Sedimentary Rocks

Sedimentary rocks are the second great rock class. Whereas igneous rocks are born hot, sedimentary rocks are born cool at the Earth's surface, mostly under water. They usually consist of layers or strata, hence they are also called stratified rocks. Depending on what they're made of, sedimentary rocks fall into one of three types.

Clastic Sedimentary Rocks

The most common set of sedimentary rocks consist of the granular materials that occur in sediment: mud and sand and gravel and clay. Sediment mostly consists of surface minerals — quartz and clays — that are made by the physical breakdown and chemical alteration of rocks. (Feldspar and other minerals may also be in sediment if they have not had time to break down.) These are carried away by water or wind and laid down in a different place. Sediment may also include pieces of stones and shells and other objects, not just grains of pure minerals. Geologists use the word clasts to denote particles of all these kinds, and rocks made of clasts are called clastic rocks.

Look around you at where the world's clastic sediment goes: sand and mud is carried down rivers to the sea, mostly. Sand is made of quartz, and mud is made of clay minerals. As these sediments are steadily buried over geologic time, they get packed together under pressure and low heat, not much more than 100°C. In these conditions the sediment is cemented into rock: sand becomes sandstone and clay becomes shale. If gravel or pebbles are part of the sediment, the rock that forms is conglomerate. If the rock is broken and recemented together it is called breccia. See examples of all these in the Sedimentary Rock Gallery.

It's worth noting that some rocks commonly lumped in the igneous category are actually sedimentary. Tuff is consolidated ash that has fallen from the air in volcanic eruptions, making it just as sedimentary as a marine claystone. There is some movement in the profession to recognize this truth, although I still observe convention by mentioning tuff in About Igneous Rocks.

Organic Sedimentary Rocks

Another type of sediment actually forms in the sea as microscopic organisms — plankton — build shells out of dissolved calcium carbonate or silica. Dead plankton steadily shower their dust-sized shells onto the seafloor, where they accumulate in thick layers. That material turns to two more rock types, limestone (carbonate) and chert (silica). These are called organic sedimentary rocks, although they're not made of organic material as a chemist would define it.

Another type of sediment forms where dead plant material builds up into thick layers. With a small degree of compaction, this becomes peat; after much longer and deeper burial, it becomes coal. Coal and peat are organic in both the geological and the chemical sense.

Although peat is forming in parts of the world today, the great beds of coal we mine formed during past ages in enormous swamps. There are no coal swamps around today, because conditions do not favor them. The sea needs to be much higher. Most of the time, geologically speaking, the sea is hundreds of meters higher than today and most of the continents are shallow seas. That's why we have sandstone, limestone, shale and coal over most of the central United States and elsewhere around the world's continents. (Sedimentary rocks also become exposed when the land rises. This is common around the edges of the Earth's lithospheric plates, and for more about that, see Plate Tectonics in a Nutshell.)

Chemical Sedimentary Rocks

These same ancient shallow seas sometimes allowed large areas to become isolated and begin drying up. In that setting, as the seawater grows more concentrated, minerals begin to come out of solution (precipitate), starting with calcite, then gypsum, then halite. The resulting rocks are certain limestones or dolomites, gypsum rock, and rock salt respectively. These rocks, called the evaporite sequence, are also part of the sedimentary clan.

In some cases chert can also form by precipitation. This usually happens below the sediment surface, where different fluids can circulate and interact chemically.

Diagenesis: Underground Changes

All kinds of sedimentary rocks are subject to further changes during their stay underground. Fluids may penetrate them and change their chemistry; low temperatures and moderate pressures may change some of the minerals into other minerals. These processes, which are gentle and do not deform the rocks, are called diagenesis as opposed to metamorphism (although there is no well-defined boundary between the two).

The most important types of diagenesis involve the formation of dolomite mineralization in limestones, the formation of petroleum and of higher grades of coal and the formation of many types of ore bodies. The industrially important zeolite minerals also form by diagenetic processes.

Sedimentary Rocks Are Stories

You can see that each type of sedimentary rock has a story behind it. The beauty of sedimentary rocks is that their strata are full of clues to what the past world was like. Those clues might be fossils, marks left by water currents, mudcracks or more subtle features seen under the microscope or in the lab.

From these clues we know that most sedimentary rocks are of marine origin, usually forming in shallow seas. But some sedimentary rocks formed on land: clastic rocks made on the bottoms of large freshwater lakes or as accumulations of desert sand, organic rocks in peat bogs or lake beds, and evaporites in playas. These are called continental or terrigenous (land-formed) sedimentary rocks.

Sedimentary rocks are rich in geologic history of a special kind. While igneous and metamorphic rocks also have stories, they involve the deep Earth and require intensive work to decipher. But in sedimentary rocks you can recognize, in very direct ways, what the world was like in the geologic past.

Rock Identification Tables

These tables will help you identify almost any rock you're likely to find. Read How to Look at a Rock for help with your observations.

First, decide whether your rock is igneous, sedimentary or metamorphic.

• Igneous: A tough, frozen melt with little texture or layering; mostly black, white and/or gray minerals; may look like lava (about igneous rocks)
• Sedimentary: Hardened sediment with layers (strata) of sandy or clayey stone; mostly brown to gray; may have fossils and water or wind marks (about sedimentary rocks)
• Metamorphic: Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica (about metamorphic rocks)

Next, check the rock's grain size and hardness. Then start in the left column of the appropriate table below and work your way across. Follow the links to pictures and more information. If you don't find a match, try another of the three big types.

Grain Size: "Coarse" grains are visible to the naked eye (greater than about 0.1 millimeter), and the minerals can usually be identified using a magnifier; "fine" grains are smaller and usually cannot be identified with a magnifier. (using a magnifier, identifying minerals)

Hardness: Hardness (as measured with the Mohs scale) actually refers to minerals rather than rocks, so a rock may be crumbly yet consist of hard minerals. But in simple terms, "hard" rock scratches glass and steel, usually signifying the minerals quartz or feldspar (Mohs hardness 6-7 and up); "soft" rock does not scratch a steel knife but scratches fingernails (Mohs 3-5.5); "very soft" rock does not scratch fingernails (Mohs 1-2). Igneous rocks are always hard.

Identification of Igneous Rocks

Grain Size	Usual Color	Other	Composition	Rock Type
fine	dark	glassy appearance	lava glass	Obsidian
fine	light	many small bubbles	lava froth from sticky lava	Pumice
fine	dark	many large bubbles	lava froth from fluid lava	Scoria
fine or mixed	light	contains quartz	high-silica lava	Felsite
fine or mixed	medium	between felsite and basalt	medium-silica lava	Andesite
fine or mixed	dark	has no quartz	low-silica lava	Basalt
mixed	any color	large grains in fine-grained matrix	large grains of feldspar, quartz, pyroxene or olivine	Porphyry
coarse	light	wide range of color and grain size	feldspar and quartz with minor mica, amphibole or pyroxene	Granite
coarse	light	like granite but without quartz	feldspar with minor mica, amphibole or pyroxene	Syenite
coarse	medium to dark	little or no quartz	low-calcium plagioclase and dark minerals	Diorite
coarse	medium to dark	no quartz; may have olivine	high-calcium plagioclase and dark minerals	Gabbro
coarse	dark	dense; always has olivine	olivine with amphibole and/or pyroxene	Peridotite
coarse	dark	dense	mostly pyroxene with olivine and amphibole	Pyroxenite
coarse	green	dense	at least 90% olivine	Dunite
very coarse	any color	usually in small intrusive bodies	typically granitic	Pegmatite

Identification of Sedimentary Rocks

Hardness	Grain Size	Composition	Other	Rock Type
hard	coarse	clean quartz	white to brown	Sandstone
hard	coarse	quartz and feldspar	usually very coarse	Arkose
hard or soft	mixed	mixed sediment with rock grains and clay	gray or dark and "dirty"	Wacke/ Graywacke
hard or soft	mixed	mixed rocks and sediment	round rocks in finer sediment matrix	Conglomerate
hard or soft	mixed	mixed rocks and sediment	sharp pieces in finer sediment matrix	Breccia
hard	fine	very fine sand; no clay	feels gritty on teeth	Siltstone
hard	fine	chalcedony	no fizzing with acid	Chert
soft	fine	clay minerals	foliated	Shale
soft	fine	carbon	black; burns with tarry smoke	Coal
soft	fine	calcite	fizzes with acid	Limestone
soft	coarse or fine	dolomite	no fizzing with acid unless powdered	Dolomite rock
soft	coarse	fossil shells	mostly pieces	Coquina
very soft	coarse	halite	salt taste	Rock Salt
very soft	coarse	gypsum	white, tan or pink	Rock Gypsum

Identification of Metamorphic Rocks

Foliation	Grain Size	Hardness	Usual Color	Other	Rock Type
foliated	fine	soft	dark	"tink" when struck	Slate
foliated	fine	soft	dark	shiny; crinkly foliation	Phyllite
foliated	coarse	hard	mixed dark and light	wrinkled foliation; often has large crystals	Schist
foliated	coarse	hard	mixed	banded	Gneiss
foliated	coarse	hard	mixed	distorted "melted" layers	Migmatite
foliated	coarse	hard	dark	mostly hornblende	Amphibolite
nonfoliated	fine	soft	greenish	shiny, mottled surface	Serpentinite
nonfoliated	fine or coarse	hard	dark	dull and opaque colors, found near intrusions	Hornfels
nonfoliated	coarse	hard	red and green	dense; garnet and pyroxene	Eclogite
nonfoliated	coarse	soft	light	calcite or dolomite by the acid test	Marble
nonfoliated	coarse	hard	light	quartz (no fizzing with acid)	Quartzite

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