have undergone the most chemical changes brought on by extreme pressures, heat, folding and the induction of mineral-rich fluids. In some cases, the original metamorphic rock minerals have completely changed on the atomic level to produce entirely new minerals. Metamorphic rocks end up smeared, squashed and folded into other rocks or over and upon themselves. They bend, not as a result of melting, but by the influence of high temperature and continuous pressure.

Orientation and Depth

Geologists often take photographs of rocks in their natural orientation. This allows the scientists to gauge if any uplift or movement has occurred in the area. They also note and catalog the matrix layer from where the rock was taken. They know older rocks come from deeper depths. If a geologist knows the matrix layer or sediment level, like Triassic, Jurassic or Mesozoic, they can attribute a rough age to the rock sample.