Geology Final (Ch. 5,6,8,13)

Basalt flows

At the beginning of many basaltic erup- tions, gases carry bits of lava into the air, forming a lava fountain. The airborne bits of lava cool and then fall around the vent as loose pieces of scoria. The lava fountain may be followed by or accompanied by eruption of basaltic lava flow. Fluid basaltic lava pours from the vent and flows downhill. Sometimes, the lava fills up and overtops the crater in the scoria cone. At other times, a lava flow issues from cracks near the base of the scoria cone after most of the cone has been constructed.

Basalt

Basalt is a dark mafic lava rock. Most basalt is dark gray to nearly black, and many out-crops have vesicles, as shown here. Basalts can contain some phenocrysts of dark pyroxene, green olivine, or cream-colored pla- gioclase feldspar.

How do coarse grained igneous rocks from?

Coarse-grained igneous rocks form at greater depths, where magma cools at a rate that is slow enough to allow large crystals to grow

Composite Volcano

Composite volcanoes are typically fairly symmetrical mountains thousands of meters high, with moderately steep slopes and commonly a crater at the top. They may be large, but are on average much smaller than shield volcanoes. Their name derives from the interlayering of lava flows, pyroclastic deposits, and volcanic mudflows. They consist mostly of intermediate- composition rocks such as andesite, but can also contain felsic and mafic rocks. Typically larger type of volcano.

Basalt lava

Contain less silica, are relatively fluid and have been known to travel 150 kilometers or more before congealing. Dark, fine-grained lava that lines Earth's ocean floors

How cooling history impacts texture

The larger the mineral crystals, the longer it took for the rock to cool. Ex: Pegmatite rocks take longer to cool than fine grained rocks

Texture

The size of mineral crystals in the rock

Glassy rocks:

cool so quickly, no crystals.

3. Which of the following rock types does NOT involve dissolved gas in the magma? a) tuff b) pumice c) scoria d) diorite e) vesicular basalt

d) diorite

5. Which are important ways that magma can raise through the crust? a) through fractures that can form dikes b) pieces of the wall rocks that break off and provide space c) tectonic forces can help open pathways d) magma is hotter and less dense than its surrounding e) all of these

e) all of these

Lava tubes

form when the surface of a lava flow solidifies to form an insulating roof over the hot, still- moving interior of the flow. Lava flows insulated by lava tubes can flow farther than lava flows on the surface because the lava stays hotter longer. If the tube drains, it becomes a curving, tube-shaped cave.

Porous igneous rocks (Vesicular rocks):

from a magma of lava that had a lot of gas.

Why do mafic crystals sink to the bottom of magma?

heavy mafic minerals may settle (sink) through the magma and collect in layers at the bottom of the magma chamber. This process, called crystal settling, will make lower parts of the magma chamber more mafic, leaving the remaining magma more felsic.

Porphyritic rocks:

large crystals surrounded by a matrix of fine-grained material.- cool slowly, then quickly.

Lava dome

A lava dome forms from the eruption of highly viscous lava. The high viscosity of the lava is generally due to a high silica content and causes the lava to pile up around the vent, instead of flowing away. Domes are often accompanied by several types of explosive eruptions.

Andesite

Andesite is the fine-grained equivalent of diorite. It is commonly gray or greenish, but it can also have a slight maroon or purplish tint. Andesite commonly has phenocrysts of cream-colored feldspar or dark amphibole.

Pegmatite

When magma crystallizes deep within the crust, a crystallizing magma may contain enough dissolved water that it grows exceptionally large crystals. If the crystals are larger than several centimeters (they can be meters across), the rock is pegmatite.

Lava flow

When magma erupts onto the surface and flows away from a vent, it creates a lava flow. Erupted lava can be fairly fluid, flowing downhill like a fast river of molten rock. Some lava flows are not so fluid and travel only a short distance before solidifying.

How does adding water effect melting point of rocks?

When you add water, it moves melting temp of rocks (lowers!)

AA lava

a type of rough-surfaced lava flow, formed when the lava breaks apart into a mass of jumbled rocks as it flows. AA flows occur in open channels or as irregularly shaped flows. Angular blocks of hardened lava tumble down the front of the flow as it moves. An aa flow has a very rough surface covered with dark, jagged rocks.

1. An igneous pegmatite indicates that: a) there was water in the magma b) the rock cooled slowly and then quickly c) the rock broke apart as it flowed d) the rock cooled quickly e) the ash and pumice were hot and became compacted

a) there was water in the magma

2. A porphyritic igneous texture indicates that: a) there was water in the magma b) the rock cooled slowly an d then quickly c) the rock broke apart as it flowed d) the rock cooled slowly e) the ash and pumice were hot and became compacted

b) the rock cooled slowly and then quickly

4. Which of the following acts to keep a rock solid (instead of melting)? a) an increase in temperature b) an increase in the vibration within a lattice c) an increase in confining pressure d) none of these

c) an increase in confining pressure

Vesicular basalt

contains abundant gas pockets (vesicles). The vesicles were gas bubbles that expanded in the magma (as pressure decreased) and were trapped when the lava solidified. Vesicles occur in lava flows and in ejected material, such as highly vesicular scoria, which represents frothy, gas-rich magma.

Welded rocks:

volcanic eruptions when pieces of rock and ash are welded together (volcanic breccia)

What Magmatism Accompanies Continental Collisions? (convergent boundaries)

- During a continental collision, one continental plate may slide beneath another continental plate. The descending continental crust gets hotter and experiences increased pressure. - Water may be released by metamorphism of water-bearing minerals and, if the descending continental crust gets hot enough, it undergoes partial melting, producing felsic magmas. - Magmas produced by continental collisions typically do not reach the surface, partly because they have to pass through thick continental crust. Also, some magmas produced have a relatively high water content compared to mantle-derived magmas and so pass through the wet solidus (and therefore solidify) as they rise. So, continental collisions, unlike other convergent boundaries, do not have many volcanoes.

Ultramafic rocks

40% of less Si. Very high in Fe and Mg, rich in olivine Coarse ultramafic: peridotite Fine ultramafic: ultramafic lava (no longer formed on earth) THE LEAST Si (<40%), 1st to crystalize, highest temp

What Happens When Subduction-Derived Magmas Encounter the Crust?

- Most magma generated above the subducting slab begins with a mafic composition because it forms by partial melting of the ultramafic mantle. Less commonly, partial melting may generate magma of intermediate composition. - If the overriding plate is a continental plate, the rising magma encounters thick continental crust that slows its upward journey. The magma heats the surrounding rocks, commonly causing localized partial melting that produces felsic or intermediate magma. - Most subduction-related magma probably never reaches the surface, but some erupts, forming clusters or belts of volcanoes. If the overlying crust is continental, the volcanoes are usually part of a mountain belt. If the overlying crust is oceanic (as shown above in part A), subduction-generated magma creates individual volcanoes along an island arc. In both settings, the subduction-generated magma mostly has an intermediate composition (andesite). Island arcs can erupt mafic magma, and continental magma can be felsic. In both cases, magmas added at depth and on the land surface thicken the crust.

How does magma form along continental rifts? (divergent boundaries)

- Solid asthenosphere rises beneath the rift and starts to melt via decompression melting (see graph below for melting in the mantle). Partial melting of the ultramafic mantle source rock yields mafic magma. - The mantle-derived mafic magma rises into the upper mantle and lower continental crust and accumulates in large magma chambers. Some of the mafic magma reaches the surface and erupts as mafic (basaltic) lava flows. - Heat from the hot mafic magma melts the adjacent continental crust, producing felsic magma. Intermediate magma forms from mixing of felsic and mafic magmas or from the assimilation of continental crust by a mafic magma. - Some felsic and intermediate magmas solidify underground as granite and related igneous rocks, while others erupt on the surface in potentially explosive volcanoes. - Melting of mantle beneath rifts is caused by decompression. The asthenosphere rises into shallower, lower pres- sure regions, and a decrease in pressure (⊲) allows the rocks to melt. This produces mafic magma that can erupt onto the surface, form- ing basalt. - This graph shows a melting curve for mafic rock (basalt) and a lower temperature melting curve for felsic rock (granite). A hot, mantle-derived mafic magma (at point A) rises into continental crust and is hotter than adjacent crust (at point C). The hot mafic magma heats the continental crust (from C to B). As the temperature of the crust crosses the felsic melting line, the granitic crust melts to produce felsic magma. The mafic magma loses heat to the crust (from A to B) and solidifies.

How Does Magma Form Along subduction zones? (ocean-ocean or ocean-continent convergent boundary)

- When an oceanic plate, composed of oceanic crust and lithospheric mantle, converges with another oceanic plate, or with a continental plate, subduction occurs. As the subducted plate descends, both pressure and temperature gradually increase. - In response to the changes in pressure and temperature, existing minerals in the subducting plate convert into new ones through the process of metamorphism. Water-bearing minerals, such as mica, break down, which forces water out of the crystalline structures. The water liberated from minerals then rises into the overlying asthenosphere. - The added water lowers the melt- ing temperature of the mantle above the subducting plate. If the temperature is high enough, melting occurs, and mantle-derived magmas rise into the overriding plate. The magma then may crystallize at depth or eventually erupt at the surface

How does magma form along mid-ocean ridges? (divergent boundaries)

-Step 1: Mantle rocks, including those in the asthenosphere, are mostly solid and crystal- line, not molten. In spite of the high pressures, the mantle's high temperatures allow these rocks to flow as a weak solid while maintaining a crystalline structure. Parts of the asthenosphere are close to their melting temperature. -Step 2: As the plates separate, solid asthenosphere rises to fill the area between the plates. As the asthenosphere rises, pressure decreases and the rock partially melts (decompression melting). A plot of decompression melting is on the next page under the heading Melting in the Mantle. -Step 3: The buoyant, mafic magma rises away from the unmelted residue in the mantle and accumulates in magma chambers in the crust and upper mantle. - Step 4: Magma rises upward through magma-filled fractures that form as the plates pull apart. Some magma erupts as lava within the rift. - Step 5: Older oceanic crust moves away from the ridge in a conveyor-belt manner as new oceanic crust forms along the axis of the ridge.

How magma forms (and how igneous rocks form)

-partial melting of source area -accumulates into rising magma body. Magma rises because its less dense than the rocks around it. -forms magma chamber (solidifies or rises). The magma may solidify in this chamber and never reach the surface, or it may reside in the chamber temporarily before continuing its journey upward. An igneous rock that solidified at a considerable depth (more than several kilometers) is referred to as a plutonic rock, and the body of rock that forms is called a pluton. -eruption as lava or ash. Volcanic ash forms when dissolved gases in the magma expand and blow the magma apart into small fragments of volcanic glass. Any igneous rock that forms on the surface is called an extru- sive rock because it forms from magma extruded onto the surface (⊲). More commonly, we simply call it a volcanic rock. *your source magma determines what will be on earth's surface

Where Does Magmatism Occur Away from Plate Boundaries?

1. Rifts in continental interiors commonly produce basaltic lava flows interpreted to be derived from melting of the lithosphere or asthenosphere. The magmatism could be caused by decompression melting as the asthenosphere rises in response to stretching and thinning of the overlying plate. Rifting can eventually split the continent in two, as the rift evolves to become a divergent plate boundary. 2. In the southwestern United States, some volcanism appears to be caused by asthenosphere encroaching on and melting the lithosphere. This process is expressed on the surface by basalt derived from melting of the lower lithosphere and by intermediate to felsic rocks derived from partial melting of the continental crust. The crust and mantle part of the lithosphere have been stretched and thinned in part of the region, breaking the landscape into a series of basins (valleys) and mountain ranges. 3. If mantle-derived magma accumulates in magma chambers in the continental crust, it may heat the crust and melt parts of it. Such melting yields felsic magmas that may or may not make it to the surface. Much mafic magma gets trapped in the lower part of the continental crust because it loses heat and solidifies as it melts the crust.

Melting by heating

A temperature increase caused by heating can melt a rock. Most rocks contain different minerals with differ- ent melting temperatures, so an increase in temperature causes only partial melting, unless temperature becomes very high. If an increase in temperature is accompanied by an increase in pressure, the higher pressure may be enough to keep the rock from melting.

What is a volcano?

A volcano is a vent where magma and other volcanic products erupt onto the surface. Many geologists reserve the term volcano for hills or mountains that have been constructed by volcanic eruptions. Some eruptions do not produce hills or mountains, and we consider them to be volcanoes, too.

Mafic rocks

About 50% Si. Rich in Ca, lots of pyroxene, plagioclase, olivine Magnesium and iron (fe) Coarse mafic: gabbro Fine mafic: basalt

Intermediate Rocks

About 65% Si quartz, feldspars, biotite, amphibole pyroxene Coarse intermediate: diorite Fine intermediate: dark grey, andesite

Melting by adding water

Adding water can significantly lower, by as much as 500°C, the temperature at which a rock will melt. On this graph, the dashed line shows the position of the melting curve if the rock contains water. Adding water moves the melting curve to lower temperatures. So, adding water to a dry rock at point E puts it on the liquid side of the melting curve, and the rock will melt.

The switch to basalt lava flows from scoria cones

After most of the scoria cone is built, magma that contains less gas reaches the surface and erupts nonexplosively as a lava flow. Taking the easiest way out of the vent, the magma can squeeze out near the base of the scoria cone rather than rising to erupt from the summit crater. Some scoria cones are not accompanied by a lava flow, and vice versa.

What types of igneous rocks form long mid-ocean ridges? (divergent boundaries)

All mafic, forms by partial melting of the ultramafic mantle. -The upper part of oceanic crust consists of basaltic lava flows. When such lavas are erupted into water, they form a series of overlapping mounds called pillows. These distinctive rocks, called PILLOW BASALTS, have in some cases become uplifted above sea level, where we can now observe them. - quick cool. -Countless thin, vertical intrusions of finely crystalline basalt cut across the pillow basalts from below. These thin intrusions, called DIKES, are so closely spaced that they are called sheeted dikes. Each dike represents a thin, tabular conduit through which magma passed. Most dikes are oriented parallel to the oceanic rift and perpendicular to the direction of spreading. -Sheeted dikes merge downward into GABBRO, the coarsely crystalline equivalent of basalt. The gabbro represents magma chambers beneath the rift and locally displays layers formed by settling of light-colored and dark crystals. - The base of the gabbro is the base of the oceanic crust, below which are ULTRAMAFIC ROCKS of the mantle. The mantle rocks show evidence of having been partially melted to form all of the overlying mafic rocks in the crust (pillow basalt, sheeted dikes, and gabbro)

How Does Water Get into a Subduction Zone?

An oceanic plate being subducted was formed originally along a mid-ocean ridge, where seawater flows into the hot crust and forms water-bearing metamorphic minerals. These water-bearing minerals, shown with blue spots in the oceanic crust, travel with the plate as it moves away from the ridge. Once formed, oceanic crust is slowly covered by sediment derived from continents, islands, and creatures living in the sea. As time passes, the sedimentary layer thickens. This sediment contains trapped seawater and minerals, including clay, that have water in their mineral structure. The oceanic crust with its liquid water within the sediment and the water bound within minerals can eventually reach a subduction zone, where the oceanic crust is subducted. Only the water trapped in minerals gets deep enough to cause melt- ing. Liquid water within sediment probably is driven off farther up the subduction zone.

Medium grained

Crystals that are mms but not cms. Phaneritic.

Coarse grained

Crystals that are mms to cms in length. Phaneritic.

Fine grained

Crystals that are not visible. Aphanitic. Result from magma that solidifies too rapidly to grow crystals visible in outcrop.

Diorite

Diorite contains more mafic minerals than does granite. It is intermediate between felsic and mafic compositions. It generally contains plagioclase feld- spar and amphibole, and it can contain biotite or pyroxene. A rock in between diorite and gran- ite is granodiorite.

How Do Gases Affect Magma?

Dissolved gas is held in magma by pressure. As magma approaches the surface, (under less pressure), gas forms bubbles. Released gas propels eruption and forms ash.

Impact of dissolved gasses on magma

ERUPTIONS OF BASALTIC MAGMA can form a variety of rock types and landforms. This variety is largely con- trolled by the gas in the magma, because gas affects the style of eruption and the solidification of lava

Early formation of a scoria cone

Early Formation of a Scoria Cone—If basaltic magma contains enough dissolved gas, the gas comes out of solution as the magma approaches the surface.The gas expands dramatically and propels clots of frothy lava out of the conduit, piling up around the vent and forming a scoria cone. This generally occurs early in a basaltic eruption because the magma has not had time to degas in the magma chamber.

Composition, Viscosity, and Eruptive Style

Felsic and intermediate magma have more silicate chains, and the chains are longer, restricting the flow of the magma and making it more viscous. The high viscosity of felsic and intermediate lavas produces steep volcanic domes and steep composite volcanoes. Magma in domes, composite volcanoes, and large volcanic calderas can trap gas and erupt explosively, producing hot, gas-propelled pyroclastic eruptions of volcanic ash, tephra, and rock fragments. As a result, these volcanoes produce a mix of pyroclastic rocks and lava flows, mostly of felsic and intermediate composition. Composition controls viscosity, eruptive style, the shape of the volcano, and the rock types that compose that volcano.

Why do felsic crystals float to the top of magma?

Felsic crystals may be less dense than magma and so may float upward. This makes the top of the magma chamber more felsic.

How do fine and medium grained igneous rocks form?

Fine-grained igneous rocks form if the magma only has enough time to grow small crystals. This commonly occurs when magma solidifies on the surface in a thick lava flow or at shallow depths beneath the surface, because cooling in these settings is fairly rapid. Medium- grained rocks form deeper, where cooling occurs more slowly

How do porphyritic rocks from?

For a porphyritic texture to form, magma needs sufficient time in a subsurface magma chamber to grow visible crystals. Later, the magma rises to just below or on the surface, where the remaining magma solidifies rapidly into the fine-grained matrix around the larger crystals (phenocrysts).

Igneous rocks

Form from molten material magma under earth's surface

Gabbro

Gabbro is a coarsely crystalline, mafic rock. It typically is dark and consists of pyroxene and other mafic minerals, along with light-gray, calcium-rich plagioclase feldspar. Feldspar-rich varieties are lighter col- ored, and some gabbro has olivine.

Granite

Granite is a coarsely crystalline, light-colored igneous rock. The light color is due to an abundance of the light- colored, felsic minerals feldspar and quartz. Most granites also contain some biotite (black mica), and some contain light- colored mica (muscovite) and garnet.

What Type of Magmatic Activity Occurs at Hot Spots?

Hot Spots and Mantle Plumes - Most hot spots are considered to be the crustal expression of a rising plume of hot mantle material. There is current debate about how deep in the mantle such plumes originate, but some may begin at the core-mantle boundary and ascend all the way through the lower mantle and into the asthenosphere. - The movement of plumes through the mantle is commonly compared to the rising blobs within a lava lamp. The teardrop shape of the blobs nicely matches experimental models of mantle plumes, but, unlike the liquid substance in lava lamps, the plume and surrounding mantle are both solid. Like the blobs in the lamp, a mantle plume rises because it is hotter and less dense than material around it. Hot Spots in Oceans - When magma generated by a mantle plume encounters the litho- sphere, it spreads out along the boundary. There, it causes melting of the overlying lithosphere, and additional melting occurs by decompression. Magma from the lithosphere and plume can reach the surface, creating large volcanoes on the seafloor. This is occurring on the Big Island in Hawaii and on the seafloor farther to the southeast. - A lithospheric plate may be moving above a plume that is anchored in the deep mantle. An active volcano overlies the hot spot, but volcanic activity will cease once the volcano has moved off the hot spot. The hot spot creates a succession of volcanoes along a linear chain of islands and seamounts as the overlying plate moves across the hot spot. Hot Spots in Continents - When a rising mantle plume encounters continental lithosphere, its high temperatures cause melting. If the melting occurs in the lower part of the lithosphere (in the mantle), it produces mostly mafic magma. If the mantle-derived magma causes melting in the upper part of the lithosphere (in the crust), it can generate felsic magma. - Mafic and felsic magmas may mix in the crust to produce intermediate magmas. Consequently, when crustally derived magmas make it to the surface, the resulting eruptions may be of many types. If the magma is felsic, there is a tendency for the eruptions to be explosive and to form a large volcanic depression called a caldera. If the magma is more mafic, the eruptions will be less explosive.

Complete melting of source area

If a magma was generated by complete melting of the source region, it would have a composition identical to that of the source. For a number of reasons, complete melting is not common.

Examples of crust melting

If a more felsic source area, such as continental crust, is melted, the magma will be felsic. If an intermediate source is almost completely melted, the magma will have an intermediate composition, but partial melting of such a source more commonly produces a felsic magma. Most mafic magma is derived by partial melting of the mantle.

Welded tuff

If tuff gets buried while still hot, as within a thick pyroclastic flow, the weight of overlying materials compacts the ash and pumice into lenses, as shown here, forming welded tuff. Tuff commonly contains angular fragments of older rocks, which do not compact.

Phaneritic texture

Individual mineral crystals are visible to the naked eye. Includes pegmatite, course-grained, medium grained. cools underground.

Pegmatite

Large, crystals cm to m in length (water needs to be associated). Water allows atoms to move quickly in magma and form massive crystals. greater than 2m long. Phaneritic. *the larger the mineral crystals, the longer it took for the rock to cool.

Assimilation

Mafic magma is hotter than the melting temperature of felsic rocks, so mafic magma can melt felsic wall rocks. If wall rocks around a magma melt, they may be incorporated into the magma, a pro- cess called assimilation.

What Determines How Far a Magma Can Rise Toward Earth's Surface?

Magma Pressure: Pressure from the weight of the overly- ing rocks is directed in toward the magma from all sides. The pressure pushes the magma into any available openings and drives it toward the sur- face. The confining pressure exerted on the magma decreases as the magma rises higher into the crust. Density: Differences in density drive the flow of magma. Mafic magma generated from partial melting of the mantle is less dense than the surrounding solid rocks and so rises. When the rising mafic magma reaches the base of the crust, its density may be greater than that of the crustal rocks. The mafic magma may then stop and form a magma chamber within or below the crust Gas Pressure: Magma contains dissolved gases, such as carbon dioxide, sulfur dioxide, and water vapor (steam). As magma rises to shallower levels, decreasing pressure allows the gases to form bubbles in the magma. If this occurs, the density of the magma decreases and the magma rises faster. If magma initially has a low content of dissolved gases, bubbles may not form or assist the magma upward. Stress: Tectonic stress can help the magma to open steep fractures that provide a pathway to the surface. Alternatively, stress can trap a magma at depth by keeping fractures closed or by creating horizontal fractures that direct a magma sideways.

Lava

Magma on Earth's surface

Worlds largest volcano

Mauna Loa, the central mountain, is the world's largest volcano. It rises 9,000 m (29,500 ft) above the seafloor and is 4,170 m (13,680 ft) above sea level. From seafloor to peak, Mauna Loa is Earth's tallest mountain. Nearby Mauna Kea is an inactive shield volcano and the site of astronomical observatories. Shield volcanoes, like Mauna Loa, have broad, gentle slopes because the basaltic magma had a relatively low viscosity that allowed it to flow downhill and spread out. Shield volcanoes contain mostly dark-colored, basaltic magma with local hills and layers of reddish and black scoria.

Aphanitic texture

Mineral crystals are not visible to the naked eye. Fine grained. Cools underground. Result from magma that solidifies too rapidly to grow crystals visible in outcrop.

Differences between mafic and felsic minerals

Minerals melt at different temperatures—felsic minerals melt before mafic ones. Minerals crystallize in the opposite order from which they melt—mafic minerals generally crystallize before felsic ones.

How Does Viscosity (more silica in magma) Affect Eruptions?

More viscous: Felsic magmas contain a lot of silica, and so they are relatively viscous. The high viscosity prevents gas from escaping easily. Gas builds up in the magma and, when it expands, greatly increases the pressure on the surrounding rock. This can cause explosive eruptions. Less viscous: Less viscous magma, such as one with a basaltic composition, allows gas bubbles to escape relatively easily. This can lead to a fairly nonexplosive eruption, such as this basaltic lava flow that flows smoothly downhill from the vent.

Partial melting of source area

Most rocks melt by partial melting as some minerals melt before others. Felsic minerals melt at lower temperatures than mafic minerals, so partial melting produces a magma that is more felsic than the source. For example, partial melting of a mafic source can yield an intermediate magma. (Ultra mafic can melt to mafic, int, or felsic; mafic can melt to int. or felsic; int can melt to felsic; felsic only melts to felsic.)

Obsidian

Obsidian is a shiny volcanic glass that is normally a medium gray to black color. Most obsidian has a composition equivalent to that of rhyolite. It forms when a lava flow cools too rapidly to form crystals. Obsidian commonly has bands, and some contain phenocrysts or fragments.

Eruption column (pyroclastic eruption)

Other explosive eruptions eject a mixture of volcanic ash, pumice, and rock fragments into the air. Such airborne material is called tephra, and tephra particles that are sand-sized or smaller are volcanic ash. Ash mostly forms when bubbles blow apart bits of magma. Tephra is derived from pumice, fragmented volcanic glass, and shattered preexisting rocks.

How composition is used to classify igneous rocks

Our classification of igneous rocks mostly considers the composition of the rocks (e.g., percentage of felsic versus mafic minerals) and the size of crystals in the rock. (size of crystal depends on how fast or slow the rock cools)

How Do Pressure and Temperature Change with Depth?

Overall, pressure and temperature increase with depth. The temperature increase with depth is called the geothermal gradient, and is somewhat variable. For every km you go beneath earth's surface, temp increases 20-40 degrees. Pressure = force that compresses a rock. The pressure on a rock at some depth in Earth is the force exerted by the weight of rock above it. This force increases with depth and is mostly balanced by forces pushing in from the sides. The core of the earth is 3000-5000 degrees, but still solid because of the pressure.

Pahoehoe lava

Pahoehoe is a type of lava flow that has an upper surface with small billowing folds that form a "ropy" tex- ture. A pahoehoe lava flow is usually fed by a lava tube and grows as a series of tongues. As the front of the flow solidifies, the lava breaks out and forms a new tongue, as shown here. Pahoehoe lava flows relatively smoothly and easily compared to aa.

How does pegmatite form?

Pegmatite may form if magma is relatively water rich. The dissolved water allows atoms to migrate farther and faster and so helps large crystals to grow. This generally occurs near the sides and top of a magma chamber and in local pockets within the magma. Most pegmatite forms at moderate to deep levels within Earth's crust.

Peridotite

Peridotite is the main coarsely crystalline ultra-mafic rock. Compared to mafic rocks, it contains more magnesium- rich and iron-rich minerals, especially green olivine and dark pyroxene. The variety shown here is all olivine. The upper mantle is composed of peridotit

Scoria cones

Pieces of scoria from the lava fountain gradually create a cone-shaped hill called a scoria cone (also called a cinder cone). Ejected fragments can be as small as sand grains or as large as huge boulders. Scoria cones typically form in a short amount of time, from a few months to a few years, and generally are no more than 300 m (~1,000 ft) high

Melting by decompression

Pressure decreases if a rock moves up from depth, getting closer to the surface. So a rock that is uplifted will experience a decrease in pressure. If the rock is already hot, it may melt as the pressure decreases, a process called decompression melting. For decompression melting to occur, the rock has to be fairly hot and must be uplifted fast enough so that it cannot cool significantly during uplift. If a rock is uplifted slowly, it can cool enough to stay solid. A hot, deeply buried rock cooling during uplift would stay within the solid field on the diagram. In other words, the rock would not melt.

Pumice

Pumice is a volcanic rock containing many vesicles (holes). The holes are so numerous that most pumice floats on water. The solid material in pumice begins as volcanic glass, but over time it can convert into micro- scopic crystals.

Rhyolite

Rhyolite is the fine- grained equivalent of granite. It is mostly a finely crystalline rock, but it can contain glass, volcanic ash, pieces of pumice, and variable amounts of visible crystals (phenocrysts) of quartz, K-feldspar, or biotite.

Scoria Cone Volcano

Scoria cones are cone-shaped hills several hundred meters high or higher usually with a small crater at their summit. They also are called cinder cones because they contain loose black or red, pebble-sized volcanic cinders (scoria), along with larger volcanic bombs. The scoria is basaltic or, less commonly, andesitic in composition. Some scoria cones form next to or on the flanks of composite and shield volcanoes. Typically smaller type of volcano.

Scoria

Scoria cones contain scoria, which is highly vesicular, and other fragments explosively ejected from the volcano during a lava fountain. The fragments may have been liquid or solid when ejected. Small blobs cool and solidify in the air to form scoria (cinders). Large blobs of magma and solid angular blocks are ejected as volcanic bombs.

Scoria

Scoria is a dark gray, black, or reddish volcanic rock that contains many vesicles. It usually has the composition of basalt or andesite. In outcrops, scoria consists of a jum- bled mass of rock fragments as large as several meters across.

How do shield volcanoes form?

Shield volcanoes erupt mostly low-viscosity basaltic lava and so are dominated by relatively nonexplosive outpourings of lava from fissures and vents. Early phases of eruptions are commonly marked by spectacular lava fountains in which molten rock is ejected hundreds of meters into the air from fissures or central vents. Fissure Eruption—A fissure eruption occurs when magma rises through a fracture and erupts onto the surface from a long fissure. Large volumes of lava can flow out of the fissure, and escaping gas throws smaller amounts of molten rock into the air as a fiery curtain. Lava Flow—Fluid basaltic lava typically flows downhill as a river of molten rock. Flows can divide, rejoin, spread out, or constrict as they encounter variations in topography. They can even have waves and rapids, like those in a river of water. Pillow Basalt— When fluid lava erupts into water, the lava flow grows for- ward as small, individual tongues that form rounded shapes called pillows. Pillows are reliable evidence that lava erupted into water.

Shield Volcano

Shield volcanoes have broad, gently curved slopes and can be relatively small (less than a kilometer across) or can form huge mountains tens of kilometers wide and thousands of meters high. They commonly contain a crater or line of craters and have fissures along their summit. Shield volcanoes consist mostly of basaltic lava flows with smaller amounts of scoria and volcanic ash. Typically larger type of volcano.

Why does the pressure and temperature graph slope?

Slopes because as pressure increases, more heat needs to be added to melt (pressure increases with depth)

Lava fountain (pyroclastic eruption)

Some explosive eruptions send molten lava into the air.A lava fountain, such as shown here, can accompany basaltic volcanism and results from a high initial gas content in a less viscous lava. The gas propels the lava and separates it into discrete pieces.

How volcanic ash becomes rock?

Some volcanic ash erupts vertically in a column and settles back to Earth. This ash cools significantly before accumulating on the surface. Because it is relatively cool and strong, the ash may not become welded; thus it is said to be non welded. Other volcanic ash erupts in thick clouds of hot gas, ash, and rock fragments, called pyroclastic flows, that flow rapidly downhill under the influence of gravity. The ash deposited by pyroclastic flows is very hot, and so most parts are welded to some extent.

Viscosity

Stickyness of a fluid. (ex: high viscosity = honey, low viscosity = water) Magma viscosity is affected by temperature. hotter temp means magma will have less viscosity and will spread out, lower temp means it will have higher viscosity and pile up. SILICA CONTENT: also helps determine viscosity. more silica = stickier, less silica = flows more freely

Nonwelded tuff

Tuff is a volcanic rock composed of a mix of volcanic ash, pumice, crystals, and rock fragments. If the particles of ash and pumice cool before being buried by overlying materials, the rock remains only weakly consolidated and is nonwelded tuff

Ultramafic lava

Ultramafic lavas erupted early in Earth's history, and so such rocks are preserved only in the oldest parts of some continents. The magma was very hot and commonly grew olivine or pyroxene crystals that are unusually long for a lava flow.

Felsic Rocks

Very high in silica (70-80% Si) Feldspar silica Rich in sodium and potassium Minerals: quartz, feldspars, micas, garnet Coarse felsic rocks: granite Fine felsic rocks: rhyolite THE MOST Si, THE LOWEST TEMP

How do vesicles form?

Vesicles form when gases dissolved in magma accumulate as bubbles. They can form only under low pressures on the surface or very near the surface. Many lavas are vesicular, and much of the material in volcanic ash forms when the thin walls between vesicles burst, shattering partially solidified magma into sharp particles. Most volcanic ash is broken vesicles.

How does volcanic breccia form?

Volcanic breccia can form in many ways, including from explosive eruptions of ash and rock fragments, from a lava flow that breaks apart as it partially solidifies while flowing, or from volcano-triggered mudflows and landslides on the steep and unstable slopes of the volcano.

Volcanic Dome

Volcanic domes are dome-shaped features that may be hundreds of meters high. They consist of solidified lava, which can be highly fractured or mostly intact. Domes include some volcanic ash intermixed with rock fragments derived from solidified lava in the dome. They form where felsic or intermediate magma erupts and is so viscous that it piles up around a vent. Many domes are within craters of composite volcanoes or within large calderas. Typically smaller type of volcano.

How does volcanic glass form?

Volcanic glass forms when magma erupts on the surface and cools so quickly that crystals do not have time to form. This can happen in a lava flow or in volcanic ash.

Volcanic glass

Volcanic glass is unstable, eventually changing from noncrystalline glass into rhyolite consisting of very small crystals. The conversion to rhyolite can produce blobby or layered patches of glass and rhyolite, commonly giving the rocks a some- what mottled appearance.

Volcanic Breccia

Volcanic rocks with fragments form in other ways, such as the break- ing apart of lava that solidifies during flow. Fragmental rocks also form from mixtures of volcanic rock, ash, and mud. In either case, the result- ing fragmental rock is a volcanic breccia.

nonvesicular basalt

may not contain enough gas to form bubbles, so the lava solidifies into nonvesicular basalt. A magma that forms nonvesicular basalt may have a low con- tent of gas because it started out with a low content of dissolved gas or because it lost gas somewhere along the way. Most basalt has fractures, like the ones shown here.