Aesthenosphere. A region in the upper mantle, starting about 50 miles down but with no sharp lower boundary, where the rock is very close to its melting point and is relatively easily deformed. Pockets of magma are likely present in some parts of the aesthenosphere.

Alkalic. Used to describe igneous rocks that are unusually rich in sodium or potassium.

Alpha Decay. A form of radioactive decay in which an unstable atomic nucleus emits two protons and two neutrons in the form of a helium nucleus.

Andesite. An intermediate extrusive igneous rock, found extensively in the southern Jemez.

Anorogenic. Not associated with an episode of mountain building. Used to describe certain granites or other rock formations.

Aphanitic. Colmposed of crystals too small to be seen with the unaided eye.

Archaean. The second eon of the earth's geological history, from 4 billion to 2.5 billion years ago.

Arenite. A sandstone containing less than 15% clay minerals in its pore spaces.

Arkose. A sandstone containing more than 25% feldspar clasts.

Ash. In geology, a fine powder of very small rock fragments (less than 2mm in size) produced by violent high-silica volcanism.

Back-Arc Basin. A small ocean basin that forms on the landward side of some destructive margins.

Banded Iron Formation. A rock consisting of alternating layers of silica and iron oxides, formed under water in conditions that existed only in Precambrian time.

Basalt. The most common extrusive mafic rock, found extensively in the Jemez area. Usually black to light gray in appearance.

Basanite. An  ultramafic alkalic extrusive rock with a low enough silica content that little feldspar is present. Typically consists of a mixture of clinopyroxene, nepheline, magnetite, and olivine.

Batholith. An immense body of plutonic rock covering up to hundreds of square miles and extending deep into the crust. Batholiths typically occur in old sutures and subduction zones.

Bentonite. A form of clay rich in montmorillonite that typically forms from weathered volcanic ash.

Beta Decay. A form of radioactive decay in which an unstable atomic nucleus emits an electron and a neutrino, which transforms one of the neutrons in the nucleus into a proton.

Bioturbation. Disturbance of a sedimentary bed by the activities of living organisms shortly after the bed was laid down.

Birefringence. An optical property of certain minerals, which causes images seen through the mineral to be doubled. Calcite is the most common mineral showing this property.

Block And Ash Flow. A pyroclastic flow consisting of a large volume of lithic fragments embedded in ash of similar composition. Typically formed when part of a dome or high-aspect flow of high-silica lava collapses.

Breccia. A sedimentary rock containing broken, irregular clasts over 2mm in size.

Caldera. A large bowl-shaped depression in the Earth's surface, typically produced by a massive volcanic eruption that empties an underground magma chamber whose roof then collapses.

Cambrian. The epoch from 540 to 485 million years ago. There are no formations of this age in the Jemez area.

Carbonaceous Chondrites. A type of very primitive meteorite that is virtually unchanged from the time the Solar System formed. Carbonaceous chondrites give us some of our best information on the age of the solar system and conditions at its beginnings.

Cenozoic. The present era of the Earth's geological history, from 65 million years ago to the present. It is further subdivided into the Paleogene, Neogene, and Quaternary periods, although the Paleogene and Neogene periods are still often lumped together as the Tertiary.

Core. The central part of the Earth, beginning about 1800 miles down. Composed mostly of iron with significant quantities of nickel and sulfur. The outer core is liquid while the inner core is solid.

Clast. Geological term for a fragment of rock, of any size from microscopic to as large as a bus.

Cleavage. A property of many minerals, which fracture along flat surfaces lying at a characteristic angle to each other.

Conglomerate. A sedimentary rock containing rounded clasts over 2mm in size.

Consolidated. Describes a sedimentary bed in which the clasts are closely packed together. A consolidated bed is often also well indurated, but this is not always the case.

Continental Platform. The portion of a continent consisting of stable crust covered with sedimentary rock.

Continental Shield. The central portion of a continent consisting of stable crust where Precambrian rocks are widely exposed.

Convection.  The process in which liquid or ductile solid material flows from regions of high temperature to regions of low temperature and back, thereby transporting heat energy.

Correlation. The process of matching up rock formations from distant locations that have the same age, either through comparison of fossils or through radioisotope dating.

Country Rock. The surrounding rock through which magma passes on its way to the surface. This rock is often quite different in composition from the magma.

Covalent Bond. A bond formed between two (usually nonmetal) atoms when they share one or more  pairs of electrons.

Craton. The stable central part of a continent. This usually consists of a continental shield surrounded by a continental platform.

Cross Bedding. A feature of sedimentary rocks laid down in a strong current. Thick beds are made up of thinner cross beds lying at a steep angle to the main beds.

Crust. The outer layer of the earth, which is about 25 miles thick in the Jemez area. It is high in silica compared with the underlying mantle and less dense.

Crystal. A clump of solid material composed of atoms bound together in a regular arrangement. Crystals that have plenty of room in which to form take on a characteristic shape reflecting their internal atomic arrangement.

Dacite. An intermediate-felsic extrusive igneous rock composed almost entirely of feldspar. If small amounts of quartz are present, it is described as rhyodacite.

Delamination. A process that may take place at the base of the crust, where a layer rich in iron and magnesium breaks loose and sinks into the underlying mantle.

Destructive margin. A location where oceanic crust is subducted into the mantle and destroyed.

Diabase. A hypabyssal mafic rock with a very fine salt-and-pepper grainy appearance.

Diagenesis. The process of transformation of a loose bed of sediments into sedimentary rock.

Diapir. A large body of magma deep in the crust or upper mantle that is slowly working its way towards the surface.

Diorite. An intermediate intrusive igneous rock, not common in the Jemez.

Dip. The angle a fault plane makes with the surface of the earth, such that a fault plunging straight down into the earth has a dip of 90 degrees.

Dip-slip fault. A fault where the displacement is mostly in the vertical direction.

Discomformity. A gap in the sedimentary rock record that does not involve any noticeable difference in the inclination of beds above and below the disconformity.

Dolomite. A sedimentary rock rich in magnesium, most likely formed when limestone is exposed to magnesium-rich fluids under the right conditions of temperature, pressure, and oxygen abundance.

Dome. A type of volcanic land form produced by high-silica magma. The interior is viscous magma flowing into the dome from beneath, while the outer layer of the dome is a mantle of solid rock fragments.

Downthrown. Describes the side of a fault in which the beds are lower than on the opposite side of the fault.

Ductile. Having the property of deforming rather than shattering when stressed. Many metals are ductile, which rock becomes ductile at the enormous pressure and elevated temperatures found in the interior of the earth.

Eolian. Describes a sedimentary rock laid down by wind as sand dunes.

Eon. The largest division of geological time. The recognized eons are the Hadean, Archaean, Proterozoic, and Phanerozoic.

Epicontinental Sea.  A shallow sea covering continental crust. The North Sea is a modern epicontinental sea.

Eutectic. The unique composition of the magma that forms from a particular mixture of minerals at the solidus temperature.

Exhumation. The process of bringing deeply buried rock to the surface as a result of tectonic forces.

Extrusive. Describes an igneous rock that cools on the surface of the earth and, as a result, is glassy or has very small crystals, difficult to see even with a magnifier.

Facies. A body of rock formed in a specific manner in a specific environment. It is common for there to be multiple facies within a given formation.

Fault. A fracture in the brittle upper crust of the earth, with the beds on either side displaced from each other.

Fault Trace. The intersection of a fault with the surface of the earth.

Feldspar. A family of tektosilicate minerals, all having the same basic crystal structure, that form from silica-rich magma that also contains alumina and sodium, potassium, or calcium oxide. Feldspar is very common in igneous rocks.

Feldspathoid. A family of silica-poor minerals that form in preference to feldspars from magma low in silica and high in alkali metals.

Felsic. Describes an igneous rock with a high silica content in excess 70%. The word felsic was coined to reflect the high content of FELdspar and SIlica in these rocks.

Ferric Iron. Iron that has contributed three electrons to the compounds it forms, characteristic of high-oxygen environments.

Ferrous Iron. Iron that has contributed two electrons to the compounds it forms, characteristic of low-oxygen environments.

Fertile. Rock that has a low solidus, so that it produces magma with the addition of a relatively small amount of heat.

Fluvial. Describes a sedimentary rock laid down in a river environment.

Foliated. Describes a rock in which individual mineral grains are aligned. Characteristic of metamorphic rock.

Footwall. The lower side of a fault.

Formation. A body of rock that was produced by a particular set of geologic processes over a relatively short period of time. Formations are combined into groups, and subdivided into members.

Friable. A sedimentary bed in which the clasts are so poorly cemented that the sediments crumble to the touch.

Gabbro. An intrusive mafic rock composed of relatively large crystals of plagioclase and pyroxene.

Glass. In geology, a solidified magma that cooled too quickly to form crystals. The structure is disorganized even at the atomic level.

Gneiss. A high-grade metamorphic rock that typically shows prominent foliation.

Granite. A felsic intrusive rock containing large quantities of quartz, found in the Siera Nacimiento Mountains just west of the Jemez and likely present below the surface within the Jemez.

Granodiorite. An intermediate-felsic intrusive rock containing modest amounts of quartz.

Group. A set of formations found in the same area representing a more or less continuous span of the rock record. Groups are further divided into formations.

Hadean. The first eon of the earth's geological history, from its formation to 4 billion years ago.

Half-life. The period of time in which half of a sample of a particular unstable isotope will experience radioactive decay.

Hanging wall. The upper side of a fault.

Hematite. Ferric iron oxide, Fe2O3. Widespread as a cementing mineral in sedimentary rocks. Large deposits of concentrated hematite are one of the most important ores of iron.

Hot spot. A localized region in the mantle, also called a mantle plume, where heat flow to the surface is unusually high. As tectonic plates move over a hot spot, a succession of volcanoes form over the hot spot, leaving a trail of increasingly old volcanoes in the opposite direction from the plate motion.

Hypabyssal. Describes an igneous rock that cools just under the surface of the earth and, as a result, is composed of fairly small crystals.

Ichnofossils. Traces left by the activities of organisms rather than the remains of the organisms themselves. These include things like dinosaur footprints or worm burrows.

Iddingsite. A mixture of iron oxides and hydroxides with some clay and silica formed by decomposition of olivine under surface conditions.

Igneous. Describes a rock that forms from solidifying magma, either on or beneath the surface of the earth.

Illite. A clay mineral similar to muscovite in composition and structure.

Incompatible Element. A chemical element that is not easily incorporated into common silicate minerals, either because its ions have the wrong size or the wrong charge. Incompatible elements tend to be concentrated in magma when it is produced by partial melting of source rock, and to be further concentrated in the last fraction of the magma to solidify.

Indurated. A sedimentary bed in which the rock has been hardened by the deposition of mineral cement.

Interglacial. A period of relative warmth during an ice age, in which glaciers retreat. We are living in an interglacial today.

Intermediate. Describes an igneous rock with an intermediate silica content of 52-63%. Examples are andesite and diorite.

Intermediate-felsic. Describes an igneous rock having a composition of 63%-69% silica, between that of intermediate and felsic rocks.

Intrusive Body. A body of plutonic rock, formed when magma forced its way through country rock and then solidified.

Ionic Bond. A bond formed  when an electron is transferred from a metal atom to a nonmetal atom. Because this leaves the metal atom with a positive charge and the nonmetal atom with a negative charge, the two atoms are attracted to each other by the resulting electrostatic force.

Interglacial. A period of relatively warm climate and retreating glaciers during an ice age. Modern human history has all taken place during an interglacial.

Intrusive. Describes an igneous rock that cools under the surface of the earth and, as a result, is composed of visible crystals.

Island Arc. A line of volcanic islands formed along a destructive margin above the subducting oceanic plate.

Isotope. A nucleus of a particular chemical element (as determined by the number of protons) with a particular number of neutrons. For example, all isotopes of oxygen contain eight protons, but 16O and 18O are isotopes of oxygen whose nuclei contain eight neutrons or ten neutrons respectively. The total number of protons is specified by the chemical element symbol (O) and the total number of neutrons plus protons by the superscript number.

Jemez Lineament. A zone of enhanced volcanic activity reaching at from central Arizona to northeastern New Mexico. Believed to mark the suture between two Precambrian plates.

Kaolinite. A clay mineral formed by strong weathering of illite.

Karst Topography. A distinctive kind of terrain that develops on limestone beds in wet climates, where dissolution of the limestone forms caves and sink holes.

Lacustrine. Associated with lakes, such as lacustrine sediments deposited in a lake bed.

Landslide Scarp. A concave surface on an unstable slope marking the surface where a landslide detached from the slope.

Lapilli. Rock fragments produced by volcanic activity that are between 2mm and 64mm in size.

Lava. Magma that has reached the surface of the Earth still in a liquid state.

Limestone. A sedimentary rock composed mostly of calcium carbonate, usually formed by deposition of small bits of shells of marine organisms.

Liquidus. The temperature at which a magma first begins to crystallize. Above the liquidus temperature, the magma is completely melted; at or below the liquidus, the magma is at least partially solidified.

Lithosphere. The outer rigid shell of the earth, consisting of the crust and uppermost mantle. The lithosphere averages about 80 km (50 miles) in thickness.

Mafic. Describes an igneous rock with a relatively low silica content of 45-52%. The word mafic was coined to reflect the high MAgnesium and Ferrous oxide content of these rocks. Examples are basalt and gabbro.

Magma. Molten rock, composed mostly of a mixture of silica and metal oxides with dissolved gases, such as water vapor and carbon dioxide. Magma that reaches the surface still in a liquid state is known as lava.

Magma Differentiation. The process whereby a cooling magma changes in composition as various minerals crystallize out of the magma.

Magnetite. An iron mineral with the composition Fe3O4 that is present in small quantities in many silica-poor igneous rocks.

Mantle. The portion of the earth that begins about 25 miles below the surface in the Jemez area and extends to the earth's core, 1800 miles deep. It is composed of ultramafic rock and makes up most of the volume of the Earth.

Medial Graben. A graben that forms across a resurgent dome or other area of uplift, caused by the stretching of the rock beds as they are pushed up.

Mantle Wedge. A wedge-shaped region of mantle found at a destructive margin, between the overriding plate and the subducting plate.

Megathrust earthquake. An earthquake produced at a destructive margin when overriding continental lithosphere and subducting oceanic lithosphere that have locked together suddenly break loose. Megathrust earthquakes can release stress over a region hundreds of kilometers long to produced the most powerful known earthquakes.

Member. A part of a formation that is in some way distinguishable from the rest of the formation, even though it was formed in a similar way. For example, the Bandelier Tuff Formation is subdivided into the Tshirege and the Otowi Members, corresponding to two similar caldera eruptions separated by 400,000 years in time.

Mesozoic. The second era of the Phanerozoic Eon, from 250 million to 65 million years ago. It is further subdivided into the Triassic, Jurassic, and Cretaceous periods.

Meta-. Prefix used to indicate the origin of a low-grade metamorphic rock. Thus, a metarhyolite is a metamorphosed rhyolite, a metaconglomerate is a metamorphosed conglomerate, and so on. 

Metamorphic. Rock that has been altered by heat and pressure deep in the earth's crust without actually melting.

Metamorphic facies. A distinctive collection of minerals characterizing metamorphic rock that formed within a particular range of pressure and temperature.

Mica. A phyllosilicate mineral composed of triple layers, each having two phyllosilicate sheets bound together by a layer of metal and hydroxyl and each triple layer bound more loosely to its neighbors by potassium or calcium ions. All micas have a single perfect cleavage plane parallel to the triple layers.

MIcrocline. A feldspar with the composition (K2O)(Al2O3)6(SiO2), stable at low temperatures.

Microcontinent. A small fragment of continental crust separated from the larger continents. A few microcontinents are known today, but they were probably numerous during the Archaean and early Proterozoic Eons.

Mid-Ocean Ridge. A place where oceanic crust is being pulled apart by convection in the mantle, causing mantle rocks to melt and the magma to rise to form new oceanic crust on the axis of the ridge.

Mineral. A substance that forms crystals with a characteristic composition and structure.

Molecular Fossils. Organic compounds found in geological formations that point to the existence of certain forms of life when the formation was deposited.

Monogenetic. Describes a volcanic field consisting of numerous small vents erupting mafic magma, where each vent erupts only a few times.

Montmorillonite. A clay mineral formed by weathering of plagioclase feldspar.

Mudflow. A gravity

Mudstone. A sedimentary rock formed mostly of clasts of all sizes below 0.0625 mm.

Muscovite. A form of mica with the composition KAl2(AlSi3O10)(OH)2.

Nepheline. A silica-poor feldspathoid mineral with the composition 3(Na2O)(K2O)4(Al2O3)8(SiO2).

Nesosilicate. A silicate mineral contaand more ining isolated silica tetraheda, not joined to any other silica tetraheda.

Normal fault. A fault in which the hanging wall is the downthrown side of the fault.

Oblique-slip fault. A fault where the displacement shows significant displacement in both the vertical and horizontal directions.

Oceanic Island Arc. An island arc formed where oceanic crust is subducting under oceanic rather than continental crust.

Olivine. A nesosilicate mineral with a composition ranging from Fe2SiO4 to Mg2SiO4. Olivine is a major component of the upper mantle and is often found in low-silica volcanic rocks at the surface.

Opalized. Describes a sedimentary rock in which so much amorphous silica has been deposited that the rock has become highly indurated.

Orogeny. An episode of mountain building in the geologic record. Usually associated with the collision of continents.

Paleosol. A fossil soil.

Paleozoic. The first era of the Phanerozoic Eon, dating from 540 million years ago to 250 million years ago. It is further divided into the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian periods.

Partial Melting. Production of magma through melting of only a modest percentage of the mass of a source rock.

Passive Margin. A continental margin that is not moving relative to the offshore oceanic plate, so that no particular seismic or volcanic activity takes place.

Pegmatite. A very coarse-grained plutonic rock, usually very rich in silica and incompatible elements.

Phanerozoic. The present eon of the Earth's geological history, from 540 million years ago to the present. Further divided into the Paleozoic, Mesozoic, and Cenozoic Eras.

Phase Change. A change in the chemical structure of a substance that does not involve any change in composition. Ice melting to water is a phase change. Minerals often experience phase changes from one solid structure to a more compact solid structure as the pressure increases deep in the Earth.

Phenocryst. A relatively large mineral crystal embedded in a mass of much finer crystals (the ground mass). Phenocrysts typically form before the magma is erupted as lava.

Phyllosilicate. A silicate mineral whose backbone is sheets of silica tetrahedra joined to neighboring tetraheda at just three of their four corners.

Picritic Basalt. A very low-silica basalt characterized by abundant olivine phenocrysts but without elevated alkali content. Quite rare in continental settings and not found in the Jemez.

Plagioclase. A feldspar mineral, varying in composition from CaAl2Si2O8 to NaAlSi3O8, found in a wide range of igneous rocks.

Planetesimal. One of the large bodies that merged to form the planets in the first 10 million years of the Solar System.

Plate. A region of the lithosphere that moves as a single body across the underlying aesthenosphere.

Plutonic. Describes a rock that forms far beneath the earth and, as a result, is formed of large crystals, easily seen without a magnifier.

Polymorph. Two different forms of a mineral, sharing the same composition but differing slightly in structure, that are stable at different ranges of temperature and pressure.

Porphyritic. Describes a rock composed of relatively large crystals (phenocrysts) embedded in a much finer matrix (the ground mass).

Precambrian. The period of geologic time from 4.55 billion to 540 million years ago, in which life on earth was at so primitive a state of development that few fossils are found in rocks of this age.

Primitive magma. The original magma formed by partial melting of source rock, prior to any differentiation or contamination by country rock.

Proterozoic. The third eon of the earth's geological history, from 2.5 billion to 540 million years ago. The oldest rocks in the Jemez area are Proterozoic in age.

Protolith. The original rock from which metamorphic rock has recrystallized.

Pyroclastic flow. A form of volcanic eruption consisting of a flow of extremely hot gas in which fine rock particles are suspended. The rock particles make the flow heavier than air, so that it can move downhill, often at very high speed. When the flow finally comes to rest, the rock particles settle out to form a tuff.

Pyroxene. A family of silica-poor minerals for which a typical composition is (CaO)(FeO)2(SiO2). Often very dark in color from the iron content.

Quartz. A hard mineral formed from pure silica.

Quartzite. A metamorphic rock formed almost entirely of quartz.

Reaction Rim. A zone of altered compositon around a crystal, such as pyroxene around an olivine crystal, caused by the magma reacting with the crystal during cooling.

Regression. The retreat of an ocean from off a continent over geological time scales.

Reverse fault. A fault where the footwall is the downthrown side of the fault.

Rhyodacite. An igneous extrusive rock intermediate in composition between dacite and rhyolite, with significant quartz content.

Rhyolite. A felsic extrusive rock containing significant quantities of quartz, found extensively throughout the Jemez.

Rio Grande Rift. A region running from central Colorado to northern Mexico where the crust is being slowly pulled apart, coinciding roughly with the valley of the Rio Grande.

Roof Pendant. An isolated outcrop of country rock on the exposed surface of a batholith, interpreted as part of the roof of the batholith that has not quite eroded away.

Sandstone. A sedimentary rock composed mostly of clasts that are 0.0625mm to 2mm in size.

Sedimentary. Describes rock formed from eroded mineral fragments that are cemented together at low temperature and pressure. Differs from metamorphic rock in that the original rock fragments are not significantly altered.

Seismic Discontinuity. A depth in the earth at which seismic wave velocities abruptly change. This indicates either a change in composition or a change in crystal structure with the same composition.

Seismic Profiling. A method for obtaining information about the crust and upper mantle. Sound waves are generated with explosives or heavy weights dropped into a borehole, or using a heavy vibrating plate on the surface of the ground, and their reflection is used to map underground structures. This is analogous to the use of sonar in the oceans.

Shale. A sedimentary rock formed mostly of clasts below 2 microns in size.

Shield Volcano. A volcano built from low-viscosity mafic lava flows having very low relief.

Siderophile. A chemical element which has a strong affinity for molten iron and so will have tended to be concentrated in the Earth's core.

Silica. Silicon dioxide, SiO2. The most important single component of magma, making up between 45% to 90% of the composition by weight. Magma high in silica is extremely viscous, with the viscosity decreasing as the silica fraction decreases.

Silica-oversaturated. Describes an igneous rock that has an excess of silica beyond that needed to form feldspar, so that quartz is present.

Silica-saturated. Describes an igneous rock that has just enough silica to form feldspar, with little or no quartz or feldspathoid present.

Silica-undersaturated. Describes an igneous rock that is so low in silica that feldspathoids are present.

Sill. A horizontal intrusive body, lying between beds of the country rock.

Siltstone. A sedimentary rock formed mostly of clasts between 0.0625 mm and 2 microns in size.

Solidus. The temperature at which a mixture of minerals first begins to melt. Below the solidus temperature, the mineral mixture is completely solid; at or above the solidus temperature, the mixture is at least partially melted.

Sorting. Describes whether the clasts in a sedimentary rock (apart from matrix in the poor space) are all close to the same size. A well-sorted sandstone consists of clasts of uniform size. A poorly-sorted sandstone contains clasts varying considerably in size.

Splay. One of several branches off the end of a fault which help spread the displacement over a larger area.

Stagnation. Describes the process of magma from deeper in the earth pooling at that depth in the crust where its density matches the density of the country rock.

Stratovolcano. A volcano characterized by repeated eruptions of intermediate-silica ash and lava from a single vent over a long period of time that gradually builds up a high, symmetric cone. This is the type of volcano that most closely resembles the popular image of a volcano.

Strand. A branch of a fault that rejoins the main fault further along.

Streak. The color of a mineral when finely powdered. Produced by rubbing a sample on an unglazed porcelain plate. An important diagnostic property for some non-silicate minerals, such as hematite (blood-red streak).

Striation. A characteristic of certain minerals, of which plagioclase is the most common example, in which the surface is marked by very fine parallel grooves.

Strike-slip fault. A fault where the displacement is mostly in the horizontal direction.

Subduction. The process whereby a tectonic plate is forced beneath another when the two collide.

Supereruption. A volcanic eruption of sufficient magnitude to devastate the entire region around the volcano and alter global climate. There is a scale for supereruptions called the Volcanic Explosivity Index, which uses terms like "mega-colossal" to describe points on the scale, suggesting somebody had too much time on his hands. The Valles caldera eruptions would likely rate as "mega-colossal" on this scale.

Suture. A zone of metamorphic and intrusive igneous rock that marks where two ancient plates collided and fused together. The Jemez Lineament is thought to be a suture between two Precambrian plates.

Tektosilicate. A silicate mineral in which silicon atoms are joined to each other by oxygen atoms to form a three-dimensional network. Quartz is an example of a tektosilicate. Feldspar is a tectosilicate in which some of the silicon atoms are replaced by aluminum atoms, with calcium, sodium, or potassium added to the crystal to supply the necessary electrons to allow aluminum to act like silicon.

Tephra. Fragments of rock produced by volcanic activity, such as ash, lapilli, or blocks.

Theia. A Mars-sized planetesimal thought to have impacted the Earth 30 million years after the formation of the Solar System, giving the Earth a large iron core and producing orbiting debris that coalesced into the Moon.

Thrust fault. A reverse fault with a shallow dip.

Transgression. An advance of an ocean across a continent over geological time scales.

Tuff. Any rock containing substantial volcanic ash. A welded tuff is deposited by a pyroclastic flow that is hot enough that the clasts are partially molten and weld together, while an unwelded tuff is deposited by a cooler flow in which the clasts are fully solidified. Volcanic rock eroded and reworked in a river environment can also be described as tuff.

Type Section. A geographical location where a formation was first carefully measured and described in order to assign it a formal formation name.

Ultramafic. Describes the most silica-poor igneous rocks, containing less than 45% silica. These are rare in the Jemez area.

Undepleted. Describes upper mantle rock that has had relatively little magma extracted from it, so that it still contains small amounts of alkaline metal oxides.

Underplating. A geological process in which dense magma from the upper mantle rises to the base of the crust but can go no further.

Volcanic arc. A line of volcanoes that forms above a subducting oceanic plate. When the line of volcanoes erupts through shallow water, the volcanic arc may take the form of an island arc.

Volcaniclastic. A clastic rock composed of igneous clasts that have been transported a short distance from the eruptive center in which they formed.

Volcano. A mountain built up by repeated eruptions of around a single vent or cluster of vents.

Wacke. A sandstone containing more than 15% clay minerals in its pore spaces.

Xenolith. A patch of rock within an igneous bed that is different in composition from the rest of the bed. Xenoliths typically are chunks of country rock that were entrained in the magma and did not fully melt.

Zeolitized. Describes a sedimentary rock in which hydrous aluminum silicate minerals, known as zeolites, have been deposited to cement the clasts together. This typically occurs at the top of the water table in volcanic sediments.

Zircon. A mineral with the composition ZrSiO4 that is unusually resistant to weathering. Zircons also contain traces of uranium, and the decay of the uranium to lead can be precisely measured to determine the age of the grain. The oldest known substance on Earth is individual zircon grains that formed over 4 billion years ago.


This bibliography is not exhaustive. However, I've found these books and papers particularly useful.


Blatt, Harvey, and Tracy, Robert J. 1996. Petrology: Igneous, Sedimentary, and Metamorphic. W.H. Freeman and Company. ISBN 0-7167-2438-3.

A good introduction to general petrology.

Klein, Cornelis, and Hurlbut, Cornelius S., Jr. 1993. Manual of Mineralogy (21st edition). John Wiley & Sons. ISBN 0-471-57452-X

Early versions of this standard reference go back over a century.

Levin, Harold L. 2010. The Earth Through Time (9th edition). John Wiley & Sons, Inc. ISBN 978-0-470-387740

Probably the best introduction to historical geology.

Lille, Robert J. 2005. Parks and Plates: The Geology of Our National Parks, Monuments, and Seashores. W.W. Norton and Company

Perhaps the best introduction to plate tectonics I've read. My only complaint, and it is a parochial one, is that the Jemez receive very little attention, due to Valles Caldera National Preserve not being part of the National Park Service at the time of publication.

Mack, Greg H., and Giles, Katherine A. (editors). 2004. The Geology of New Mexico: A Geologic History. New Mexico Geological Society Special Publication 11.

If you are familiar with basic geology, it's hard to recommend a better book for getting an overview of the geological history of New Mexico. The book has the readability one hopes for in a good review, with just enough technical details to please an expert without bogging down a dilettante.

Philpotts, Anthony R., and Ague, Jay. 2009. Principles of Igneous and Metamorphic Petrology (2nd edition). Cambridge University Press. ISBN 978-0521880060

Probably the definitive textbook on the subject.

Rollinson, Hugh. 2007. Early Earth Systems: A Geochemical Approach. Blackwell Publishing

A good if somewhat technical introduction to the Hadean and Archean eons. Discusses the many questions that have still not been settled and the various answers that have been proposed.

Schmincke, Hans-Ulrich. 2004. Volcanism. Springer-Verlag. ISBN 978-3-540-43650-8.

An excellent and not too technical summary of the current understanding of volcanoes. Assumes a good understanding of basic geological concepts.


Kelly, Shari. Accessed 15 October 2016, "Tsiping".

Discusses the Tsiping ruins south of Canones. 6 November 2013. "X-rays reveal inner structure of the Earth's ancient magma ocean."

Discusses the early magma ocean phase of the Hadean and the time scales for magma solidification.

Wall, Mike. 25 September 2013. "Was Ancient Earth Like Jupiter's Super-Volcanic Moon Io?"

Discusses a theory that the Earth at the end of the Hadean dissipated heat primarily through "heat pipes" rather than hot spots.

Shackley, M. Steven. 2015. "Sources of Archaeological Obsidian in the Greater Southwest."

Discusses the geological origins of obsidian found in archaeological sites.


Bailey, R.A., Smith, R.L., and Ross, C.S. 1969. "Stratigraphic Nomenclature of Volcanic Rocks in the Jemez Mountains, New Mexico". Geological Survey Bulletin 1274-P.

The classical paper on the stratigraphy of the Jemez volcanic field. Has aged remarkably well.

Galusha, Ted, and Blick, John C. 1971. "Stratigraphy of the Santa Fe Group." Bulletin of the American Musem of Natural History 144:1.

An older paper, but still valuable for its detailed discussion of the Santa Fe Group beds in the Espanola Basin.

McLemore, Virginia T, Hoffman, Gretchen. 2005. "Mineral Deposits in Rio Arriba County, New Mexico". New Mexico Geological Society, 56th Field Conference.

A good and reasonable up-to-date summary of valuable mineral deposits in Rio Arriba County, which covers the northern Jemez area.

Premo, Wayne, and Kellogg, Karl. 2005. "Timing and Origin of Proterozoic Basement Rocks in the Sierra Nacimiento Region, NW New Mexico: Evidence From SHRIMP U-Pb Zircon Geochronology and Nd Isotopic tgracer Studies". Geological Society of America 2005 Conference.

The best recent paper on radioisotope ages of the Precambrian rocks of the Sierra Nacimiento.

Waresback, Damon B., and Turbeville, B.N. 1990 "Evolution of a Plio-Pleistocene volcanogenic-alluvial fan: The Puye Formation, Jemez Mountains, New Mexico." Geological Society of American Bulletin 102:3

A good description of the Puye Formation.

Woodward, Lee A., Martinez, Ruben, DuChene, Harvey R., Schumacher, Otto L., and Reed, Richard K. 1974 "Precambrian Rocks of the Southern Sierra Nacimiento New Mexico" New Mexico Geological Society Guidebook, 25th Field Conference.

Rather dated, but describes the Precambrian rocks of the southern Sierra Nacimiento in some detail, with a simple map.

Conference Proceedings and Other Reports

The NMGS conference proceedings are genuine treasures, combining numerous geological papers with at least three day's worth of road logs. However, they are aimed at professional or serious amateur geologists and are not for the tyro.

Baur, Paul W., Kues, Barry S., Dumbar, Nelia W., Karlstrom, Karl E, and Harrison Bruce (ed.) September 1995. "Geology of the Santa Fe Region, New Mexico" New Mexico Geological Society 46th Annual Field Conference

Road logs for Santa Fe to Ghost Ranch via Cundiyo; Santa Fe to Pecos and Mineral Hill; and the Cerrillos His and Ortiz Mountains.\

Goff, Fraser; Kues, Barry S, Rogers, Margaret Anne, McFadden, Les D, and Gardner, Jamie N.. (ed.) September 1996. "The Jemez Mountains Region" New Mexico Geological Society 47th Annual Field Conference

Road logs from Bernalillo to Los Alamos, from Los Alamos through the Valles caldera, and from Los Alamos through the Dome Road to Cochiti.

Kues, Barry S., Kelley, Shari A., and Lueth, Virgil W. (ed.) September 2007. "Geology of the Jemez Region II." New Mexico Geological Society 58th Annual Field Conference

Road logs for Los Alamos to Youngsville and back via the north caldera rim; Los Alamos to the Valles caldera moat; and Los Alamos to Ponderosa.

Open-File Geologic Map Series

These are another treasure, covering most of the area around the Jemez in detail. Most of the maps are accompanied by a detailed report describing the tectonic setting, formations, and structural features (such as fault zones) in the mapped area.

Copyright ©2015 Kent G. Budge. All rights reserved.