Carbonate Petrography


25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?


The Geologic Column


Folds and Foliations


Consequences and Causes of Metamorphism

What Is a Metamorphic Rock? 

If someone were to put a rock on a table in front of you, how would you know that it is metamorphic? First, metamorphic rocks can possess metamorphic minerals, new minerals that grow in place within the solid rock only under metamorphic temperatures and pressures. In fact, metamorphism can produce a group of minerals that together make up what geologists call a “metamorphic mineral assemblage.” And second, metamorphic rocks can have metamorphic texture deﬁned by distinctive arrangements of mineral grains not found in other rock types. Commonly, the texture results in metamorphic foliation, due to the parallel alignment of platy minerals (such as mica) and/ or the presence of alternating light-coloured and dark-coloured layers. When metamorphic minerals and/or textures develop, a metamorphic rock becomes as different from its protolith as a butterﬂy is from a caterpillar. For example, metamorphism of red shale can yield a metamorphic rock consisting of aligned mica ﬂakes and brilliant garnet crystals (a in figure above), and metamorphism of a limestone composed of cemented-together fossil fragments can yield a metamorphic rock consisting of large interlocking crystals of calcite (b in figure above). The process of forming metamorphic minerals and textures takes place very slowly it may take thousands to millions of years and it involves several processes, which sometimes occur alone and sometimes together. The most common processes are: 
  • Recrystallization, which changes the shape and size of grains without changing the identity of the mineral making up the grains (a in figure above). 
  • Phase change, which transforms one mineral into another mineral with the same composition but a different crystal structure. On an atomic scale, phase change involves the rearrangement of atoms. 
  • Metamorphic reaction, or neocrystallization (from the Greek neos, for new), which results in growth of new mineral crystals that differ from those of the protolith (b in figure above). During neocrystallization, chemical reactions digest minerals of the protolith to produce new minerals of the metamorphic rock. 
  • Pressure solution, which happens when a wet rock is squeezed more strongly in one direction than in others. Mineral grains dissolve where their surfaces are pressed against other grains, producing ions that migrate through the water to precipitate elsewhere (c in figure above). 
  • Plastic deformation, which happens when a rock is squeezed or sheared at elevated temperatures and pressures. Under these conditions, grains behave like soft plastic and change shape without breaking (d in figure above). 

Caterpillars undergo metamorphosis because of hormonal changes in their bodies. Rocks undergo metamorphism when they are subjected to heat, pressure, compression and shear, and/or very hot water. Let’s now consider the details of how these agents of metamorphism operate.

Metamorphism Due to Heating 

When you heat cake batter, the batter transforms into a new material cake. Similarly, when you heat a rock, its ingredients transform into a new material metamorphic rock. Why? Think about what happens to atoms in a mineral grain as the grain warms. Heat causes the atoms to vibrate rapidly, stretching and bending chemical bonds that lock atoms to their neighbours. If bonds stretch too far and break, atoms detach from their original neighbours, move slightly, and form new bonds with other atoms. Repetition of this process leads to rearrangement of atoms within grains, or to migration of atoms into and out of grains, a process called solid-state diffusion. As a consequence, recrystallization and/or neo-crystallization take place, enabling a metamorphic mineral  assemblage to grow in solid rock. Metamorphism takes place at temperatures between those at which diagenesis occurs and those that cause melting. Roughly speaking, this means that most metamorphic rocks you ﬁnd in outcrops on continents formed at temperatures of between 250C and 850C.

Metamorphism Due to Pressure 

As you swim underwater in a swimming pool, water squeezes against you equally from all sides in other words, your body feels pressure. Pressure can cause a material to collapse inward. For example, if you pull an air-ﬁlled balloon down to a depth of 10 m in a lake, the balloon becomes signiﬁcantly smaller. Pressure can have the same effect on minerals. Near the Earth’s surface, minerals with relatively open crystal structures can be stable. However, if you subject these minerals to extreme pressure, the atoms pack more closely together and denser minerals tend to form. Such transformations involve phase changes and/or neo-crystallization.

Changing Both Pressure and Temperature 

So far, we've considered changes in pressure and temperature as separate phenomena. But in the Earth, pressure and temperature change together with increasing depth. For example, at a depth of 8 km, temperature in the crust reaches about 200C and pressure reaches about 2.3 kbar. If a rock slowly becomes buried to a depth of 20 km, as can happen during mountain building, temperature in the rock increases to more than 500C, and pressure to 5.5 kbar. Experiments and calculations show that the “stability” of certain minerals (the ability of a mineral to form and survive) depends on both pressure and temperature. When pressure and temperature increase, the original mineral assemblage in a rock becomes unstable, and a new assemblage forms out of minerals that are stable. Thus, a metamorphic rock formed at 8 km does not contain the same minerals as one formed at 20 km.

Compression, Shear, and Development  of Preferred Orientation 

Imagine that you have just built a house of cards and, being in a destructive mood, you step on it. The structure collapses because the downward push you apply with your foot exceeds the push provided by air in other directions. We can say that we have subjected the cards to compression (a in figure above). Compression ﬂattens a material (b in figure above). Shear, in contrast, moves one part of a material sideways, relative to another. If, for example, you place a deck of cards on a table, then set your hand on top of the deck and move your hand parallel to the table, you shear the deck (c in figure above). When rocks are subjected to compression and shear at elevated temperatures and pressures, they can change shape without breaking. As it changes shape, the internal texture of a rock also changes. For example, platy (pancake-shaped) grains become parallel to one another, and elongate (cigar shaped) grains align in the same direction. Both platy and elongate grains are inequant grains, meaning that the dimension of a grain is not the same in all directions; in contrast, equant grains have roughly the same dimensions in all directions (d in figure above). The alignment of inequant minerals in a rock results in a preferred orientation (e in figure above).

The Role of Hydrothermal Fluids 

Metamorphic reactions commonly take place in the presence of hydrothermal ﬂuids (very hot-water solutions). Where does the water in hydrothermal ﬂuids come from? Some of it was originally bonded to minerals in the protolith, for metamorphic reactions can release such water into its surroundings. Some of it may seep up into the protolith from a nearby igneous intrusion, or down from overlying groundwater reservoirs. Notably, under extremely high pressures and temperatures, the water of hydrothermal ﬂuids is in neither gas nor liquid state, but rather is in a “supercritical” state, meaning that it has characteristics of both gas and liquid. Such hydrothermal ﬂuids chemically react with rock; they accelerate metamorphic reactions, because atoms involved in the reactions can migrate faster through a ﬂuid than they can through a solid, and hydrothermal ﬂuids provide water that can be absorbed by minerals during metamorphic reactions. Finally, ﬂuids passing through a rock may pick up some dissolved ions and drop off others, as a bus picks up and drops off passengers, and thus can change the overall chemical composition of a rock during metamorphism. The process of changing a rock’s chemical composition by reactions with hydrothermal ﬂuids is called metasomatism.

Recognizing Depositional Environments

How Do We Recognize Depositional Environments? 

Geologists refer to the conditions in which sediment was deposited as the depositional environment. Examples include beach, glacial, and river environments. To identify depositional environments, geologists, like crime scene investigators, look for clues. Detectives may seek ﬁngerprints and bloodstains to identify a culprit. Geologists examine grain size, composition, sorting, bed-surface marks, cross bedding, and fossils to identify a depositional environment. Geological clues can tell us if the sediment was deposited by ice, strong currents, waves, or quiet water, and in some cases can provide insight into the climate at the time of deposition. With experience, geologists can examine a succession of beds and determine if it accumulated on a river ﬂoodplain, along a beach, in shallow water just offshore, or on the deep ocean ﬂoor.
Let’s now explore some examples of different depositional environments and the sediments deposited in them, by imagining that we are taking a journey from the mountains to the sea, examining sediments as we go. We will see that geologists distinguish among three basic categories of depositional environments: terrestrial, coastal, and marine.

Terrestrial (Nonmarine) Sedimentary Environments 

We begin our exploration with terrestrial depositional environments, those that develop inland, far enough away from the shoreline that they are not affected by ocean tides and waves. The sediments settle on dry land, or under and adjacent to freshwater.  
In some settings, oxygen in surface water or groundwater reacts with iron to produce rust-like iron-oxide minerals in terrestrial sediments, which give the sediment an overall reddish hue. Strata with this hue are informally called redbeds.

Glacial environments 

High in the mountains, where it’s so cold that more snow collects in the winter than melts away,  glaciers rivers or sheets of ice develop and slowly ﬂow. Because ice is a solid, it can move sediment of any size. So as a glacier moves down a valley in the mountains, it carries along all the sediment that falls on its surface from adjacent cliffs or gets plucked from the ground at its base or sides. At the end of the glacier, where the ice ﬁnally melts away, the sediment that had been in or on the ice accumulates as “glacial till” (a in figure above). Till is unsorted and unstratiﬁed it contains clasts ranging from clay size to boulder size all mixed together.

Mountain stream environments

As we walk down beyond the end of the glacier, we enter a realm where turbulent streams rush downslope in steep-sided valleys. This fast-moving water has the power to carry large clasts; in fact, during ﬂoods, boulders and cobbles can tumble down the stream bed. Between ﬂoods, when water ﬂow slows, the largest clasts settle out to form gravel and boulder beds, while the stream carries ﬁner sediments like sand and mud farther downstream (b in figure above). Sedimentary deposits of a mountain stream would, therefore, include breccia and  conglomerate.

Alluvial-fan environments

Our journey now takes us to the mountain front, where the fast-moving stream empties onto a plain. In arid regions, where there is not enough water for the stream to ﬂow continuously, the stream deposits its load of sediment near the mountain front, producing a wedge-shaped apron of gravel and sand called an alluvial fan  (c in figure above). Deposition takes place here because when the stream pours from a canyon mouth and spreads out over a broader region, friction with the ground causes the water to slow down, and slow-moving water does not have the power to move coarse sediment. The sand here still contains feldspar grains, for these have not yet weathered into clay. Alluvial-fan sediments become arkose and conglomerate.

Sand-dune environments

If the climate is very dry, few plants can grow and the ground surface lies exposed. Strong winds can move dust and sand. The dust gets carried away, and the resulting well-sorted sand can accumulate in dunes. Thus, thick layers of well-sorted sandstone, in which we can ﬁnd large cross beds, are relicts of desert sand-dune environments (d in figure above).

River (ﬂuvial) environments

In climates where streams ﬂow, we ﬁnd several distinctive depositional environments. Rivers transport gravel, sand, silt, and mud. The coarser sediments tumble along the bed in the river’s channel and collect in cross-bedded, rippled layers while the ﬁner sediments drift along, suspended in the water. This ﬁne sediment settles out along the banks of the river, or on the ﬂoodplain, the ﬂat land on either side of the river that is covered with water only during ﬂoods. On the ﬂoodplain, mud layers dry out between ﬂoods, leading to the formation of mud cracks. River sediments lithify to form sandstone, siltstone, and shale. Typically, the coarser sediments of channels are surrounded by layers of ﬁne-grained ﬂoodplain deposits, so in cross section, the channel has a lens-like shape (e in figure above). Geologists commonly refer to river deposits as ﬂuvial sediments, from the Latin word ﬂuvius, for river.

Lake environments

In temperate climates, where water remains at the surface throughout the year, lakes form. In lakes, the relatively quiet water can’t move coarse sediment; any coarse sediment brought into the lake by a stream settles out at the stream’s outlet. Only ﬁne clay makes it out into the centre of the lake, where it settles to form mud on the lake bed. Thus, lake sediments typically consist of ﬁnely  laminated shale (f in figure above). 

At the mouths of streams that empty into lakes, small deltas may form. A delta is a wedge of sediment that accumulates where moving water enters standing water. Deltas were so named because the map shape of some deltas resembles the Greek letter delta ($), as we discuss further in Chapter 14. In 1885, an American geologist named G. K. Gilbert showed that such deltas contain three components (figure above): topset beds composed of gravel, foreset beds of gravel and sand, and silty bottomset beds.

Coastal and Marine Environments 

Along the seashore, a variety of distinct coastal environments occur; the character of each reﬂects the nature of the sediment supply and the climate. Marine environments start at the high-tide line and extend offshore, to include the deep ocean ﬂoor. The type of sediment deposited at a location depends on the climate, water depth, and whether or not clastic grains are available.

Marine delta deposits

After following the river downstream for a long distance, we reach its mouth, where it empties into the sea. Here, the river builds a delta of sediment out into the sea. River water stops ﬂowing when it enters the sea, so sediment settles out. Large deltas are much more complex than the lake examples that Gilbert studied, for they include many different sedimentary environments including swamps, channels, ﬂoodplains, and submarine slopes. Sea-level changes may cause the positions of the different environments to move with time. Thus, deposits of an ocean-margin delta produce a great variety of sedimentary rock types (a in figure above).

Coastal beach sands

Now we leave the delta and wander along the coast. Oceanic currents transport sand along the coastline. The sand washes back and forth in the surf, so it becomes well sorted (waves winnow out silt and clay) and well rounded, and because of the back-and-forth movement of ocean water over the sand, the sand surface may become rippled (b in figure above). Thus, if you ﬁnd well-sorted, medium grained sandstone, perhaps with ripple marks, you may be looking at the remnants of a beach environment.

Shallow-marine clastic deposits

From the beach, we proceed offshore. In deeper water, where wave energy does not stir the sea ﬂoor, ﬁner sediment can accumulate. Because the water here may be only meters to a few tens of meters deep, geologists refer to this depositional setting as a shallow-marine environment. Clastic sedimentary layers that accumulate in this environment tend to be ﬁne-grained, well-sorted, well rounded silt, and they are inhabited by a great variety of organisms such as mollusks and worms. Thus, if you see beds of siltstone and mudstone containing marine fossils, you may be looking at shallow-marine clastic deposits.

Shallow-water carbonate environments

In shallow marine settings relatively free of clastic sediment, warm, clear, nutrient-rich water hosts an abundance of organisms. Their shells, which consist of carbonate minerals, make up most of the sediment that accumulates (a and b in figure above). The nature of carbonate sediment depends on the water depth. Beaches collect sand composed of shell fragments; lagoons (protected bodies of quiet water) are sites where carbonate mud accumulates; and reefs consist of coral and coral debris. Farther offshore of a reef, we can ﬁnd a sloping apron of reef fragments. Shallow-water carbonate environments transform into various kinds of limestone.

Deep-marine deposits

We conclude our journey by sailing far offshore. Along the transition between coastal regions and the deep ocean, turbidity currents deposit graded beds. In the deep-ocean realm, only ﬁne clay and plankton provide a source for sediment. The clay eventually settles out onto the deep-sea ﬂoor, forming deposits of ﬁnely laminated mudstones, and plankton shells settle to form chalk (from calcite shells; a and b in figure above) or chert (from siliceous shells). Thus, deposits of mudstone, chalk, or bedded chert indicate a deepmarine origin.

Sedimentary structures

Sedimentary structures

Geologists use the term sedimentary structure for the layering of sedimentary rocks, for surface features on layers formed during deposition, and for the arrangement of grains within layers. Here, we examine some of the more important types.

Bedding and Stratiﬁcation 

Let’s start by introducing the jargon for discussing sedimentary layers. A single layer of sediment or sedimentary rock with a recognizable top and bottom is called a bed; the boundary between two beds is a bedding plane; several beds together constitute strata (singular stratum, from the Latin stratum, meaning pavement); and the overall arrangement of sediment into a sequence of beds is bedding, or stratiﬁcation. From the word strata, we derive other words, such as stratigrapher (a geologist who specializes in studying strata) and stratigraphy (the study of the record of Earth history preserved in strata). In some outcrops, stratiﬁcation can be quite subtle. But commonly, successive beds have different colours, textures, and resistance to erosion, so bedding gives outcrops a striped appearance (a in figure above).
Why does bedding form? To ﬁnd the answer, we need to think about how sediment accumulates. Changes in the climate, water depth, current velocity, or the sediment source control the type of sediment deposited at a location at a given time. For example, on a normal day a slow- moving river may carry only silt, which collects on the riverbed (b in figure above). During a flood, the river flows faster and carries sand and pebbles, so a layer of sandy gravel forms over the silt layer. Then, when the flooding stops, more silt buries the gravel. If this  succession of sediments become lithified and exposed for you to see, they appear as alternating beds of siltstone and sandy conglomerate. During geologic time, long-term changes in a depositional environment can take place. Thus, a given sequence of strata may differ markedly from sequences of strata above or below. 

A sequence of strata that is distinctive enough to be traced as a unit across a fairly large region is called a stratigraphic formation, or simply a formation (a in figure above). For example, a region may contain a succession of alternating sandstone and shale beds deposited by rivers, overlain by beds of marine limestone deposited later when the region was submerged by the sea. A stratigrapher might identify the sequence of sandstone and shale beds as one formation and the sequence of limestone beds as another. Formations are often named after the locality where they were ﬁrst found and studied. A map that portrays the distribution of stratigraphic formations is called a geologic map (b in figure above).

Ripple Marks, Dunes, and Cross Bedding: Consequences of Deposition in a Current 

Many clastic sediments accumulate in moving ﬂuids (wind, rivers, or waves). Fascinating sedimentary structures develop at the interface between the sediment and the ﬂuid. These structures are called bedforms. Bedforms that develop at a given location reﬂect such factors as the velocity of the ﬂow and the size of the clasts. Though there are many types of bedforms, we’ll focus on only two ripple marks and dunes. The growth of both produces cross bedding, a special type of lamination within beds. Ripple marks are relatively small (generally no more than a few centimetres high), elongated ridges that form on a bed surface at right angles to the direction of current ﬂow. You can ﬁnd ripples on modern beaches and preserved on bedding planes of ancient rocks (figure above). Dunes are relatively large, elongate ridges built of sediment transported by a current. In effect, dunes are “mega-ripples.” Dunes on the bed of a stream may be tens of centimeters high, and wind-formed dunes of deserts may be tens to over 100 meters high. 

If you examine a vertical slice cut into a ripple or dune, you will ﬁnd distinct internal laminations that are inclined at an angle. Such laminations are called cross beds. To see how cross beds develop, imagine a current of air or water moving uniformly in one direction (a in figure above). The current erodes and picks up clasts from the upstream part of the bedform and deposits them on the downstream or leeward face of the crest. Sediment builds up until gravity causes it to slip down the leeward face. With time, the dune or ripple builds in the downstream direction. The surface of the slip face establishes the shape of the cross beds. Eventually, a new cross-bedded layer builds out over a pre-existing one. The boundary between two successive layers is called the “main bedding,” and the internal curving surfaces within the layer constitute the cross bedding (b and c in figure above).

Turbidity Currents and Graded Beds 

Sediment deposited on a submarine slope tends to be unstable. For example, an earthquake or storm might disturb this sediment and cause it to slip downslope and mix with water to create a murky, turbulent cloud. This cloud is denser than clear water and thus ﬂows downslope like an underwater avalanche (a to c in figure above). We call this moving submarine suspension of sediment a turbidity current. Downslope, the turbidity current slows, and the sediment that it has carried starts to settle out. Larger grains sink faster through a ﬂuid than do ﬁner grains, so the coarsest sediment settles out ﬁrst. Progressively ﬁner grains accumulate on top, with the ﬁnest sediment (clay) settling out last. This process forms a graded bed that is, a layer of sediment in which grain size varies from coarse at the bottom to ﬁne at the top. Geologists refer to a deposit from a turbidity current as a turbidite.

Bed-Surface Markings 

A number of features develop on the surface of a bed as a consequence of events that happen during deposition or soon after, while the sediment layer remains soft. Such bed-surface markings include the following: 

  • Mud cracks: If a mud layer dries up after deposition, it cracks into roughly hexagonal plates that typically curl up at their edges. We refer to the openings between the plates as mud cracks (a and b figure above). 
  • Scour marks: As currents flow over a sediment surface, they may erode small troughs, called scour marks, parallel to the current flow. 
  • Fossils: Fossils are relicts of past life. Some fossils are shell imprints or footprints on a bedding surface. 
Burial and lithiﬁcation of bed-surface markings can preserve them in the stratigraphic record.

Why Study Sedimentary Structures? 

Sedimentary structures are not just a curiosity, but are important clues that help geologists understand the environment in which sedimentary beds were deposited. For example, the presence of ripple marks and cross bedding indicates that layers were deposited in a current, the presence of mud cracks indicates that the sediment layer was exposed to the air and dried out, and graded beds indicate deposition by turbidity currents. Also, fossil types can tell us whether sediment was deposited along a river or in the deep sea, for different species of organisms live in different environments. In the next section of this chapter, we examine these environments in greater detail.

Classes of sedimentary rocks

Classes of sedimentary rocks

Geologists divide sedimentary rocks into four major classes, based on their mode of origin. 
(1) Clastic sedimentary rock consists of cemented-together clasts, solid fragments and grains broken off of preexisting rocks (the word comes from the Greek klastos, meaning broken); (2) biochemical sedimentary rock consists of shells; (3) organic sedimentary rock consists of carbon-rich relicts of plants or other organisms; and (4) chemical sedimentary rock is made up of minerals that precipitated directly from water solutions. Let’s now look at these major classes in more detail.

Clastic Sedimentary Rocks Formation

Nine hundred years ago, a thriving community of Native Americans inhabited the high plateau of Mesa Verde, Colorado. In hollows beneath huge overhanging ledges, they built multistory stone-block buildings that have survived to this day. Clearly, the blocks are solid and durable they are, after all, rock. But if you were to rub your thumb along one, it would feel gritty, and small grains of quartz would break free and roll under your thumb, for the block consists of quartz sand grains cemented together. Geologists call such rock a sandstone. Sandstone is an example of clastic sedimentary rock. It consists of loose clasts, known as detritus, that have been stuck together to form a solid mass. The clasts can consist of individual minerals (such as grains of quartz or ﬂakes of clay) or of fragments of rock (such as pebbles of granite). Formation of sediment and its transformation into clastic sedimentary rock takes place via the following ﬁve steps.

  • Weathering: Detritus forms by disintegration of bedrock into separate grains due to physical and chemical weathering. 
  • Erosion: Erosion refers to the combination of processes that separate rock or regolith (surface debris) from its substrate. Erosion involves abrasion, falling, plucking, scouring, and dissolution, and is caused by moving air, water, or ice. 
  • Transportation: Gravity, wind, water, or ice carry sediment. The ability of a medium to carry sediment depends on its viscosity and velocity. Solid ice can transport sediment of any size, regardless of how slowly the ice moves. Very fast-moving, turbulent water can transport coarse fragments (cobbles and boulders), moderately fast-moving water can carry only sand and gravel, and slow-moving water carries only silt and clay. Strong winds can move sand and dust, but gentle breezes carry only dust. 
  • Deposition: Deposition is the process by which sediment settles out of the transporting medium. Sediment settles out of wind or moving water when these fluids slow, because as the velocity decreases, the fluid no longer has the ability to carry sediment. Sediment is deposited by ice when the ice melts. 
  • Lithiﬁcation: Geologists refer to the transformation of loose sediment into solid rock as lithiﬁcation. The lithification of clastic sediment involves two steps. First, once the sediment has been buried, pressure caused by the weight of overlying material squeezes out water and air that had been trapped between clasts, and clasts press together tightly, a process called compaction. Compacted sediment may then be stuck together to make coherent sedimentary rock by the process of cementation. Cement consists of minerals (commonly quartz or calcite) that precipitate from groundwater and fill the spaces between clasts. 

Classifying clastic sedimentary rocks

Say that you pick up a clastic sedimentary rock and want to describe it sufﬁciently so that, from your words alone, another person can picture the rock. What characteristics should you mention? Geologists ﬁnd the following characteristics most useful. 
  • Clast size. Size refers to the diameter of fragments or grains making up a rock. Names used for clast size, listed in order from coarsest to finest, are: boulder, cobble, pebble, sand, silt, and clay. 
  • Clast composition. Composition refers to the makeup of clasts in sedimentary rock. Clasts may be composed of rock fragments or individual mineral grains. 
  • Angularity and sphericity. Angularity indicates the degree to which clasts have smooth or angular corners and edges. Sphericity, in contrast, refers to the degree to which the shape of a clast resembles a sphere. 
  • Sorting. Sorting of clasts indicates the degree to which the clasts in a rock are all the same size or include a variety of sizes. Well-sorted sediment consists entirely of sediment of the same size, whereas poorly-sorted sediment contains a mixture of more than one clast size. 
  • Character of cement. Not all clastic sedimentary rocks have the same kind of cement. In some, the cement consists  predominantly of quartz, whereas in others, it consists predominantly of calcite. 

With these characteristics in mind, we can distinguish among several common types of clastic sedimentary rocks. This table provides common rock names specialists sometimes use other, more precise names based on more complex classiﬁcation schemes. The size, angularity, sphericity, and sorting of clasts depends on the medium (water, ice, or wind) that transports the clasts and, in the case of water or wind, on both the velocity of the medium and the distance of transport. The composition of the clasts depends on the composition of rock from which the clasts were derived, and on the degree of chemical weathering that the clasts have undergone. Thus, the type of clasts that accumulate in a sedimentary deposit varies with location. To see how, let’s follow the fate of rock fragments as they gradually move from a cliff face in the mountains via a river to the seashore. Different kinds of sediment develop along the route. Each kind, if buried and lithiﬁed, would yield a different type of sedimentary rock.

To start, imagine that some large blocks of granite tumble off a cliff and slam into other blocks already at the bottom. The impact shatters the blocks, producing a pile of angular fragments. If these fragments were to be cemented together before being transported very far, the resulting rock would be breccia (a in above figure). Later, a storm causes the fragments (clasts) to be carried away by a turbulent river. In the water, clasts bang into each other and into the riverbed, a process that shatters them into still smaller pieces and breaks off their sharp edges. As the clasts get carried downstream, they gradually become rounded pebbles and cobbles. When the river water slows, these clasts stop moving and form a mound or bar of gravel. Burial and lithiﬁcation of these rounded clasts produces conglomerate (b in above figure). If the gravel stays put for a long time, it undergoes chemical weathering. As a consequence, cobbles and pebbles break apart into individual mineral grains, eventually producing a mixture of quartz, feldspar, and clay. Clay is so ﬁne that ﬂowing water easily picks it up and carries it downstream, leaving sand containing a mixture of quartz and some feldspar grains this sediment, if buried and lithiﬁed, becomes arkose (c in above figure). Over time, feldspar grains in sand continue to weather into clay so that gradually, during successive events that wash the sediment downstream, the sand loses feldspar and ends up being composed almost entirely of durable quartz grains. Some of the sand may make it to the sea, where waves carry it to beaches, and some may end up in desert dunes. This sediment, when buried and lithiﬁed, 
becomes quartz sandstone (d figure below). Meanwhile, silt and clay may accumulate in the ﬂat areas bordering streams, regions called ﬂoodplains that become inundated only during ﬂoods. And some silt and mud settles in a wedge, called a delta, at the mouth of the river, or in lagoons or mudﬂats along the shore. The silt, when lithiﬁed, becomes siltstone, and the mud, when lithiﬁed, becomes shale or mudstone (e figure below).

Biochemical Sedimentary

Rocks The Earth System involves many interactions between living organisms and the physical planet. Numerous organisms have evolved the ability to extract dissolved ions from seawater to make solid shells. When the organisms die, the solid material in their shells survives. This material, when lithiﬁed, comprises biochemical sedimentary rock. Geologists recognize several different types of biochemical sedimentary rocks, which we now describe.

Limestone (biochemical)

A snorkeler gliding above a reef sees an incredibly diverse community of coral and algae, around which creatures such as clams, oysters, snails (gastropods), and lampshells (brachiopods) live, and above which plankton ﬂoat (a figure above). Though they look so different from each other, many of these organisms share an important characteristic: they make solid shells of calcium carbonate (CaCO3). The CaCO3 crystallizes either as calcite or aragonite. (These minerals have the same composition, but different crystal structures.) When the organisms die, the shells remain and may accumulate.  
Rocks formed dominantly from this material are the biochemical version of limestone. Since the principal compound making up limestone is CaCO3, geologists refer to limestone as a type of carbonate rock. Limestone comes in a variety of textures, because the material that forms it accumulates in a variety of ways. For example, limestone can originate from reef builders (such as coral) that grew in place, from shell debris that was broken up and transported, from carbonate mud, or from plankton shells that settled like snow out of water. Because of this variety, geologists distinguish among fossiliferous limestone, consisting of visible fossil shells or shell fragments (b figure above); micrite, consisting of very ﬁne carbonate mud; and chalk, consisting of plankton shells. Experts recognize many other types as well. Typically, limestone is a massive light-gray to darkbluish-gray rock that breaks into chunky blocks it doesn't look much like a pile of shell fragments (c figure above). That’s because several processes change the texture of the rock over time. For example, water passing through the rock not only precipitates cement but also dissolves some carbonate grains and causes new ones to grow.

Chert (biochemical).

If you walk beneath the northern end of the Golden Gate Bridge in California, you will find outcrops of reddish, almost porcelain-like rock occurring in 3- to 15-cm-thick layers (a figure above). Hit it with a hammer, and the rock would crack to form smooth, spoon-shaped (conchoidal) fractures. Geologists call this rock biochemical chert; it’s made from cryptocrystalline quartz (crypto is Greek for hidden), meaning quartz grains that are too small to be seen without the extreme magnification of an electron microscope. The chert beneath the Golden Gate Bridge formed from the shells of silica-secreting plankton that accumulated on the sea floor. Gradually, after burial, the shells dissolved, forming a silica-rich gel. Chert then formed when this gel solidified. 

Organic Sedimentary Rocks 

We've seen how the mineral shells of organisms (CaCO3 or SiO2) can accumulate and lithify to become biochemical sedimentary rocks. What happens to the “guts” of the organisms the cellulose, fat, carbohydrate, protein, and other organic compounds that make up living matter? Commonly, this organic debris gets eaten by other organisms or decays at the Earth’s surface. But in some environments, the organic debris settles along with other sediment and eventually gets buried. When lithiﬁed, organic-rich sediment becomes organic sedimentary rock. Since the dawn of the industrial revolution in the early 19th century, coal, one type of organic sedimentary rock, has provided the fuel of modern industry and transportation, for the organic chemicals in the rock yield energy when burned. Coal is a black, combustible rock consisting of over 50 to 90% carbon. The remainder consists of oxygen, nitrogen, hydrogen, sulphur, silica, and minor amounts of other elements. Typically, the carbon in coal occurs in large, complex organic molecules made of many rings note that the carbon does not occur in CaCO3. Coal forms when plant remains have been buried deeply enough and long enough for the material to become compacted and to lose signiﬁcant amounts of volatiles (hydrogen, water, CO2, and ammonia); as the volatiles seep away, a concentration of carbon remains  (b figure above).

Chemical Sedimentary Rocks 

The colourful terraces, or mounds, that grow around the vents of hot-water springs; the immense layers of salt that underlie the ﬂoor of the Mediterranean Sea; the smooth, sharp point of an ancient arrowhead these materials all have something in common. They all consist of rock formed primarily by the precipitation of minerals from water solutions. We call such rocks chemical sedimentary rocks. They typically have a crystalline texture, partly formed during their original precipitation and partly when, at a later time, new crystals grow at the expense of old ones through a process called recrystallization. In some examples, crystals are coarse. In others, they are too small to see. Geologists distinguish among many types of chemical sedimentary rocks, primarily on the basis of composition.

Evaporites: the products of salt-water evaporation.  

In 1965, two daredevil drivers in jet-powered cars battled to be the ﬁrst to set the land speed record of 600 mph. On November 7, Art Arfons, in the Green Monster, peaked at 576.127  mph; but eight days later Craig Breedlove, driving the Spirit of America, reached 600.601 mph. Travelling at such speeds, a driver must maintain an absolutely straight line; any turn will catapult the vehicle out of control. Thus, high-speed trials take place on extremely long and ﬂat racecourses. Not many places can provide such conditions the Bonneville Salt Flats of Utah do. The salt ﬂats formed when an ancient salt lake evaporated. Under the heat of the Sun, the water turned to vapour and drifted up into the atmosphere, but the salt that had been dissolved in the water stayed behind. Salt precipitation occurs where salt-water becomes supersaturated, meaning that it has exceeded its capacity to contain more dissolved ions. In supersaturated salt-water, ions bond to form solid grains that either settle out of the water or grow on the ﬂoor of the water body. Supersaturated salt-water develops where evaporation removes water from a water body faster than the rate at which new water enters. This process takes place in desert lakes and along the margins of restricted seas (figure above). For thick deposits of salt to form, large volumes of water must evaporate. Because salt deposits form as a consequence of evaporation, geologists refer to them as evaporites. The speciﬁc type of salt minerals comprising an evaporite depends on the amount of evaporation. When 80% of the water evaporates, gypsum forms; and when 90% of the water evaporates, halite precipitates. 

Travertine (chemical limestone).  

Travertine is a rock composed of crystalline calcium carbonate (CaCO3) formed by chemical precipitation from groundwater that has seeped out at the ground surface either in hot- or cold-water springs, or on the walls of caves. What causes this precipitation? It happens, in part, when the groundwater degasses, meaning that some of the carbon dioxide that had been dissolved in the groundwater bubbles out of solution, for removal of carbon dioxide encourages the precipitation of carbonate. Precipitation also occurs when water evaporates, thereby increasing the concentration of carbonate. Various kinds of microbes live in the environments in which travertine accumulates, so biologic activity may also contribute to the precipitation process. Travertine produced at springs forms terraces and mounds that are meters or even hundreds of meters thick, such as those at Mammoth Hot Springs (a in figure above). Travertine also grows on the walls of caves where groundwater seeps out (b in figure above). In cave settings, travertine builds up beautiful and complex growth forms called speleothems.


Another carbonate rock, dolostone, differs from limestone in that it contains the mineral dolomite (CaMg[CO3]2), which contains equal amounts of calcium and magnesium. Where does the magnesium come from? Most dolostone forms by a chemical reaction between solid calcite and magnesium-bearing groundwater. Much of the dolostone you may ﬁnd in an outcrop actually originated as limestone but later changed into dolostone as dolomite crystals replaced calcite. This change may take place beneath lagoons along a shore soon after the limestone formed, or a long time later, after the limestone has been buried deeply.

Chert (replacement).  

A tribe of Native Americans, the Onondaga, once lived off the land in eastern New York State. Here, outcrops of limestone contain layers or nodules (lenses or lumps) of a black chert (a in figure above). Because of the way it breaks, the tribe’s artisans could fashion sharpedged tools (arrowheads and scrapers) from this chert, so the Onondaga collected it for their own toolmaking industry and for use in trade with other people. Unlike the deep sea (biochemical) chert described earlier, the chert collected by the Onondaga formed when cryptocrystalline quartz gradually replaced calcite crystals within a body of limestone long after the limestone was deposited; geologists call such material “replacement chert.” 
Chert comes in many colours (black, white, red, brown, green, gray), depending on the impurities it contains. Petriﬁed wood is chert that forms when silica-rich sediment, such as ash from a volcanic eruption, buries a forest. The silica dissolves in groundwater, and then later precipitates as cryptocrystalline quartz within wood, gradually replacing the wood’s cellulose. The chert deposit retains the shape of the wood and the growth rings within it. Some chert, known as agate, precipitates in concentric rings inside hollows in a rock and ends up with a striped appearance, caused by variations in the content of impurities incorporated in the chert (b in figure above).


What is Soil?

What is soil?. If you've ever had the chance to dig in a garden, you've seen first hand that the material in which ﬂowers grow looks and feels different from beach sand or potter’s clay. We call the material in a garden “dirt” or, more technically, soil. Soil consists of rock or sediment that has been modiﬁed by physical and chemical interaction with organic material, rainwater, and organisms over time. Soil is one of our planet’s most valuable resources, for without it there could be no agriculture, forestry, ranching, or home gardening.

How Does Soil Form?

How does soil form?. Three processes taking place at or just below the surface of the Earth contribute to soil formation. First, chemical and physical weathering produces loose debris, new minerals (such as clay), and ions in solution. Second, rainwater percolates through the debris and carries dissolved ions and clay ﬂakes downward. The region in which this downward transport takes place is called the zone of leaching, because leaching means extracting, absorbing, and removal. Farther down, new mineral crystals precipitate directly out of the water or form by reaction of the water with debris. Also, the water leaves behind its load of ﬁne clay. The region in which new minerals and clay collect is the zone of accumulation. Third, microbes, fungi, plants, and animals interact with sediment by producing acids that weather grains, by absorbing nutrient atoms, and by leaving behind organic waste and remains. Plant roots and burrowing animals (insects, worms, and gophers) churn and break up the soil, and microbes metabolise minerals and organic matter and release chemicals. Because different soil-forming processes operate at different depths, soils typically develop distinct zones, known as soil horizons, arranged in a vertical sequence called a soil proﬁle. Let’s look at an idealised soil proﬁle, from top to bottom, using a soil formed in a temperate forest as our example. The highest horizon is the O-horizon (the preﬁx stands for organic), so called because it consists almost entirely of humus (plant debris) and contains barely any mineral matter. Below the O-horizon, we ﬁnd the A-horizon, in which humus has decayed further and has mixed with mineral grains (clay, silt, and sand). Water percolating through the A-horizon causes chemical weathering reactions to occur and produces ions in solution and new clay minerals. Downward-moving water eventually carries soluble chemicals and ﬁne clay deeper into the subsurface. The O- and  A-horizons constitute dark-gray to blackish-brown topsoil, the fertile portion of soil that farmers till for planting crops. (In some places, the A-horizon grades downward into the E-horizon, a soil level that has undergone substantial leaching but has not yet mixed with organic material.) Beneath the A-horizon (or the A- and E-horizons) lies the B- horizon. Ions and clay accumulate in the B-horizon, or subsoil. Note from our description that the O-, A-, and E-horizons  make up the zone of leaching, whereas the B-horizon makes up the zone of accumulation. Finally, at the base of a soil proﬁle we ﬁnd the C-horizon, which consists of material derived from the substrate that’s been chemically weathered and broken apart, but has not yet undergone leaching or accumulation. The C-horizon grades downward into unweathered bedrock, or into unweathered sediment. As farmers, foresters, and ranchers well know, the soil in one locality can differ greatly from the soil in another, in terms of composition, thickness, and texture. Indeed, crops that grow well in one type of soil may wither and die in another nearby. Such diversity exists because the make up of a soil depends on several soil-forming factors:
  • Climate: Large amounts of rainfall and warm temperatures accelerate chemical weathering and cause most of the soluble elements to be leached. Small amounts of rainfall and cooler temperatures result in slower rates of weathering and leaching, so soils take a long time to develop and can retain unweathered minerals and soluble components. Climate is the single most important factor in determining the nature of soils that develop. 
  • Substrate composition: Some soils form on basalt, some on granite, some on volcanic ash, and some on recently deposited quartz silt. These different substrates consist of different materials, so the soils formed on them end up with different chemical compositions. 
  • Slope steepness: A thick soil can accumulate under land that lies flat. But on a steep slope, weathered rock may wash away before it can evolve into a soil. Thus, all other factors being equal, soil thickness increases as the slope angle decreases. 
  • Wetness: Depending on the details of local topography and on the depth below the surface at which groundwater occurs, some soil is wetter than other soil in the same region. Wet soils tend to contain more organic material than do dry soils. 
  • Time: Because soil formation is an evolutionary process, a young soil tends to be thinner and less developed than an old soil. The rate of soil formation varies greatly with environment. 
  • Vegetation type: Different kinds of plants extract or add different nutrients and quantities of organic matter to a soil. Also, some plants have deeper root systems than others and help prevent soil from washing away.

Soil Classiﬁcation 

Soil scientists worldwide have struggled mightily to develop a rational scheme for classifying soils. Not all schemes utilize the same criteria, and even today there is not worldwide agreement on which works best. In the United States, a country that includes many climates at mid-latitudes, many soil scientists use the U.S. Comprehensive Soil Classiﬁcation System, which distinguishes among 12 orders of soil based on the physical characteristics and environment of soil formation. Canadians use a different scheme focusing only on soils that develop north of the 40th parallel. The Canadian  scheme works well for cooler, high-latitude climates. As we've noted, rainfall and vegetation play a key role in determining the type of soil that forms. For example, in deserts, where there is very little rainfall and sparse vegetation, an aridisol forms. (In older classiﬁcations, these were known as “pedocal” soils.) Aridisols have no O-horizon (because there is so little organic material), and the A-horizon is thin. Soluble minerals, speciﬁcally calcite, that would be washed away entirely if there were more rainfall, instead accumulate in the B-horizon. In fact, capillary action may bring calcite up from deeper down as water evaporates at the ground surface. The calcite locally cements clasts together in the B-horizon to form a rock-like mass called caliche or calcrete. In temperate environments, an alﬁsol forms this soil has an O-horizon, and because of moderate amounts of rainfall, materials leached from the A-horizon accumulate in the B-horizon. (In older classiﬁcations, these were known as “pedalfer” soils.) In a tropical climate, oxisols develop. Here, so much rainfall percolates down into the ground that all reactive minerals in the soil undergo chemical weathering, producing ions and clay that ﬂush downward. This process leaves an A-horizon that contains substantial amounts of stable iron-oxide, aluminium-oxide, and aluminium-hydroxide residues. The resulting soil tends to be brick-red and is traditionally called laterite.
U.S. Department of Agriculture map of soil types around the world.

Soil Erosion 

As we have seen, soils take time to form, so soils capable of supporting crops or forests are a natural resource worthy of protection. However, agriculture, overgrazing, and clear cutting have led to the destruction of soil. Crops rapidly remove nutrients from soil, so if they are not replaced, the soil will not contain sufﬁcient nutrients to maintain plant life. When the natural plant cover disappears, the surface of the soil becomes exposed to wind and water. Actions such as the impact of falling raindrops or the rasping of a plow break up the soil at the surface, with the result that it can wash away in water or blow away as dust. When this happens, soil erosion, the removal of soil by running water or by wind, takes place. In some localities, erosion carries away almost six tons of soil from an acre of land per year. Human activities can increase rates of soil erosion by 10 to 100 times, so that it far exceeds the rate of soil formation. Droughts exacerbate the situation. For example, during the 1930s a succession of droughts killed off so much vegetation in the American plains that wind stripped the land of soil and caused devastating dust storms. Large numbers of people were forced to migrate away from the Dust Bowl of Oklahoma and adjacent areas. The consequences of rainforest destruction have particularly profound effects on soil. In an established rain forest, lush growth provides sufﬁcient organic debris so that trees can grow. But if the forest is logged, or cleared for a griculture, the humus rapidly  disappears, leaving laterite that contains few nutrients. Crop plants consume whatever nutrients remain so rapidly that the soil becomes infertile after only a year or two, useless for agriculture and unsuitable for regrowth of rainforest trees.