Geological materials as evidence
Geological materials, including rocks, minerals, fossils, soil particles, and humus, serve as crucial evidence in both criminal and civil investigations. These materials possess distinct characteristics—such as origin, color, hardness, and chemical composition—that forensic geoscientists analyze to trace their sources and link them to specific locations, including crime scenes. The classification of rocks into igneous, sedimentary, and metamorphic categories provides a foundational understanding of their formation processes and characteristics. For instance, igneous rocks can be intrusive or extrusive, with varying crystal sizes depending on their cooling rates.
Sedimentary rocks form from accumulated sediments, while metamorphic rocks are produced under heat and pressure from existing rock types. The analysis of geological materials also extends to human-made products like glass and concrete, which retain the properties of their natural components. Careful collection and examination of these samples, using advanced techniques and instruments, are critical for establishing their relevance to investigations. Overall, geological materials play a significant role in forensic science, providing valuable insights into the context of various cases.
Subject Terms
Geological materials as evidence
DEFINITION: Natural earth materials such as rocks, minerals, fossils, petroleum, soil particles, and humus.
SIGNIFICANCE: Geological materials have signature characteristics that make them very useful in the investigation of many criminal and civil cases. Comparisons of earth materials can allow investigators to locate the origins of evidence and pinpoint the locations of crime scenes.
Forensic geoscientists examine geological materials as well as some human-made products (such as glass, bricks, and concrete) to determine the origins of these materials and to compare them with samples collected from crime scenes. Because earth materials have specific characteristics and origins, these materials can provide very useful evidence in criminal as well as civil investigations.
Diversity of Geological Evidence
The many different kinds of rocks have clearly distinguishable origins, colors, sizes, crystalline structures, hardnesses, and chemical compositions. All rocks may be classified into one of three major groups: igneous, sedimentary, or metamorphic. Igneous rocks are formed by the cooling of molten magma as the magma moves from the earth’s mantle to near the air at the surface; the crystal formation in such rocks varies with the cooling time. When the magma cools very slowly, very large crystals are formed. Rocks of this type are called intrusive igneous rocks (an example is granite). In contrast, the expelled magma from volcanic eruptions cools rapidly, forming what are called extrusive igneous rocks. Some of these have very fine crystals (such as basalt), and others have no crystals and an amorphous or noncrystalline appearance (such as pumice).
Sedimentary rocks, which include sandstone, shale, and limestone, are made of loose sediments deposited by wind or water that have become cemented together and hardened over time. Metamorphic rocks, which include quartzite, slate, and marble, are formed when igneous or sedimentary rocks are altered by heat, temperature, pressure, and chemical reactions as they are compressed by the earth’s crust before they sink to the hot mantle.
Every rock is a mixture of minerals, which are pure substances that exhibit unique properties. Granite is composed of three minerals: quartz, feldspar, and mafic minerals. Basalt and pumice rocks contain feldspar and mafic minerals, but no quartz. Quartz—the major component in sands and glass—is light-colored pure crystalline silicon dioxide without aluminum, iron, magnesium, calcium, sodium, or potassium. In feldspar, 25 to 50 percent of the silicon is replaced by aluminum, and potassium, sodium, or calcium are present but no magnesium or iron. Feldspar weathers slowly to form silt and clay; it is a minor component of sands. Mafic minerals (such as olivine and pyroxene) contain silicon and high amounts of magnesium, iron, or both. They weather rapidly to form silt and clay. Crystals of mafic minerals are dark in color because of their high magnesium or iron content, compared with the light-colored crystals of feldspar. Sandstone is composed mostly of sand deposits cemented by a silicon dioxide mineral called silica, and limestone is composed of sand, silt, and clay deposits cemented by a calcium carbonate mineral called lime.
In the weathering of rocks and minerals, physical, chemical, and biological processes form particles of sand (the largest), silt, and clay (the smallest), the inorganic components of soil. When rocks are wet, mosses grow on them, and microorganisms such as bacteria and fungi feed on rocks by releasing their enzymes to dissolve the rocks’ surfaces and then absorbing elements from the rocks. Rock fragments and dead tissues of organisms that accumulate on rock surfaces can serve as media for seed germination and the growth of plants. As these processes continue over time, a layer of soil particles develops above the rocks. The dead organisms, including decomposed plant and animal residues, become humus, the organic matter component of soil.
Synthetic products made from geological materials retain the characteristics of those materials when they break down. Bricks, for example, are made of clay minerals. Concrete is made from a mixture of Portland cement (limestone mixed with clay or shale), water, and sand.
Sample Collection and Examination
Like all types of evidence samples, forensic geological samples must be collected with care. Records must be kept regarding when, where, and how each sample was collected, and proper procedures for taking samples must be followed. The details of the case being investigated generally dictate the kinds of samples collected. For example, to enable a determination of whether a particular automobile was present at a crime scene, technicians would collect separate samples of rocks, minerals, and chunks of soil from several places under the vehicle (such as fenders, tires, and tailpipe) for comparison with similar materials found at the crime scene.
When forensic geoscientists examine evidence samples, they use a variety of techniques and instruments to determine the physical and chemical properties of the materials and to make comparisons. With soil samples, they look at particle size distribution—that is, the percentages of sand, silt, and clay that make up the samples. These scientists use stereomicroscopes to characterize the size and color of geological samples and petrographic microscopes to identify minerals and examine rock textures. Among the tools used to determine the chemistry and structure of rocks are scanning electron microscopes and mass spectrometers (instruments that measure the quantity of atoms, or groups of atoms, in a sample based on their mass). Forensic geoscientists also use X-ray diffraction to identify clay minerals and other crystalline materials based on their crystal structures.
Bibliography
Fitzpatrick, Robert W. and Laurance J. Donnelly. "An Introduction to Forensic Soil Science and Forensic Geology: A Synthesis." Geological Society, London, Special Publications, vol. 492, 19 July 2021, doi.org/10.1144/SP492-2021-81. Accessed 14 Aug. 2024.
"Introduction to Forensic Geology." University of Massachusetts Lowell, 2024, faculty.uml.edu/Nelson‗Eby/Forensic%20Geology/Exercises/Introduction%20to%20Forensic%20Geology.pdf. Accessed 14 Aug. 2024.
Martin, Michael. Earth Evidence. Mankato, Minn.: Capstone Press, 2007.
Murray, Raymond C. Evidence from the Earth: Forensic Geology and Criminal Investigation. Missoula, Mont.: Mountain Press, 2004.
Perry, Dale L., ed. Instrumental Surface Analysis of Geologic Materials. New York: VCH, 1990.
Pye, Kenneth. Geological and Soil Evidence: Forensic Applications. Boca Raton, Fla.: CRC Press, 2007.
Tarbuck, Edward J., Frederick K. Lutgens, and Dennis Tasa. Earth Science. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.