Geology!

Caldera

A caldera is a collapse feature triggered by the emptying of the magma chamber beneath the volcano, usually as the result of a large volcanic eruption. If enough magma erupts, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above.

Crater

A crater is a basin of a roughly circular form within which occurs a vent from which magma erupts as gases, lava, and ejecta.

Volcano

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano Island off Sicily (in Italy) which in turn, was named after Vulcan, the Roman god of fire.

Decompression Melting

At divergent plate margins (such as mid-ocean ridges, or MOR) magma is generated by a process called decompression melting (figure 4.2). Melting of the rocks takes place as a result of a decrease in pressure and hence the name decompression melting. Figure 4.2. As hot mantle rock ascends, it continually moves into zones of lower pressure. This drop in confining pressure can initiate decompression melting, even without additional heat. Here are some of the activities that lead to the generation of magma by decompression melting: Convective motions in upper mantle cause plate divergence at MOR & subduction at trenches Combined slab pull & divergence cause the thinning of the lithosphere (oceanic crust), resulting in a lower pressure at MOR, which promotes the upward motion of hot mantle rock As hot mantle rock continues to rise, to 40 miles or shallower depth, the pressure conditions become conducive for the first magma to form by the partial melting of the peridotite source rock - Decompression melting!

World Volcanoes

Distribution of the world volcanoes is not random. A look at volcanic activity maps of the world (fig. 4.8) shows that of the more than 800 active volcanoes that have been recorded most are located within the circum-pacific belt known as the Ring of Fire. A second group of volcanoes, such as those occurring in Hawaii and Iceland, are confined to the deep ocean basin. A third group includes those volcanic eruptions which are irregularly distributed in the interiors of the continents such as the Columbia River Basalts and the explosive eruptions in Yellowstone. Until the late 1960s, geologists had no explanation for the apparently haphazard distribution of continental volcanoes, nor were they able to account for the almost continuous chain of volcanoes that circles the margins of the Pacific basin. With the development of the theory of plate tectonics, the picture was greatly clarified. Recall that magma originates in the upper mantle. The basic connection between plate tectonics and volcanism is that plate motions provide the mechanisms by which mantle rocks melt to generate magma.

Extrusive Igneous Rocks

Extrusive igneous rocks (Fig. 4.1) are formed at the Earth's surface as a result of the melting of rocks within the mantle and crust. Extrusive igneous rocks cool and solidify quicker than intrusive igneous rocks. Since the rocks cool very quickly they are fine grained. Basalt is an example of extrusive or volcanic igneous rock.

Magma Types

FELSIC (silicic): 66-76% SiO2 (SiO2=silica) INTERMEDIATE: 52-66% SiO2 MAFIC (basic): 45-52% SiO2 Magma viscosity Magma viscosity refers to magma's resistance to flow. Magma/lava containing high amounts of SiO2 has high viscosity. Silica tends to polymerize, i.e. it links up to form long silicate chains whose presence slows down the flow. Thus, felsic magmas are more viscose than mafic magmas (Table 4.1).

Organic Sedimentary Rocks

Fossiliferous Limestone: mostly calcite cement with some shells Coquina: predominantly shells with a minor amount of calcite cement; if the shells are microscopic, the rock is called Chalk Coal forms from the accumulation of large amounts of plant materials that become lithified over millions of years Diatomite consists of minute quartz formed by the accumulation of the frustules (like the calcite shells of other sea organisms) of microscopic single-celled organisms called diatoms Chert is formed when diatoms & radiolarians accumulate in the sea floor as silica-rich ooze and after burial the shells gradually dissolve forming silica-rich solutions which then precipitate to form chert

Agents of Metamorphism

Heat as a metamorphic agent Earth's internal heat comes mainly from energy that is continually being released by radioactive decay and from thermal energy generated during the formation of our planet. The temperature of the Earth increases with depth at a rate known as the geothermal gradient. In the upper crust, this increase in temperature averages between 20 degrees Celsius and 30 degrees Celsius per kilometer. Thus, rocks that formed at Earth's surface will experience a gradual increase in temperature as they move to greater depths. When buried to a depth of about 8 kilometers (5 miles), where temperatures are about 2000 degrees Celsius, clay minerals tend to become unstable and begin to recrystallizes into new minerals, such as chlorite and muscovite that are stable in this environment. Environments where rocks may be carried to great depths and heated include convergent plate boundaries where slabs of sediment-laden oceanic crust are being subducted. In addition, rocks may become deeply buried in large basins where gradual subsidence results in very thick accumulations of sediment. Such locations, exemplified by the Gulf of Mexico, are known to develop metamorphic conditions near the base of the pile. Furthermore, continental collisions, which result in crustal thickening due to folding and faulting, cause rocks to become deeply buried where elevated temperatures may trigger melting of hot rocks. Heat may also be transported from the mantle into even the shallowest layers of the crust by rising magmas (intrusions). Rising mantle plumes, upwelling at mid-ocean ridges, and magma generated by the melting of mantle rocks at subduction zones are three examples. Anytime magma forms and rises towards the surface, metamorphism generally occurs. When magma intrudes relatively cool rocks at shallow depths, the host rock is "baked". Pressure as a metamorphic agent Pressure is the force per unit area applied on a surface; P=F/A. Below you have some common examples to help you visualize how much pressure is applied onto rocks to produce metamorphism: Pressure at sea level =1 Atmosphere (=1 bar) Pressure of air inside an automobile tire is 2 bars (~ 30 psi) Pressure in deepest part of the ocean is about 1000 bars (=1 kbar) Pressure under one mile of rock is about 500 bars Most metamorphic rocks were metamorphosed at pressures of 2 to 12 kbar (depth of up to 25 miles). Pressure like temperature also increases with depth at a rate of 300 bars/km (1 kbar per 3.3 km). Buried rocks are subjected to confining pressure, where the forces are applied equally in all directions. The deeper you go in the ocean, the greater the confining pressure. The same is true for rock that is buried. Confining pressure causes the spaces between mineral grains to close, producing a more compact rock having a greater density. At greater depths, confining pressure may cause minerals to recrystallize into new minerals that display a more compact crystalline form. Confining pressure does not, however, fold and deform rocks. In addition to confining pressure, rocks may be subjected to directed pressure. This occurs, for example, at convergent plate boundaries where slabs of lithosphere collide. Here the forces that deform rock are unequal in different directions and are referred to as differential stress. Unlike confining pressure, which "squeezes" rock equally in all directions, differential stresses are greater in one direction than in others. Rocks subjected to differential stress are shortened in the direction of greater stress and elongated, or lengthened, in the direction perpendicular to that stress. As a result, the rocks involved are often folded or flattened (similar to stepping on a rubber ball). Along convergent plate boundaries the greatest differential stress is directed roughly horizontal in the direction of plate motion, and the least pressure is in the vertical direction. Consequently, in these settings the crust is greatly shortened (horizontally) and thickened (vertically). Although differential stresses are generally small when compared to confining pressure, they are important in creating the various textures exhibited by metamorphic rocks (see metamorphic textures below).

Igneous Rock Types

Igneous Rock Types The major types of igneous rocks are listed in figure 4.6, and some of the common rocks are shown in figure 4.7 A, B, and C. Figure 4.6 Classification of major igneous rocks based on mineral composition and texture. Coarse- grained rocks are plutonic, solidifying deep underground. Fine- grained rocks are volcanic, or solidify as shallow, thin plutons. Ultramafic rocks are dark, dense rocks, composed almost entirely of minerals containing iron and magnesium. Although relatively rare at or near Earth's surface, these rocks are major constituents of the upper mantle.

Chemical Sedimentary Rocks

In contrast to detrital rocks, which form from the solid products of weathering, chemical sediments derive from ions that are carried in solution to lakes and seas. This material does not remain dissolved in the water indefinitely, however. Some of it precipitates to form chemical sediments. These become rocks such as limestone and chert. Others are formed when lake or sea water evaporates. Rocks such as rock salt and gypsum which form this way are also called evaporites. Evaporites: Rock Salt (NaCl) Rock Gypsum (CaSO4.2H2O) Precipitates: Limestone forms from CaCO3 in sea water Mg may replace some of the Ca in limestone to form Dolomite Chert forms when microcrystalline quartz precipitates from Si and O dissolved in water

Intrusive Plutonic Rock

Intrusive igneous rocks (Fig. 4.1) are formed from magma that cools and solidifies within the Earth's crust. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a result these rocks are made up of large minerals (also called coarse-grained minerals) such that they can be identified with the naked eye. Granite is an example of intrusive or plutonic igneous rock. Fig. 4.1 Igneous rock textures. A. Igneous rocks that crystallize at or near Earth's surface cool quickly and often exhibit a fine- grained (aphanitic) texture. B. Coarse-grained (phaneritic) igneous rocks form when magma slowly crystallizes at depth. C. During a volcanic eruption in which silica- rich lava is ejected into the atmosphere, a frothy glass called pumice may form. D. A porphyritic texture results when magma that already contains some large crystals migrates to a new location where the rate of cooling increases. The resulting rock consists of larger crystals (phenocrysts) embedded within a matrix of smaller crystals (groundmass). (Photos by E. J. Tarbuck)

Lava

Lava is molten rock expelled by a volcano during eruption. This molten rock is formed in the interior of the Earth. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 °C to 1,200 °C (1,300 °F to 2,200 °F). Although lava is quite viscous, with about 100,000 times the viscosity of water, it can flow great distances before cooling and solidifying. A lava flow is a moving outpouring of lava, which is created during a non-explosive effusive eruption. When it has stopped moving, lava solidifies to form igneous rock. The term lava flow is commonly shortened to lava. Explosive eruptions produce a mixture of volcanic ash and other fragments called tephra, rather than lava flows. Lava is the extrusive equivalent of magma.

Magma

Magma Magma is molten rock (or melt) that is found beneath the surface of the Earth, and may also exist on other terrestrial planets (e.g. Mercury, Venus, or Mars). Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma is capable of intrusion into adjacent rocks, extrusion onto the surface as lava, and explosive ejection as tephra to form pyroclastic rock. Magma is a complex high-temperature fluid substance. Temperatures of most magmas are in the range 700°C to 1300°C (1300°F to 2400°F), but very rare carbonatite melts may be as cool as 600°C, and komatiite melts may have been as hot as 1600°C. Most are silicate solutions.

Cooling Time of Magma and Lava

Magma closer to the Earth's surface cools in weeks to months, while magma located at greater depth cools in centuries to a million years. Lava flow cools in few days to months Ash thrown up into the air during violent volcanism cools in minutes

Metamorphism

Metamorphism refers to solid-state changes to rocks in Earth's interior produced by increased heat (200-800 degrees Celsius) and pressure (2-12Kbar). Changes that occur due to the increased heat and pressure involve rock composition and/or texture (figure 6.1). Heat and pressure are thus the main agents of metamorphism. The term "solid-state" implies that a primary (parent) rock which will be changed to a metamorphic rock shouldn't melt; rather it must remain hot but solid. If it melts, the resulting rock is no more metamorphic, rather it will be an igneous rock (remember how igneous rocks form - they form from melt). When rocks become extremely hot, the individual anions (charged ions/elements) that constitute the minerals tend to slightly move from their fixed positions, thereby reacting with other anions and form new minerals. This is how metamorphic minerals and rocks form. The formation of metamorphic minerals and textures takes place very slowly (thousands to millions of years) and it involves one or many of the following processes: Examples of Changes in composition and texture that occur during metamorphism Metamorphic reaction (Neocrystallization): involves chemical reactions that decompose older minerals and produce new minerals. E.g. Metamorphism of impure limestone (made up of mainly calcite (CaCO3) plus some quartz (SiO2) leads to the formation of Wollastonite (CaSiO3): SiO2 + CaCO3 --> CaSiO3 + CO2 Recrystallization: changes the internal structure of minerals (manifested as well formed crystal shape) without changing the chemical composition of the minerals. E.g.1. Quartz in sandstone recrystallizes into quartzite with new interlocking quartz crystals E.g.2. Clay minerals in sedimentary rocks recrystallize into andalusite, kyanite, or sillimanite

Sedimentary Rock

OVERVIEW: 90-95% of the Earth's crust consists of igneous and metamorphic rocks. And most of the Earth's solid surface consists of either sediment or sedimentary rock. Sediments cover almost the entire ocean floor. Thus, while sediment and sedimentary rocks make up only a small percentage of Earth's crust, they are concentrated at or near the surface. Because of this unique position, sediment and the rock layers that they eventually form contain evidence of past conditions and events at the surface. Furthermore, it is sedimentary rocks that contain fossils, which are vital tools in the study of the geologic past. Thus, this group of rocks provides geologists with much of the basic information they need to reconstruct the details of Earth's history. Such study is not only of interest for its own sake but has practical value as well. Coal, which provides a significant portion of our electrical energy, is classified as a sedimentary rock. Moreover, other major energy sources - oil, natural gas, and uranium - are derived from sedimentary rocks. So are the major sources of iron, aluminum, manganese, and phosphate fertilizer, plus numerous materials essential to the construction industry such as cement and aggregate. Sediments and sedimentary rocks are also the primary reservoir of groundwater. Thus, an understanding of this group of rocks and the processes that form and modify them is basic to locating additional supplies of many important resources. The module begins with a brief examination of the processes of weathering, mass wasting, and erosion. The two forms of weathering, i.e. mechanical and chemical, are investigated. Examples of mass wasting will be discussed from Southern California (mainly LA County). This will be followed by the examination of the various detrital and chemical sedimentary rocks, including shale, sandstone, conglomerate, limestone, coal, and rock salt. Having examined the types of rocks, the ways that sediment becomes rock are investigated. Following a look at the classification of sedimentary rocks, the module concludes with a discussion of the mineral and energy resources that are associated with these rocks.

Sedimentary Rock

Sedimentary Rock - Rock that forms at or near the surface of the Earth by processes that include weathering, erosion, accumulation and cementation of sediments, or by the precipitation of minerals from water solution. Sediment consists of: Loose fragments of rocks or minerals broken off of bedrock Mineral crystals that precipitate directly out of water, and Shells that formed when organisms extract ions from wate

Sed Rock Past Environments

Sedimentary Rocks & Past Environments Sedimentary rocks contain evidence of past environments: Cross-bedded Sandstone → Desert Environment Coal → Dense forest, swamp Environment Rock salt, gypsum → Tropical, marine Environment Till & "erratic" → Glacial Environment A glacial erratic is a piece of rock that deviates from the size and type of rock native to the area in which it rests. "Erratics" are carried by glacial ice, often over distances of hundreds of kilometers. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta (Canada).

Bowens Reaction Series

The Bowen's reaction series is a conceptual chart that shows the sequence in which minerals would crystallize from melt (magma or lava). According to this chart, minerals tend to crystallize in a systematic fashion based on their melting points. As shown in figure 4.5 the first mineral to crystallize from a melt is olivine. Further cooling generates calcium-rich plagioclase feldspar as well as pyroxene, and so forth down the diagram. During the crystallization process, the composition of the liquid portion of the melt continually changes. For example, at the stage when about a third of the melt has solidified, the melt will be nearly depleted of iron, magnesium, and calcium because these elements are constituents of the earliest-formed minerals. The removal of these elements from the melt will cause it to become enriched in sodium and potassium. Further, because the original melt contained 50% silica (SiO2), assuming it was a basaltic magma, the crystallization of the earliest-formed mineral, olivine, which is only about 40% silica, leaves the remaining melt richer in silica. Thus, the silica component of the melt becomes enriched as the magma evolves. Bowen also demonstrated that if the solid components of magma remain in contact with the remaining melt, they will chemically react and evolve into the next mineral in the sequence shown in figure 4.5. Figure 4.5. Bowen's reaction series shows the sequence in which minerals crystallize from a magma. Compare this figure to the mineral composition of the rock groups in Figure 4.6. Note that each rock group consists of minerals that crystallize in the same temperature range.

Igneous Hazards

The following igneous products may cause volcanic hazards. Lava flow, and Ash flow/fall and Pyroclastic flow Pyroclastic flow is a fast-moving current of hot gas and rock, which travels away from the volcano at speeds generally greater than 100 km/h (60mph). Lahar The word lahar comes from Indonesia, referring to mudflows that are volcanic in origin. Lahars are similar to pyroclastic flows but contain more water. Lahars form when pyroclastic flows that contain ash and rocks are mixed with water from snow, ice or rainfall. Gas & Dust If the gaseous emissions and dust (very fine ash) from volcanoes reach the stratosphere, they absorb sunlight, resulting in cool Earth's T; if the gases are acidic, they also result in acid rain To learn more about volcanic hazards, watch the following short videos:

Detrimental Sed Rocks

There are a variety of ways that the products of weathering are transported, deposited, and transformed into solid rock. As a result, three categories of sedimentary rocks are recognized. These are detrital, chemical, and organic sedimentary rocks. Detrital Sedimentary Rocks Detrital sedimentary rocks are predominantly composed of mineral grains & rock fragments; quartz and clay minerals being the chief constituents. In the section under "Weathering", we have noted that clay minerals are the most abundant products of the chemical weathering of silicate minerals, especially the feldspars. The other common mineral, quartz, is abundant because it is extremely durable and very resistant to chemical weathering. Thus, when igneous rocks such as granite are attacked by weathering processes, individual quartz grains are freed. Other common minerals in detrital rocks are feldspars and micas. Because chemical weathering rapidly transforms these minerals into new substances, their presence in sedimentary rocks indicates that erosion and deposition were fast enough to preserve some of the primary minerals from the source rock before they could be decomposed. Detrital sedimentary rocks are classified by grain roundness, grain size, sorting, and mineralogy. Grain Roundness Roundness refers to the degree of sharpness of the grain edges. Examples: Conglomerate: Detrital sedimentary rock made up of rounded rock fragments (figure 5.1) Breccia: Detrital sedimentary rock made up of angular rock fragments (figure 5.1) Figure 5. 1. Conglomerate (top) made up rounded rock fragments, and breccia (bottom) made of angular fragments. Grain Size Table 6.1. shows the various grain sizes and particles names that correspond to the given size. Students do not have to memorize the size ranges, rather simply note the three simplified sediment names gravel, sand and mud (in that order from largest to smallest sizes) and corresponding rock names. Examples: Conglomerate or breccia are made up of gravel-sized sediments Sandstone is made up of sand-sized sediments Shale, mudstone and siltstone are made up of mud-sized sediments Sorting Sorting refers to the degree of similarity in grain size. If the majority of the sediments are of similar sizes, we say they are well-sorted; on the other hand, if made up of mixed sizes (large and small), we say the sediments are poorly sorted. Mineralogy (Composition) This parameter is especially important in classifying sandstone varieties. Thus, we have: Quartz sandstone: if quartz makes up > 95% of the sediments, Arkose: if it contains appreciable quantities of feldspar (> 25%), and Greywacke: if it contains abundant rock fragments + matrix (>15%).

Volcanoes of California

Volcanoes of California Mount Shasta The largest of the Cascade volcanoes Constructed of at least four overlapping volcanoes Has been frequently active throughout the Holocene (last 10,000 years) Historical eruption in 1786. Lassen volcanic center A concentration of volcanic features covering much of Lassen National Park. The massive lava dome forming Lassen Peak was constructed about 25,000 years ago and was the site of California's most recent eruption during 1914-1917. Medicine Lake volcano 50-km-wide Geology: obsidian & basaltic lava flows Latest eruptions: about 1000 years ago (rhyolite & dacite at Glass Mt.) Mammoth Lakes Reside on the edge of the Long Valley Caldera. The area is geologically active, with hot springs and rhyolite domes that are less than 1000 years old.

The Formation of Sedimentary Rocks

We have already said that sedimentary rocks are formed when loose sediments get compacted and cemented they turn in to hard materials, called sedimentary rocks. The processes involved include the following: (1) Weathering, (2) Erosion, (3) Deposition (accumulation), (4) Diagenesis (compaction and cementation).

4 Types of Sedimentary Rock Formated

Weathering Weathering refers to the decomposition or breakdown of rock due to contact with air, moisture, & living organisms. There are two types of weathering. Mechanical weathering: breaks intact rock into unconnected grains or chunks Chemical weathering: refers to the chemical reactions that alter or destroy minerals when rock comes in contact with water solutions or air. Chemical weathering is most effective in areas of warm, moist climates. High amounts of water & higher temperature generally cause chemical reactions to run faster. Thus, warm humid climates generally have more highly weathered rock, and rates of weathering are higher than in cold dry climates. Example: limestones in a dry desert climate are very resistant to weathering, but limestones in tropical climate weather very rapidly. Erosion In the previous section on weathering, we have seen that when rocks come in contact with air & water they get disintegrated (physically and chemically) and produce a layer of rock and mineral fragments, also called regolith. If the regolith also contains organic matter, water and air, we call this soil. The process that moves regolith (weathered material) and deposits them elsewhere is known as erosion. It usually occurs due to transport by wind, water, or ice; when weathered material is transported down-slope under the force of gravity, it is called mass wasting. Although rock fragments, mineral grains, and soil particles move only a short distance during each rainfall, substantial quantities eventually leave the fields and make their way downslope to a stream. Once in the stream these weathered material (rocks fragments, mineral grains and soil particles), which can now be called sediments, are transported downstream and eventually deposited. The amount of weathered material currently transported to the sea by rivers is about 24 billion metric tons per year. In many regions the rate of erosion is significantly greater than the rate of weathering. Mass wasting is an important part of the erosional process, as it moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. Deposition After weathered materials are weathered, they get eroded and transported by water, wind or glacial ice to the locations where they accumulate. The transported material includes rock fragments, mineral grains, soil particles, and soluble constituents. Deposition (accumulation) of solid particles occurs when wind and water currents slow down and as glacial ice melts. In sedimentary processes, deposition refers to solids settling out of a fluid (air or water). The mud on the floor of a lake, a delta at the mouth of a river, a gravel bar in a stream bed, the particles in a desert sand dune, are examples. The deposition of material dissolved in water is not related to the strength of wind or water currents. Rather, ions in solution are removed when chemicals or temperature changes cause material to crystallize and precipitate or when organisms remove dissolved material to build shells. Diagenesis As deposition continues, older sediments are buried beneath younger layers and gradually converted to sedimentary rock (lithified) by compaction and cementation (by calcite, silica, and iron oxide). These and other changes are referred to as diagenesis, a collective term for all of the changes (short of metamorphism) that take place in texture, composition, and other physical properties after sediments are deposited.