Deposition Associated with Glaciation

Deposition Associated with Glaciation 

The Glacial Conveyor 

The glacial conveyor and the formation of lateral and medial moraines on glaciers.

Glaciers can carry sediment of any size and, like a conveyor belt, transport it in the direction of flow (that is, toward the toe;  figure above a). The sediment load either falls onto the surface of the glacier from bordering cliffs or gets plucked and lifted from the substrate and incorporated into the moving ice. Geologists refer to a pile of debris carried by or left by glaciers as a moraine. Sediment dropped on the glacier’s surface moves with the ice and becomes a stripe of debris. Stripes formed along the side edges of the glacier are lateral moraines. When a glacier melts, lateral moraines lie stranded along the side of the glacially carved valley, like bathtub rings. Where two valley glaciers merge, the debris constituting two lateral moraines merges to become a medial moraine, running as a stripe down the interior of the composite glacier (figure above b). Trunk glaciers created by the merging of many tributary glaciers contain several medial moraines. Sediment transported to a glacier’s toe by the glacial conveyor accumulates in a pile at the toe and builds up to form an end moraine.

Oil and Gas

Oil and Gas

What Are Oil and Gas? 

For reasons of economics and convenience, industrialized societies today rely primarily on oil (petroleum) and natural gas for their energy needs. Oil and natural gas, both fossil fuels, consist of hydrocarbons, chain-like or ring-like molecules made of carbon and hydrogen atoms. Chemists consider hydrocarbons to be a type of organic chemical.

Some hydrocarbons are gaseous and invisible, some resemble a watery liquid, some appear syrupy, and some are solid. The viscosity (ability to flow) and the volatility (ability to evaporate) of a hydrocarbon product depend on the size of its molecules. Hydrocarbon products composed of short chains of molecules tend to be less viscous (meaning they can flow more easily) and more volatile (meaning they evaporate more easily) than products composed of long chains, because the long chains tend to tangle up with each other. Thus, short-chain molecules occur in gaseous form (natural gas) at room temperature, moderate-length-chain molecules occur in liquid form (gasoline and oil), and long-chain molecules occur in solid form (tar).

The Geologic Column

The Geologic Column

Global correlation of strata led to the development of the geologic column.

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history (figure above a, b). The column is divided into segments, each of which represents a specific interval of time. The largest subdivisions break Earth history into the Hadean, Archean, Proterozoic, and Phanerozoic Eons. (The first three together constitute the Precambrian.) The suffix zoic means life, so Phanerozoic means visible life, and Proterozoic means first life. (It wasn’t until after the eons had been named that geologists determined that the earliest life, cells of Bacteria and Archaea, appeared in the Archean Eon.) The Phanerozoic Eon is subdivided into eras. In order from oldest to youngest, they are the Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life) Eras. We further divide each era into periods and each period into epochs.

Taxonomy and Identification of fossils

Taxonomy and Identification of fossils

The study of how to identify and name organisms is taxonomy. Taxonomic classification of fossils follows the same principles used for the classification of living organisms and has a hierarchy of divisions. These principles were first proposed in the 18th century by Carolus Linnaeus, a Swedish biologist.

Fossilization

Fossilization

What Kinds of Rocks Contain Fossils? 

Most fossils are found in sediments or sedimentary rocks. Fossils form when organisms die and become buried by sediment, or when organisms travel over or through sediment and leave imprints or debris. The degree of preservation of a fossil reflects the context of burial. For example, rocks formed from sediments deposited under anoxic (oxygen-free) conditions in quiet water (such as lake beds or lagoons) can preserve particularly fine specimens. In contrast, rocks made from sediments deposited in high-energy environments where strong currents tumble shells and bones and break them up contain at best only small fragments of fossils mixed with other clastic grains. Fossils sometimes occur in volcaniclastic rocks, but they are not found in intrusive igneous rocks and tend to be destroyed by metamorphism.

Basins and Domes in Cratons

Basins and Domes in Cratons 

North America’s craton consists of a shield, where Precambrian rock is exposed, and a platform, where Paleozoic sedimentary rock covers the Precambrian.

A craton consists of crust that has not been affected by orogeny for at least about the last 1 billion years. As a result, cratons have cooled substantially, and therefore have become relatively strong and stable. Geologists divide cratons into two provinces: shields, in which Precambrian metamorphic and igneous rocks crop out at the ground surface, and cratonic platforms, where a relatively thin layer of Phanerozoic sediment covers the Precambrian rocks (figure above).

Mountain Topography

Mountain Topography 

Leonardo da Vinci, the Renaissance artist and scientist, enjoyed walking in the mountains, sketching ledges and examining the rocks he found there. In the process, he discovered marine shells (fossils) in limestone beds cropping out a kilometre above sea level, and he suggested that the rock containing the fossils had risen from below sea level up to its present elevation. Modern geologists agree with Leonardo, and they now refer to processes causing the surface of the Earth to move vertically from a lower to a higher elevation as uplift. In this section, we look at why uplift occurs, how erosion carves rugged landscapes out of uplifted crust, and why Earth’s mountains can’t get much higher than Mt. Everest.

Mountain Building

Mountain Building

Before plate tectonics theory became established, geologists were just plain confused about how mountains formed. In the context of the new theory, however, the many processes driving mountain building became clear: mountains form primarily in response to convergent-boundary deformation, continental collisions, and rifting. Since collision zones, rifts, and plate boundaries are linear, mountain belts are linear. Below, we look at these different settings and the types of mountains and geologic structures that develop in each one.

Folds and Foliations

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Geometric characteristics of folds.

Not all folds look the same some look like arches, some look like troughs, and some have other shapes. To describe these shapes, we must first label the parts of a fold (figure above a). The hinge refers to a line along which the curvature is greatest, and the limbs are the sides of the fold that display less curvature. The axial surface is an imaginary plane that contains the hinges of successive layers and effectively divides the fold into two halves. With these terms in hand, we distinguish among the following: 

• Anticlines, synclines, and monoclines: Folds that have an arch-like shape in which the limbs dip away from the hinge are called anticlines (figure above a), whereas folds with a trough-like shape in which the limbs dip toward the hinge are called synclines (figure above b). A monocline has the shape of a carpet draped over a stair step (figure above c). 
• Non-plunging and plunging folds: If the hinge is horizontal, the fold is called a non-plunging fold, but if the hinge is tilted, the fold is called a plunging fold (figure above d). 
• Domes and basins: A fold with the shape of an overturned bowl is called a dome, whereas a fold shaped like an upright bowl is called a basin (figure above e, f). Domes and basins both display circular outcrop patterns that look like bull’s-eyes the oldest layer occurs in the centre of a dome, whereas the youngest layer is located in the centre of a basin. 

Characteristics of folds on outcrops and in the landscape.

Using these terms, now see if you can identify the various folds shown in figure above a–e.

Formation of Folds 

Fold development in flexural-slip and passive flow-folding.

Folds develop in two principal ways (figure above a, b). During formation of flexural-slip folds, a stack of layers bends, and slip occurs between the layers. The same phenomenon happens when you bend a deck of cards to accommodate the change in shape, the cards slide with respect to each other. Passive-flow folds form when the rock, overall, is so soft that it behaves like weak plastic and slowly flows; these folds develop simply because different parts of the rock body flow at different rates. 

Folding is caused by several different processes, as illustrated by the following cross sections.

Why do folds form? Some layers wrinkle up, or buckle, in response to end-on compression (figure above a–d). Others form where shear stress gradually shifts one part of a layer up and over another part. Still others develop where rock layers move up and over step-like bends in a fault and must curve to conform with the fault’s shape. Finally, some folds form when new slip on a fault causes a block of basement to move up so that the overlying sedimentary layers must warp.

Tectonic Foliation in Rocks 

In an undeformed sandstone, the grains of quartz are roughly spherical, and in an undeformed shale, clay flakes press  together into the plane of bedding so that shales tend to split parallel to the bedding. During ductile deformation, however, internal changes take place in a rock that gradually modify the original shape and arrangement of grains. For example, quartz grains may transform into cigar shapes, elongate ribbons, or tiny pancakes, and clay flakes may recrystallize or reorient so that they lie at an angle to the bedding. Overall, deformation can produce inequant grains and can cause them to align parallel to each other. We refer to layering developed by the alignment of grains in response to deformation as tectonic foliation. 

The development of tectonic foliation in rock.

We introduced foliation, such as slaty cleavage, schistosity, and gneissic layering, while discussing the effects of metamorphism. Here we add to the story by noting that such foliation forms in response to flattening and shearing in ductilely deforming rocks in other words, foliation indicates that the rock has developed a strain under metamorphic conditions (figure above a, b).
Credits: Stephen Marshak (Essentials of Geology)

Brittle Structures

Brittle Structures 

Joints and Veins 

Examples of joints and veins.

If you look at the photographs of rock outcrops, you’ll notice thin dark lines that cross the rock faces. These lines represent traces of natural cracks along which the rock broke and separated into two pieces during brittle deformation.  Geologists refer to such natural cracks as joints (figure above a, b). Rock bodies do not slide past each other on joints. Since joints are roughly planar structures, we define their orientation by their strike and dip, as described in (Describing the Orientation of Geologic Structures).

Seismic Study of Earth’s Interior

Seismic Study of Earth’s Interior 

Let’s now utilize your knowledge of seismic velocity, refraction, and reflection to see how each of the major layer boundaries inside the Earth was discovered.

Discovering the Crust-Mantle Boundary 

Discovery of the Moho.

The concept that seismic waves refract at boundaries between different layers led to the first documentation of the core-mantle boundary. In 1909, Andrija Mohorovicic, a Croatian seismologist, noted that P-waves arriving at seismometer stations less than 200 km from the epicentre travelled at an average speed of 6 km per second, whereas P-waves arriving at seismometers more than 200 km from the epicentre travelled at an average speed of 8 km per second. To explain this observation, he suggested that P-waves reaching nearby seismometers followed a shallow path through the crust, in which they travelled relatively slowly, whereas P-waves reaching distant seismometers followed a deeper path through the mantle, in which they travelled relatively rapidly (figure above a, b).

The Movement of Seismic Waves Through the Earth

The Movement of Seismic Waves Through the Earth 

Wave Fronts and Travel Times 

The propagation of earthquake waves.

The energy released by an earthquake moves through rock in the form of waves, just as waves propagate outward from the impact point of a pebble on the surface of a pond. The boundary between the rock through which a wave has passed and the rock through which it has not yet passed is called a wave front. In 3-D, a wave front expands outward from the earthquake focus like a growing bubble. We can represent a succession of waves in a drawing by a series of concentric wave fronts. The changing position of an imaginary point on a wave front as the front moves through rock is called a seismic ray. You can picture a seismic ray as a line drawn perpendicular to a wave front; each point on a curving wave front follows a slightly different ray (figure above a). The time it takes for a wave to travel from the focus to a seismometer along a given ray is the travel time along that ray.

Defining the “Size” of Earthquakes

Defining the “Size” of Earthquakes 

Some earthquakes shake the ground violently, whereas others can barely be felt. Seismologists have developed two scales to define size in a uniform way, so that they can systematically describe and compare earthquakes. The first scale focuses on the severity of damage at a locality and is called the Mercalli Intensity scale. The second focuses on the amount of ground motion at a specific distance from the epicentre, as measured by a seismometer, and is called the magnitude scale.

How Do We Measure and Locate Earthquakes?

How Do We Measure and Locate Earthquakes?

Most news reports about earthquakes provide information on the size and location of an earthquake. What does this information mean, and how do we obtain it? What’s the difference between a large earthquake and a minor one? How do seismologists locate an epicentre? To answer these questions we must first understand how a seismometer works and how to read the information it provides.

What Causes Earthquakes?

What Causes Earthquakes?

To the causes of earthquakes, Ancient cultures offered a variety of explanations for seismicity (earthquake activity), most of which involved the action or mood of a giant animal or god. Scientific study suggests that seismicity instead occurs for several reasons, including: 

• the sudden formation of a new fault (a fracture or rupture on which sliding occurs) 
• sudden slip on an already existing fault
• a sudden change in the arrangement of atoms in rock  minerals 
• movement of magma in, or explosion of, a volcano
• a giant landslide 
• a meteorite impact
• an underground nuclear-bomb test

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, where do most earthquakes occur are along faults slip. 

Earthquake hypocenters and epicentres.

The place within the Earth where rock ruptures and slips, or the place where an explosion occurs, is the hypocenter or focus of the earthquake. Energy radiates from the focus. The point on the surface of the Earth that lies directly above the focus is the epicentre, so maps can portray the position of epicentres (figure above a, b). Since slip on faults causes most earthquakes, we focus our discussion on faults.
How earthquakes happen? Where do most earthquakes occur? Why do earthquakes happen? How do earthquakes happen? Where are earthquakes most likely to occur? Why do earthquakes happen?

Faults in the Crust 

Examples of fault displacement on the San Andreas fault in California.

At first glance, a fault may look simply like a fracture or break that cuts across rock or sediment. But on closer examination, you may be able to see evidence of sliding that occurred on a fault. For example, the rock adjacent to the fault may be broken up into angular fragments or may be pulverized into tiny grains, due to the crushing and grinding that can accompany slip, and the surface of a fault may be polished and grooved as if scratched by a rasp. In some localities, a fault cuts through a distinct marker (a sedimentary bed, an igneous dike, or a fence); where this happens, the end of the marker on one side of the fault is offset relative to the end on the other side. The distance between two ends of the marker, as measured along the fault surface in the direction of slip, is the fault’s displacement (figure above a, b). Many faults are completely underground, and will be visible only if exposed by erosion of overlying rock. But some faults intersect and offset the ground surface, producing a step called a fault scarp (figure below a). The ground surface exposure of a fault is called the fault line or fault trace. 

The basic types of fault. Fault types are distinguished from one another by the direction of slip relative to the fault surface.

19th-century miners who encountered faults in mine tunnels referred to the rock mass above a sloping fault plane as the hanging wall, because it hung over their heads, and the rock mass below the fault plane as the footwall, because it lay beneath their feet. The miners described the direction in which rock masses slipped on a sloping fault by specifying the direction that the hanging wall moved in relation to the footwall, and we still use these terms today. When the hanging wall slips down the slope of the fault, it’s a normal fault. When the hanging wall slips up the slope, it’s a reverse fault if steep, and a thrust fault if shallowly sloping (figure above a–c). Strike-slip faults are near-vertical planes on which slip occurs parallel to an imaginary horizontal line, called a strike line, on the fault plane no up or down motion takes place on such faults (figure above d).

Faults are found in many locations but don’t panic! Not all of them are likely to be the source of earthquakes. Faults that have moved recently or are likely to move in the near future are called active faults (and if they generate earthquakes, news media sometimes refer to them as “earthquake faults”). Faults that last moved in the distant past and probably won’t move again in the near future are called inactive faults.

Generating Earthquake Energy: Stick-Slip 

What is the relationship between faulting and earthquakes? Earthquakes can happen either when rock breaks and a new fault forms, or when a pre-existing fault suddenly slips again. Let’s look more closely at these two causes. 

A model representing the development of a new fault. Rupturing can generate earthquake-like vibrations.

• Earthquakes due to fault formation: Imagine that you grip each side of a brick-shaped block of rock with a clamp. Apply an upward push on one of the clamps and a downward push on the other. By doing so, you have applied a “stress” to the rock. (Stress refers to a push, pull, or shear.) At first, the rock bends slightly but doesn't break (figure above a). In fact, if you were to stop applying stress at this stage, the rock would return to its original shape. Geologists refer to such a phenomenon as elastic behaviour the same phenomenon happens when a rubber band returns to its original shape or a bent stick straightens out after you let go. Now repeat the experiment, but bend the rock even more. If you bend the rock far enough, a number of small cracks or breaks start to form. Eventually the cracks connect to one another to form a fracture that cuts across the entire block of rock (figure above b). The instant that this fracture forms, the block breaks in two and the rock on one side suddenly slides past the rock on the other side, and any elastic bending that had built up is released so the rock straightens out or rebounds (figure above c). Because sliding occurs, the fracture has become a fault. A fault can’t slip forever, for friction eventually slows and stops the movement. Friction, defined as the force that resists  sliding on a surface, is caused by the existence of bumps on surfaces these bumps act like tiny anchors and snag on the opposing surface. 
• Earthquakes due to slip on a pre-existing fault: Once a fault comes into being, it is a scar in the Earth’s crust that can remain weaker than surrounding, intact crust. When stress builds sufficiently, it overcomes friction and the pre-existing fault slips again. This movement takes place before stress becomes great enough to cause new fracturing of surrounding intact rock. Note that after each slip event, friction prevents the fault from slipping again until stress builds again. Geologists refer to such alternation between stress buildup and slip events (earthquakes) as stick-slip  behaviour. 

The breaking of rock that occurs when a fault slips, like the snap of a stick, generates earthquake energy. The concept that earthquakes happen because stresses build up, causing rock adjacent to the fault to bend elastically until slip on the fault occurs is called the  elastic-rebound theory. 

Of note, the major earthquake (or “mainshock”) along a fault may be preceded by smaller ones, called foreshocks, which possibly result from the development of the smaller cracks in the vicinity of what will be the major rupture. Smaller earthquakes, called aftershocks, occur in the days to months following a large earthquake. The largest aftershock tends to be ten times smaller than the mainshock, and most are even smaller. Aftershocks happen because slip during the  mainshock does not leave the fault in a perfectly stable configuration. For example, after the mainshock, irregularities on one side of the fault surface, in their new position, may push into the opposing side and generate new stresses. Such stresses may become large enough to cause a small portion of the fault around the irregularity to slip again, or may trigger slip in a nearby fault.

The Amount of Slip during an Earthquake 

How much of a fault surface slips during an earthquake? The answer depends on the size of the earthquake: the larger the earthquake, the larger the slipped area and the greater the displacement. For example, the major earthquake that hit San Francisco, California, in 1906 ruptured a 430-km-long (measured parallel to the Earth’s surface) by 15-km-deep (measured perpendicular to the Earth’s surface) segment of the San Andreas fault. Thus, the area that slipped was almost 6500 km2. During the 2011 Tohoku earthquake an area 300 km long by 100 km wide (30,000 km2) slipped. 

The amount of slip varies along the length of a fault the maximum observed displacement during the 1906 earthquake was 7 m, in a strike-slip sense. Slip on a thrust fault that caused the 1964 Good Friday earthquake in southern Alaska reached a maximum of 12 m, and the maximum slip during the Tohoku earthquake was over 20 m. Smaller earthquakes, such as the one that hit Northridge, California, in 1994, resulted in only about 0.5-m slip even so, this earthquake toppled homes, ruptured pipelines, and killed 51 people. The smallest-felt earthquakes result from displacements measured in millimetres to centimetres. 

Although the cumulative movement on a fault during a human life span may not amount to much, over geologic time the cumulative movement becomes significant. For example, if earthquakes occurring on a strike-slip fault cause 1 cm of displacement per year, on average, the fault’s movement will yield 10 km of displacement after 1 million years.

Credits: Stephen Marshak (Essentials of Geology)

Where Does Metamorphism Occur?

Where Does Metamorphism Occur? 

So far, we've discussed the nature of changes that occur during metamorphism, the agents of metamorphism (heat, pressure, compression and shear, and hydrothermal fluids), the rock types that form as a result of metamorphism, and the concepts of metamorphic grade and metamorphic facies. With this background, let’s now examine the geologic settings on Earth where metamorphism takes place, as viewed from the perspective of plate tectonics theory.

Because of the wide range of possible metamorphic environments, metamorphism occurs at a wide range of conditions in the Earth. You will see that the conditions under which metamorphism occurs are not the same in all geologic settings. That’s because the geothermal gradient (the relation between temperature and depth), the extent to which rocks endure compression and shear during metamorphism, and the extent to which rocks interact with hydrothermal fluids all depend on the geologic environment.

Types of Metamorphic Rocks

Types of Metamorphic Rocks 

Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals. 

Foliated Metamorphic Rocks 

To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers.

Sedimentary Basins

Sedimentary Basins

The sedimentary veneer on the Earth’s surface varies greatly in thickness. If you stand in central Siberia or south-central Canada, you will find yourself on igneous and metamorphic basement rocks that are over a billion years old sedimentary rocks are nowhere in sight. Yet if you stand along the southern coast of Texas, you would have to drill through over 15 km of sedimentary beds before reaching igneous and metamorphic basement. Thick accumulations of sediment form only in special regions where the surface of the Earth’s lithosphere sinks, providing space in which sediment collects. Geologists use the term subsidence to refer to the process by which the surface of the lithosphere sinks, and the term sedimentary basin for the sediment-filled depression. In what geologic settings do sedimentary basins form? An understanding of plate tectonics theory provides the answers.

Relation of Volcanism to Plate Tectonics

Relation of Volcanism to Plate Tectonics 

A map showing the distribution of volcanoes around the world and the basic geologic settings in which volcanoes form, in the contact of plate tectonics theory.

Different styles of volcanism occur at different locations on Earth. Most eruptions occur along plate boundaries, but major eruptions also occur at hot spots (figure above). We’ll now look at the settings in which eruptions occur, in the context of plate tectonics theory and see why different kinds of volcanoes form in different settings.

How Do You Describe an Igneous Rock?

How Do You Describe an Igneous Rock? 

Different parameters are used to describe an igneous rock which are described in detail.

Characterizing Color and Texture 

If you wander around a city admiring building facades, you’ll find that many facades consist of igneous rock, for such rocks tend to be very durable. If you had to describe one of these rocks to a friend, what words might you use? You would  probably start by noting the rock’s colour. Overall, is the rock dark or light? More specifically, is it gray, pink, white, or black? Describing colour may not be easy, because some igneous rocks contain many visible mineral grains, each with a different colour; but even so, you’ll probably be able to characterize the overall hue of the rock. Generally, the colour reflects the rock’s composition, but it isn't always so simple, because colour may also be influenced by grain size and by the presence of trace amounts of impurities. (For example, the presence of a small amount of iron oxide gives rock a reddish tint.) Next, you would probably characterize the rock’s texture. A description of igneous texture indicates whether the rock consists of glass, crystals, or fragments. If the rock consists of crystals or fragments, a description of texture also specifies the grain size. Here are the common terms for defining texture:

How Do Extrusive and Intrusive Environments Differ?

How Do Extrusive and Intrusive Environments Differ? 

With a background on how melts form and freeze, we can now introduce key features of the two settings intrusive and extrusive in which igneous rocks form.

Extrusive Igneous Settings 

Different volcanoes extrude molten rock in different ways. Some volcanoes erupt streams of low-viscosity lava that flood down the flanks of the volcano and then cover broad swaths of the countryside. When this lava freezes, it forms a relatively thin lava flow. Such flows may cool in days to months. In contrast, some volcanoes erupt viscous masses of lava that pile into rubbly domes. And still others erupt explosively, sending clouds of volcanic ash and debris skyward, and/or avalanches of ash tumbling down the sides of the volcano.

Studying Rock

Studying Rock 

Outcrop Observations 

The study of rocks begins by examining a rock in an outcrop. If the outcrop is big enough, such an examination will reveal relationships between the rock you’re interested in and the rocks around it, and will allow you to detect layering. Geologists carefully record observations about an outcrop, then break off a hand specimen, a fist-sized piece, that they can examine more closely with a hand lens (magnifying glass). Observation with a hand lens enables geologists to identify sand-sized or larger grains, and may enable them to describe the texture of the rock.

Thin-Section Study 

Studying rocks in thin section.

The Basis of Rock Classification

The Basis of Rock Classification 

Examples of three major rock groups.

Beginning in the 18th century, geologists struggled to develop a sensible way to classify rocks, for they realized, as did miners from centuries past, that not all rocks are the same. Classification schemes help us organize information and remember significant details about materials or objects, and they help us recognize similarities and differences among them. By the end of the 18th century, most geologists had accepted the genetic scheme for classifying rocks that we continue to use today. This scheme focuses on the origin (genesis) of rocks. Using this approach, geologists recognize three basic groups: (1) igneous rocks, which form by the freezing (solidification) of molten rock (figure above a); (2) sedimentary rocks, which form either by the cementing together of fragments (grains) broken off preexisting rocks or by the precipitation of mineral crystals out of water solutions at or near the Earth’s surface (figure above b); and (3) metamorphic rocks, which form when pre-existing rocks change character in response to a change in pressure and temperature conditions (figure above c). Metamorphic change occurs in the solid state, which means that it does not require melting. In the context of modern plate tectonics theory, different rock types form in different geologic settings (figure below).