CSET Requirement 4.1a: Compare and contrast the properties of rocks based on physical and chemical conditions in which rocks are formed, including plate tectonic processes
Sedimentary Rocks
The key to understanding sedimentary rocks is to realize that all sedimentary processes of weathering, erosion, transportation, and deposition are aimed at one goal - reaching the final end products of all sedimentary processes, quartz sand, shale (clay), and limestone (CaCO3). Weathering is the breakdown of one mineral/rock into another. Quartz for all intents and purposes, does not weather and will survive in the system relatively unscathed. Erosion is the process by which soil and rock are removed from the Earth's surface by natural processes such as wind or water flow, and then transported. During transportation, the sand and clay, beginning as a poorly sorted mixture, are separated more and more as they travel down stream away from the source. Quartz sand, which rolls and bounces along the bottom, does not transport as easily as clay which travels in suspension. The result is, during transportation these three weathering products do not transport equally well, and become separated. The final separation takes place at the ocean shoreline.
Imagine an average continental igneous rock that contains quartz, and feldspar, and mafic minerals, a representative sampling of an igneous rock. Now imagine we are going to do every sedimentary process to that rock that it is possible to do, including complete weathering, complete erosion and complete transportation, sorting and deposition. The results are always the same - quartz sandstone (at the beach level), shale (near continental shelf), and limestone (far continental shelf deeper under sea).
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.
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). Another type of sediment rock forms where limestone settle and combines with calcite and makes chalk. Calcite is calcium carbonate and forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates. Sedimentary rocks become exposed when the land rises. This is common around the edges of the Earth's lithospheric plates.
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 evaporites are also part of the sedimentary clan.
Imagine an average continental igneous rock that contains quartz, and feldspar, and mafic minerals, a representative sampling of an igneous rock. Now imagine we are going to do every sedimentary process to that rock that it is possible to do, including complete weathering, complete erosion and complete transportation, sorting and deposition. The results are always the same - quartz sandstone (at the beach level), shale (near continental shelf), and limestone (far continental shelf deeper under sea).
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.
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). Another type of sediment rock forms where limestone settle and combines with calcite and makes chalk. Calcite is calcium carbonate and forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates. Sedimentary rocks become exposed when the land rises. This is common around the edges of the Earth's lithospheric plates.
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 evaporites are also part of the sedimentary clan.
Metamorphic Rock
Metamorphic rocks are what happens when sedimentary and igneous become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape-changed") rocks.
Heat and pressure usually work together, because both rise as you go deeper in the Earth. At high temperatures and pressures, most rocks break down and change into a different assemblage of minerals that are stable in the new conditions. The clay minerals of sedimentary rocks are a good example. Clays form as feldspar and mica break down in the conditions at the Earth's surface. With heat and pressure they slowly return to mica and feldspar. Even with their new mineral assemblages, metamorphic rocks may have the same overall chemistry they had before metamorphism.
Fluids are an important agent of metamorphism. Every rock contains some water, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated by clay minerals as they change back to feldspar and mica. This water can become so charged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. Strain refers to any change in the shape of rocks due to the force of stress. Movement on a fault zone is one example.
Under greater heat and pressure, when metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called foliation, is important to observe when identifying a metamorphic rock. As strain increases, the foliation becomes more intense, and the minerals sort themselves into thicker layers. The foliated rock types that form under these conditions are called schist or gneiss depending on their texture. Schist is finely foliated whereas gneiss is organized in wide bands of minerals.
Heat and pressure usually work together, because both rise as you go deeper in the Earth. At high temperatures and pressures, most rocks break down and change into a different assemblage of minerals that are stable in the new conditions. The clay minerals of sedimentary rocks are a good example. Clays form as feldspar and mica break down in the conditions at the Earth's surface. With heat and pressure they slowly return to mica and feldspar. Even with their new mineral assemblages, metamorphic rocks may have the same overall chemistry they had before metamorphism.
Fluids are an important agent of metamorphism. Every rock contains some water, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated by clay minerals as they change back to feldspar and mica. This water can become so charged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. Strain refers to any change in the shape of rocks due to the force of stress. Movement on a fault zone is one example.
Under greater heat and pressure, when metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called foliation, is important to observe when identifying a metamorphic rock. As strain increases, the foliation becomes more intense, and the minerals sort themselves into thicker layers. The foliated rock types that form under these conditions are called schist or gneiss depending on their texture. Schist is finely foliated whereas gneiss is organized in wide bands of minerals.
Igneous Rocks
Igneous rocks begin as hot, fluid material, and the word "igneous" comes from the Latin for fire. This material may have been lavaerupted at the Earth's surface, or magma (unerupted lava) at shallow depths, or magma in deep bodies (plutons). Rock formed of lava is called extrusive, rock from shallow magma is called intrusive and rock from deep magma is called plutonic.
Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges (Plate Divergence on picture), where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.
You can tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of a piece of bread as a similar example. Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture And pillow lava is a lumpy formation created by extruding lava underwater.
The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Ferrum), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.
The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic. The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually. In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle.
Igneous rocks form in three main places: where lithospheric plates pull apart at mid-ocean ridges (Plate Divergence on picture), where plates come together at subduction zones and where continental crust is pushed together, making it thicker and allowing it to heat to melting.
You can tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive rocks cool quickly (over periods of seconds to months) and have invisible or very small grains. Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Because they solidified from a fluid state, igneous rocks tend to have an equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the texture of a piece of bread as a similar example. Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture And pillow lava is a lumpy formation created by extruding lava underwater.
The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Ferrum), hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic rock. So granite is felsic and plutonic.
The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic. The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually. In general, granitic rocks are less dense than basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle.