Sedimentary, igneous, and metamorphic rocks make up the three main types of rocks found on Earth. Their formation processes give them distinct properties, including differences in relative hardness. But are sedimentary rocks always softer than igneous or metamorphic rocks?
Sedimentary rocks form from the accumulation and cementation of mineral and organic particles on the Earth's surface. The sediments compact under pressure and cement together to create sedimentary rock. Examples include shale, sandstone, limestone and conglomerate. Igneous rocks originate as magma or lava and cool into solid form. Granite, basalt, and obsidian are common igneous rocks. Metamorphic rocks arise when existing rocks undergo changes from intense heat, pressure, or chemical reactions. Slate, marble, gneiss and schist represent some metamorphic rocks.
Hardness denotes a mineral's resistance to scratching or abrasion. Geologists often use the Mohs hardness scale to classify and compare mineral hardness. The Mohs scale lists common minerals in order of increasing hardness, from talc at 1 to diamond at 10. The relative hardness between minerals can be assessed by which mineral can visibly scratch the other.
In general, igneous and metamorphic rocks tend to be harder than sedimentary rocks. Igneous rocks crystallize from molten material and metamorphic rocks form under immense heat and pressure, allowing formation of hard mineral crystals. Sedimentary rocks originate from mechanical and chemical weathering processes that break down existing rock into sediments. Fragments of various hardness then reunify into sedimentary rock.
However, hardness varies widely within each rock type based on mineral composition. For example, granite is an intrusive igneous rock with visible crystals of quartz, feldspar, mica, and other minerals. The quartz content gives granite an overall hardness around 6 or 7 on the Mohs scale. Basalt is an extrusive igneous rock with fine-grained mineral crystals and a Mohs hardness of 6. Both are harder than the sedimentary rock limestone, composed chiefly of calcite mineral, which has a Mohs hardness of 3.
Shale is a soft sedimentary rock with Mohs hardness from 2 to 4, while quartzite is a metamorphic rock derived from sandstone that may have a hardness of 7 due to high quartz content. Non-foliated metamorphic rocks like quartzite and marble tend to be harder than foliated metamorphic rocks like slate and schist which split into thin sheets. So individual sedimentary rocks may be softer or harder than individual igneous or metamorphic rocks.
Additionally, sedimentary rocks can be cemented together by hard minerals like silica. Mineral cement increases overall strength and hardness. Well-cemented sandstone and conglomerate are harder than poorly cemented mudstone or shale. Sedimentary breccia composed of angular cemented fragments can also be very hard and resistant to erosion.
So, while igneous and metamorphic rocks tend to exceed sedimentary rocks in hardness on average, substantial overlap exists between the rock types. Factors like mineral composition, grain size, and cements result in a wide hardness range even within a single rock type. Soft and hard examples can be found among sedimentary, igneous and metamorphic rocks. Absolute hardness depends on the specific rock's makeup, not just its overarching classification. Evaluating hardness on a case-by-case basis is required to determine relative hardness between any two rock samples.
