Abiogenic. Describes anything formed through nonliving
Acritarchs. An extinct family of tiny organisms that may be among the earliest eukaryotes.
Aeromagnetic Survey. The measurement of magnetic fields
above the earth's surface by low-flying aircraft. Originally
developed to detect ore bodies, it can also trace basement
structures hidden below nonmagnetic sedimentary beds.
Alkalic. Used to describe igneous rocks that are unusually
rich in sodium or potassium. Not all the alkali metal can be
incorporated into feldspar, so less common silica-rich minerals
form, such as feldspathoids.
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.
Amphibole. A family of double chain inosilicate
minerals common in igneous rocks, containing water in their
Andesite. An intermediate extrusive igneous rock, found extensively in the southern Jemez.
Angular Unconformity. An unconformity in which the
formations above the unconformity lie at an angle to those below.
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
Ash. In geology, a fine powder of very small rock fragments (less than 2mm in size) produced by violent high-silica volcanism.
Asthenosphere. 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.
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
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 an antineutrino, which transforms one of the neutrons in the nucleus into a proton.
Bimodal. In the broadest sense, refers to any statistical
distribution with two distinct peaks. In geology, usually refers
to a volcanic center where high silica and low silica rocks are
present with little intermediate silica lava.
Biogenic. Describe anything produced by the activities of
Bioturbation. Disturbance of a sedimentary bed by the
activities of living organisms shortly after the bed was laid
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.
Calc-alkaline. Describes a suite of igneous
rocksmoderately enriched in the alkali metals, potassium and
sodium, and the alkaline earths, calcium and magnesium. Other
distinctive chemical characteristics include a high aluminum
content and a tendency to steadily decrease in iron content as the
silica content increases.
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
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
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.
Contact Metamorphism. Metamorphism due to local heating by
a nearby intrusion of magma.
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 solid 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
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.
Cyanobacteria. A large group of chlorophyll-containing
bacteria capable of generating oxygen by photosynthesis. Formerly
known as blue-green algae.
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. An unconformity that does not involve any
noticeable difference in the inclination of beds above and below
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
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, while 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.
Eukaryotes. One of the three main domains of life.
Eukaryotic cells have true nuclei and a complex internal
structure. They include modern plants, animals, and fungi.
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 and makes up the
bulk of the earth's crust.
Feldspathoid. A family of silica-poor minerals that form
in preference to feldspars from magma low in silica and high in
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
Ferrous Iron. Iron that has contributed two
electrons to the compounds it forms, characteristic of low-oxygen
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
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
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
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.
Hartzburgite. A rare rock type sometimes found as
xenoliths in lava flows. It is believed to be the solid residue
left when mantle rock is partially melted.
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.
Hornblende. A generic term for a black iron-bearing amphibole of variable composition, common in igneous rocks.
Hornfels. A very tough metamorphic rock formed from
sedimentary rock metamorphosed by the heat of a nearby magma
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
Igneous. Describes a rock that forms from solidifying magma, either on or beneath the surface of the earth.
Igneous suite. Families of igneous rocks having a similar
origin. Each suite comes from its own distinctive source rock
subject to a particular degree and type of partial melting.
Illite. A clay mineral similar to muscovite in composition
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.
Index Fossil. A fossil species with distinctive
characteristics that appears suddenly in the fossil record at a
precise point in time, allowing a sequence of beds to be dated
from its appearance. Index fossils are now used to define the
starting poiint of most time intervals in the geologic time scale.
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
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. Describes an igneous rock that crystallized
slowly underground, so that it is formed of visible crystals.
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.
Inosilicate. A mineral build around a backbone of single
or double chains of silica tetrahedra.
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.
Isostacy. The balance of buoyant forces in the Earth's
crust, which causes mountains to form over low-density rock to
balance its buoyancy, or crust to subside under ice sheets.
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
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.
Lamprophyre. A rare igneous rock low in silica and very
high in potassium thought to be formed by very slight melting of
the upper mantle.
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.
Leptite. A form of metamorphic rock rich in feldspar and
quartz. Also known as feldspathic schist.
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.
Lithological discontinuity. A line dividing areas where
the rock beds show distinctive features, such as composition, age,
or isotope ratios.
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
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.
Mantle Wedge. A wedge of mantle above subducting
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.
Mass Extinction. A point in the fossil record where a
signficant fraction of families of living organisms abruptly
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
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.
Metasomatism. Change in the composition of a body of rock
from fluids from a nearby magma intrusion.
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.
Miogeocline. The region of sediment accumulation along a
passive continental crust following rifting.
Migmatite. A rock consisting of layers of dark (melanosome)
and light (leucosome) minerals with a characteristic
pattern, thought to represent a rock that was once partially
melted deep underground.
Mineral. A substance composed of crystals with a
characteristic composition and structure.
Mohorovičić discontinuity. The seismic discontinuity that defines the boundary between the crust and mantle. Often referred to in the English-speaking world as the Moho.
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 containing isolated silica tetraheda, not joined to any other silica tetrahedra.
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
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.
Onlap. A set of beds near the geographical limit of a
sedimentary formation where the younger beds extend further over
the underlying formations than the older beds. Usually indicates
subsidence in the region when the beds were laid down.
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. This produces a belt of mountains underlain by deformed rock called an orogen.
Paleomagnetism. The study of magnetic fields of ancient
rock beds, which give clues to the location of the beds relative
to the magnetic poles when the beds were formed.
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.
Pelitic. Describes sedimentary rock rich in clay minerals,
or metamorphic rocks transformed from clay-rich sedimentary rock.
Peneplain.A very flat surface produced by extended erosion
during a time of tectonic stability.
Peraluminous. Describes a rock containing more aluminum
than can be incorporated in feldspar, so that mica or more unusual
aluminum-rich minerals are present.
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.
Pinching Out. Describes where a formation thins to a knife
edge and disappears, leaving the beds above and below in direct
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.
Plate Tectonics. The unifying theory of modern geology,
which holds that the crust and upper mantle (the lithosphere)
is broken into several large plates that are carried along
by currents in the underlying mantle.
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.
Province. Used in two overlapping senses. A Precambrian
province is a region of crust formed in a single short period of
time before being assembled into larger continents, such as the
Yavapai Province. A tectonic province is an area of crust
characterized by a particular history since its formation, such as
the Colorado Plateau.
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
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.
Refractory. Describes a solid material that has a very high melting point.
Regional Metamorphism. Metamorphism over a broad region
caused by a widespread tectonic event, usually mountain building.
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.
Rift Valley. A valley formed by rifting of the crust.
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.
Schistose. Describes a rock that easily splits into thinly
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
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
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.
Slab Pull. The force exerted by the weight of subducting crust at destructive margins, thought by some geologists to be the driving force for plate tectonics.
Slope-Forming Unit. A rock unit composed of relatively
soft sedimentary rock, such as mudstones, shales, or poorly
cemented conglomerates or sandstones, that erodes to a relatively
gentle slope compared with more resistant beds above and below.
Sole Mark. Any distinctive marking left on the surface of
a sediment bed by running water.
Solid Solution Series. A mineral in which one chemical
element freely substitutes for another chemical element, so that
the mineral has a range of compositions. For example, plagioclase
feldspar forms a solid solution series ranging from pure albite
(NaAlSi3O8) to pure anorthite (CaAl2Si2O8).
Any number of calcium atoms can substitute for sodium atoms in the
structure, so long as a matching aluminum atom simultaneously
substitutes for a silicon atom.
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.
Spreading Center. A location such as a mid-ocean
ridge where two lithosphere plates are separating and rising
mantle rock melts to produce new crust in the zone of separation.
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.
Stratigraphy. The branch of geology concerned with
dividing rock beds into groups, formations, and members in order
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.
Stromatolite. A stony structure, typically around a meter in height and diameter, built up by a microbial mat dominated by cyanobacteria.
Subalkaline. Describes igneous rock in which the alkali
metal content is low enough that it is all incorporated into
Subduction. The process whereby a tectonic plate is forced beneath another when the two collide.
Suite. A body of rock that has been so distorted, by
metamorphism or igneous intrusion, that the chronological order of
the beds is difficult to discern.
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.
Tholeiitic. Describes a suite of igneous rock formed in a
reduced environment, so that olivine rather than magnetite
crystallizes out early in its differentiation.
Thrust fault. A reverse fault with a shallow dip.
Transgression. An advance of an ocean across a continent over geological time scales.
Trench Rollback. The process in which the point of
subduction of a subducting plate migrates in the direction of the
subducting plate. This often stretches the overriding plate to
produce a back-arc basin.
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.
Uncomformity. A gap in the sedimentary rock record where
older beds are in contact with younger beds with a significant age
difference across the gap.
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
Unit Cell. The smallest piece of a crystal that contains
the basis for its entire structure. A unit cell is always a
parallelepiped, that is, a six-sided box whose sides are always
parallel to the opposite side but need not necessarily be
perpendicular to the adjoining sides.
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.
BooksCarmichael, Ian S.E; Turner, Francis J.; and Verhoogen, John. 1974. Igneous Petrology. McGraw-Hill, Inc. ISBN 0-07-009987-1
A somewhat dated but still very thorough and
useful text on the basics of igneous petrology.
Kelley, Vincent C., and Northrop, Stuart A.
1975. Geology of Sandia Mountains and Vicinity, New Mexico.
Memoir 29, New Mexico Bureau of Mines and Mineral Resources.
Describes the geology of the Sandia Mountains,
which border the Jemez region to the south. Includes some
interesting history of early geologic work in New Mexico.
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.
Kues, Barry S., Lewis, Claudia J. and Lueth,
Virgil W. 2014. A Brief History of Geological Studies in New
Mexico. New Mexico Geological Society Special Publication
12. ISBN 978-1-58546-011-3
A good review of the history of geological
exploration and research in New Mexico.
Kutsche, Paul, and Van Ness, John R. 1981. Cañones:
Values, Crisis and Survival in a Northern New Mexico Village.
An anthropological study of a small northern New
Mexico village. Gives valuable insight into the traditional
Hispanic culture in the area.
Levin, Harold L. 2010. The Earth Through
Time (9th edition). John Wiley & Sons, Inc. ISBN
Probably the best introduction to historical
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.
Nédelec, Anne, and Bouchez, Jean-Luc (tr. Peter Bowden). 2015. Granites: Petrology, Structure, Geological Setting, and Metallogeny. Oxford University Press. ISBN 978-0-19-870561-1.
An excellent if somewhat technical discussion of
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.
Nava, Margaret. March 2008. "Exploring Gilman Canyon." The Sandoval Signpost.
An interesting history of the Gilman Tunnels.
Phys.org. 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.
Baars, D.L. 1974. "Permian
rocks of north-central New Mexico" . New Mexico Geological
Society Annual Fall Field Conference Guidebook Series. p. 167-169. Retrieved 5 June 2019.
A nice summary of the Permian formations of the Jemez region.
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.
Krainer, Karl, and Lucas, Spencer G. 2005
"Lithofacies of the Pennsylvanian Osha Canyon Formation at the
Jemez Mountains, New Mexico" New Mexico Geological
Society 56th Annual Field Conference. pp. 135-144.
The best recent paper on the Osha Canyon Formation.
Lucas, Spencer G; Krainer, Karl; Chaney, Dan S.; Dimichele, William A.; Voigt, Sebastian; Berman, David S.; and Henrici, Amy C. 2013. "The Lower Permian Abo Formation in Central New Mexico." Carboniferous-Permian Transition in Central New Mexico. New Mexico Museum of Natural History and Science, Bulletin 59.
The best recent paper on the Abo Formation.
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.
Whitmeyer, Steven J., and Karlstrom, Karl E.
2007 "Tectonic model for the Proterozoic growth of North America."
Geosphere 3:4, pp. 220-259
The best summary of the Precambrian assembly of
North America I've read.
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
Woodward, Lee A., McLelland, Douglas, and
Husler, J.W. 1977. "Precambrian Rocks of the Northern Part of the
Nacimiento Uplift, New Mexico" New Mexico Geological Society
Guidebook, 28th Field Conference.
Rather dated, but describes the Precambrian
rocks of the northern Sierra Nacimiento in some detail, with a
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, the papers are aimed at professional or serious amateur geologists.
Baldridge, W. Scott; Dickerson, Patricia Wood; Riecker, Robert E. and Zidek, Jiri (ed.) October 1984. "Rio Grande Rift: Northern New Mexico" New Mexico Geological Society 35th Annual Field Conference
Road logs for Picuris Range; Amalia; and the Rio
Grande Gorge and Ojo Caliente.
Bauer, 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. Though slightly out of date, this is one
of the finest of the conference reports, with some of the best
Koning, Daniel J., Karlstrom, Karl E., Kelley, Shari A., and Lueth, Virgil W. (ed) September 2011. "Geology of the Tusas Mountains and Ojo Caliente." New Mexico Geological Society 62nd Annual Field Conference
Road logs for the Ojo Caliente and Tusas
Mountains, including many hiking trip options.
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.
National Geologic Map Database
A few quadrangles of the Jemez area, particularly in the westernmost Jemez, are not yet part of the Open-File site. These can often be found in the National Geologic Map Database, although you first need to know the quadrangle name (which can be obtained from the Open-File site index map.)
Hayden, F.V. 1869. United States Geological
Survey of Colorado and New Mexico.
The report of the Hayden Survey of the Front
Range from Denver to Santa Fe. Hayden was a careful and skilled
observer and the report makes excellent reading.
Herrick, C.L. 1900. "Report of a
Geological Reconnoissance [sic] in western Soccorro and Valencia
Counties, New Mexico." American Geologist 25:6
An interesting report on field work during the
transition from classical geological surveys to modern detailed
studies. C.L.Herrick was an early president of the University of
Wheeler, George M. and others. 1875-1889. Report
upon United States geographical surveys west of the one
The report of the Wheeler surveys. Not nearly as
readable as the Hayden report, with much bureaucratese; but
includes the first good (for their day) maps of the Jemez region.
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