Igneous, sedimentary and metamorphic rocks

A rock is a naturally occurring aggregate of one or more minerals, or of natural glass or organic matter. Geologists group rocks into three families by how they formed: igneous rocks crystallize from molten material, sedimentary rocks form from the debris or chemical precipitates of other rocks, and metamorphic rocks form when existing rocks are transformed by heat, pressure and fluids. For an engineer, the name of a rock is useful mainly as a shorthand for its likely strength, durability, layering, permeability and problems. A granite, a shale and a limestone of the same size behave very differently in a cut slope, a tunnel or a foundation, and knowing the family and the name gives you a first prediction before any testing.

The rock cycle

The three families are linked by the rock cycle. Magma cools to form igneous rock. Igneous rock exposed at the surface weathers and erodes; the sediment is transported, deposited, buried and cemented into sedimentary rock. Deep burial or mountain building heats and squeezes rocks into metamorphic rock, and enough heat can melt them back into magma. Any rock can follow many paths through the cycle, and the processes run at rates from years for erosion of soft deposits to hundreds of millions of years for burial and uplift.

Igneous rocks

Igneous rocks are classified by texture, which reflects how fast the melt cooled, and by composition, which reflects which minerals crystallized. Intrusive (plutonic) rocks cooled slowly at depth and have coarse grains visible to the eye. Extrusive (volcanic) rocks cooled quickly at or near the surface and are fine-grained or glassy; some contain gas holes called vesicles, and pyroclastic rocks are made of fragments blown out of volcanoes.

Fresh, massive igneous rocks such as granite, diorite, gabbro and basalt are among the strongest construction materials, with unconfined compressive strengths often above 100 MPa. Their problems usually come from discontinuities and weathering rather than from the intact rock. Granite can weather in place to a deep, sandy residual soil with corestones of fresh rock, which makes rock-head levels highly irregular. Columnar basalt is broken by cooling joints into polygonal columns that can topple from cut faces. Volcanic sequences often contain weak layers of ash (tuff), weathered flow tops, buried soils and vesicular zones that are far more permeable and weaker than the dense flow interiors.

Eastern Washington is underlain by the Columbia River Basalt Group, a huge stack of lava flows erupted roughly 17 to 6 million years ago. Each flow has a dense interior broken into columns and an upper zone that is often vesicular, rubbly or weathered, and some flows are separated by sedimentary interbeds. The dense interiors make excellent foundations and quarry rock, while the flow tops and interbeds are the main aquifers in the region and can be weak zones in cuts. Granite and metamorphic rocks are also found around Spokane, beneath and beside the basalts.

Sedimentary rocks

Sedimentary rocks cover about three quarters of the continental surface, though they form only a thin veneer over igneous and metamorphic basement. They are classified as clastic, made of fragments of other rocks, or chemical and biochemical, made of minerals precipitated from water, often with the help of organisms.

Clastic rocks are named by grain size. Conglomerate and breccia contain gravel-sized clasts, rounded in conglomerate and angular in breccia. Sandstone is made of sand-sized grains, most often quartz, held together by a cement of silica, calcite or iron oxide; the type and amount of cement largely determine its strength. Siltstone and mudstone are made of silt and clay; when a fine-grained rock splits readily along closely spaced bedding planes it is called shale.

Chemical and biochemical rocks include limestone, made mostly of calcite, often from shells and skeletal debris; dolostone, made of dolomite; chert, made of microcrystalline silica; evaporites such as rock salt and gypsum, which precipitate when seawater or lake water evaporates; and coal, made of compressed plant material.

Sedimentary rocks are typically layered. Bedding planes, where deposition paused or changed, are surfaces of weakness and often of higher permeability. Thin clay or bentonite seams within a sequence can be extremely weak, especially if they have been sheared by earlier movement, and many large landslides in sedimentary rocks slide along such seams. Shales and mudstones may look strong in a fresh core but break down within days or weeks of exposure to wetting and drying, a process called slaking. Engineers test this with the slake durability test and protect exposed shale faces and foundation subgrades promptly.

Much of the interior United States is underlain by flat-lying sedimentary rocks, from the Paleozoic limestones and shales of the Midwest to the Cretaceous shales of the northern Great Plains. Limestone regions such as central Kentucky, Tennessee, Missouri and Florida are known for karst. In the Northeast and Midwest, glacial deposits cover much of the bedrock. In eastern Washington, sedimentary rocks are found mainly as interbeds within the basalt and as younger basin fills.

Metamorphic rocks

Metamorphism changes a rock's minerals and texture without melting it. Heat and pressure cause new minerals to grow, and directed pressure aligns platy and elongated minerals into a parallel fabric called foliation. Metamorphic rocks are classified first by whether they are foliated.

Foliated rocks form a series of increasing metamorphic grade. Slate forms from shale at low grade and splits into thin, flat sheets along slaty cleavage. Phyllite has a silky sheen from fine mica. Schist has visible mica flakes and splits easily along its foliation, making it strongly anisotropic. Gneiss, at high grade, shows light and dark mineral bands and is much stronger and less fissile than schist.

Non-foliated rocks form when the minerals are equidimensional or the pressure was uniform. Marble forms from limestone or dolostone and, like its parent, is soluble. Quartzite forms from quartz sandstone and is one of the hardest, most durable and most abrasive rocks known. Hornfels forms by contact metamorphism next to intrusions and is dense and strong.

The key engineering issue with foliated rocks is anisotropy. Strength, stiffness and permeability parallel to the foliation may be a fraction of the values across it. A schist slope dipping out of a cut along its foliation can fail where the same rock dipping into the slope would stand steeply. Talc, chlorite and graphite schists are especially weak. Serpentinite, formed by the alteration of ultramafic rocks, is often sheared, slippery, and may contain naturally occurring asbestos.

Rock strength and simple properties

For preliminary assessment, engineers estimate rock strength from field tests and standard descriptions before laboratory testing. The unconfined compressive strength (UCS) of intact rock ranges from less than 1 MPa for very weak mudstones to more than 250 MPa for quartzite and fresh basalt. Field estimates use simple tests: whether the rock can be peeled with a knife, scratched, broken by hand, or needs one or many hammer blows. The point load test, which squeezes a core or lump between two cones, gives an index that correlates roughly with UCS and is quick enough to run on many samples in the core shed.

Two other simple properties appear in almost every rock description: unit weight and porosity. Most common rocks have unit weights of about 25 to 28 kN/m³; porous sandstones and weathered rocks are lighter, and mafic rocks heavier. Porosity, the volume of voids as a fraction of total volume, ranges from under 1 percent for fresh granite to 30 percent or more for poorly cemented sandstone and chalk. High porosity generally means lower strength, higher permeability and greater susceptibility to frost damage.

# Point load index and estimated UCS for a set of core tests
import math

tests = [  # (rock, core diameter mm, failure load kN)
    ("sandstone", 54, 7.8),
    ("sandstone", 54, 6.9),
    ("mudstone",  54, 1.1),
    ("basalt",    63, 21.5),
]
K = 22  # site-specific conversion factor should be confirmed by UCS tests

def descr(ucs):
    for limit, name in [(1, "extremely weak"), (5, "very weak"), (25, "weak"), (50, "medium strong"),
                        (100, "strong"), (250, "very strong")]:
        if ucs < limit:
            return name
    return "extremely strong"

for rock, d, p in tests:
    i_s = p * 1000 / d**2
    i_s50 = i_s * (d / 50) ** 0.45
    ucs = K * i_s50
    print(f"{rock:10s} Is(50) = {i_s50:5.2f} MPa  UCS ~ {ucs:5.0f} MPa  ({descr(ucs)})")

sandstone  Is(50) =  2.77 MPa  UCS ~    61 MPa  (strong)
sandstone  Is(50) =  2.45 MPa  UCS ~    54 MPa  (strong)
mudstone   Is(50) =  0.39 MPa  UCS ~     9 MPa  (weak)
basalt     Is(50) =  6.01 MPa  UCS ~   132 MPa  (very strong)

The size correction matters because smaller cores give slightly higher indices than larger ones. Standard practice corrects every result to the 50 mm reference size before comparing tests or converting to UCS. Point load tests are also run axially and on irregular lumps, with an equivalent diameter calculated from the specimen dimensions, which makes the test useful on broken core that could never be trimmed for a laboratory UCS test.

Rock as a construction material

Rock is excavated not only to make room for structures but also to be used: as riprap for erosion protection, as armor stone on breakwaters, as crushed aggregate for concrete and pavements, and as rockfill in embankments and dams. Each use needs durability as well as strength. Riprap and armor stone must resist freeze–thaw cycles, wetting and drying, and abrasion by waves and ice without splitting along hidden seams. Common acceptance tests include the specific gravity and absorption of the stone, soundness tests that simulate freezing or salt crystallization, abrasion tests such as the Los Angeles or micro-Deval tests, and visual inspection for cracks, bedding planes and weathering. Dense igneous rocks, quartzite and sound limestone generally perform well; shales, poorly cemented sandstones and weathered or micaceous rocks often fail within a few winters.