Basin and Range
Encyclopedia
The Basin and Range Province is a vast physiographic region
defined by a unique topographic expression. Basin and Range topography
is characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The region covers most of the western United States
, extends into northwestern Mexico
and is mostly desert
, as well as numerous ecoregions. The physiography
of the province is the result of tectonic extension
that began around 17 Ma (million years ago) in Early Miocene
time.
Clarence Dutton
famously compared the many narrow parallel mountain ranges that distinguish the unique topography of the Basin and Range to an "army of caterpillars marching toward Mexico", which is a helpful way to visualize the overall appearance of the region. The Basin and Range province should not be confused with The Great Basin
, which is a sub-section of the greater Basin and Range physiographic region defined by its unique hydrological characteristics (internal drainage).
. In the United States
, it is bordered on the west by the eastern fault scarp
of the Sierra Nevada mountain range and spans over 500 miles (804.7 km) to its eastern border marked by the Wasatch Fault
, the Colorado Plateau
and the Rio Grande Rift
. The Basin and Range Province extends north to the Columbia Plateau
and south as far as the Trans-Mexican Volcanic Belt
in Mexico
, though the southern boundaries of the Basin and Range are debated. Evidence suggests that the less-recognized southern portion of the Basin and Range Province is bounded on the east by the Laramide
Thrust Front of the Sierra Madre Oriental
and on the west by the Gulf of California
and Baja Peninsula with notably less faulting apparent in the Sierra Madre Occidental
in the center of the southernmost Basin and Range Province. In the adjacent figure the province includes the Great Basin
section (22a, including part of the Mojave Desert), the Sonoran Desert
(22b) & Salton Trough (22c) sections, the Mexican Altiplano (22d), and Sacramento section (22e).
, which is composed of crust
and upper mantle
. Extensional environments like the Basin and Range are characterized by listric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing a horst and graben
geometry, where horst refers to the upthrown fault block and graben to the down dropped fault block.
The average crustal thickness of the Basin and Range Province is approximately 30 – 35 km and is comparable to extended continental crust
around the world. The crust in conjunction with the upper mantle comprises the lithosphere
. The base of the lithosphere beneath the Basin and Range is estimated to be about 60 – 70 km. Opinions vary regarding the total extension of the region, however the median estimate is about 100% total lateral extension. Total lateral displacement in the Basin and Range varies from 60 – 300 km since the onset of extension in the Early Miocene
with the southern portion of the province representing a greater degree of displacement than the north. Evidence exists to suggest that extension initially began in the southern Basin and Range and propagated north over time.
of the Farallon Plate
under the west coast of the North American continental plate which stimulated the thickening of the crust. Most of the pertinent tectonic plate movement associated with the Basin and Range Province occurred in Neogene
time and continues to the present. By Early Miocene
time, much of the Farallon Plate had been consumed, and the seafloor spreading ridge that separated the Farallon Plate from the Pacific Plate
(East Pacific Rise
) approached North America. In the Middle Miocene
, the East Pacific Rise was subducted beneath North America ending subduction
along this part of the Pacific margin; however, the Farallon Plate continued to subduct into the mantle
. The movement at this boundary divided the East Pacific Rise and spawned the San Andreas
transform fault
, generating an oblique strike-slip component. Today, the Pacific Plate moves north-westward relative to North America, a configuration which has given rise to increased shearing along the continental margin
.
The tectonic activity responsible for the extension in the Basin and Range is a complex and controversial issue among the geoscience community. The most accepted hypothesis suggests that crustal shearing associated with the San Andreas Fault
caused spontaneous extensional faulting similar to that seen in the Great Basin. However, plate movement alone does not account for the high elevation of the Basin and Range region. The western United States is a region of high heat flow
which lowers the density of the lithosphere and stimulates isostatic
uplift as a consequence. Lithospheric regions characterized by elevated heat flow are weak and extensional deformation
can occur over a broad region. Basin and Range extension is therefore thought to be unrelated to the kind of extension produced by mantle upwelling
which may cause narrow rift zones, such as the Afar Triple Junction
. Geologic processes that elevate heat flow are varied, however some researchers suggest that heat generated at a subduction zone is transferred to the overriding plate as subduction proceeds. Fluids along fault zones then transfer heat vertically through the crust. This model has led to increasing interest in geothermal systems
in the Basin and Range, and requires consideration of the continued influence of the fully subducted Farallon plate in the extension responsible for the Basin and Range Province.
(MCC), an idea that was first developed based on studies in the Basin and Range Province. A metamorphic core complex occurs when lower crust is brought to the surface as a result of extension. MCCs in the Basin and Range were not interpreted as being related to crustal extension until after the 1960s. Since then, similar deformational patterns have been identified in MCCs in the Basin and Range and has led geologists to examine them as a group of related geologic features formed by Cenozoic crustal extension. The study of metamorphic core complexes has provided valuable insight into the extensional processes driving Basin and Range formation.
the Farallon Plate
subduction
-associated compressive forces of the Laramide
, Sevier
and Nevada orogenies ended, plate interactions changed from orthogonal compression to oblique strike-slip, and volcanism in the Basin and Range Province flared up (Mid-Tertiary ignimbrite flare-up
). It is suggested that this plate continued to be underthrust until about 19 Ma, at which time it was completely consumed and volcanic activity ceased, in part. Olivine basalt from the oceanic ridge erupted around 17 Ma and extension began.
petroleum
, the Basin and Range province supplies nearly all the copper
and most of the gold
, silver
, and barite
mined in the United States
.
United States physiographic region
The list of continental United States Physiographic regions identifies the 8 regions, 25 provinces, and 85 sections. The system dates to Nevin Fenneman's paper Physiographic Subdivision of the United States, published in 1917...
defined by a unique topographic expression. Basin and Range topography
Basin and Range
The Basin and Range Province is a vast physiographic region defined by a unique topographic expression. Basin and Range topography is characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins...
is characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The region covers most of the western United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, extends into northwestern Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
and is mostly desert
Desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...
, as well as numerous ecoregions. The physiography
Physical geography
Physical geography is one of the two major subfields of geography. Physical geography is that branch of natural science which deals with the study of processes and patterns in the natural environment like the atmosphere, biosphere and geosphere, as opposed to the cultural or built environment, the...
of the province is the result of tectonic extension
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
that began around 17 Ma (million years ago) in Early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
time.
Clarence Dutton
Clarence Dutton
Clarence Edward Dutton was an American geologist and US Army officer. Dutton was born in Wallingford, Connecticut on May 15, 1841...
famously compared the many narrow parallel mountain ranges that distinguish the unique topography of the Basin and Range to an "army of caterpillars marching toward Mexico", which is a helpful way to visualize the overall appearance of the region. The Basin and Range province should not be confused with The Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
, which is a sub-section of the greater Basin and Range physiographic region defined by its unique hydrological characteristics (internal drainage).
Geography
The Basin and Range Province includes much of western North AmericaNorth America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, it is bordered on the west by the eastern fault scarp
Fault scarp
A fault scarp is the topographic expression of faulting attributed to the displacement of the land surface by movement along faults. They are exhibited either by differential movement and subsequent erosion along an old inactive geologic fault , or by a movement on a recent active fault...
of the Sierra Nevada mountain range and spans over 500 miles (804.7 km) to its eastern border marked by the Wasatch Fault
Wasatch Fault
The Wasatch Fault is an earthquake fault located primarily on the western edge of the Wasatch Mountains in the U.S. state of Utah. The fault is 240 miles long, stretching from southern Idaho, through northern Utah, before terminating in central Utah near the town of Fayette...
, the Colorado Plateau
Colorado Plateau
The Colorado Plateau, also called the Colorado Plateau Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 337,000 km2 within western Colorado, northwestern New Mexico,...
and the Rio Grande Rift
Rio Grande Rift
The Rio Grande Rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuahua, Mexico in the south. The rift zone consists of four...
. The Basin and Range Province extends north to the Columbia Plateau
Columbia Plateau
The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbia River...
and south as far as the Trans-Mexican Volcanic Belt
Trans-Mexican volcanic belt
The Trans-Mexican Volcanic Belt also known as the Transvolcanic Belt and locally as the Sierra Nevada , is a volcanic belt that extends 900 km from west to east across central-southern Mexico...
in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
, though the southern boundaries of the Basin and Range are debated. Evidence suggests that the less-recognized southern portion of the Basin and Range Province is bounded on the east by the Laramide
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
Thrust Front of the Sierra Madre Oriental
Sierra Madre Oriental
The Sierra Madre Oriental is a mountain range in northeastern Mexico.-Setting:Spanning the Sierra Madre Oriental runs from Coahuila south through Nuevo León, southwest Tamaulipas, San Luis Potosí, Querétaro, and Hidalgo to northern Puebla, where it joins with the east-west running Eje Volcánico...
and on the west by the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
and Baja Peninsula with notably less faulting apparent in the Sierra Madre Occidental
Sierra Madre Occidental
The Sierra Madre Occidental is a mountain range in western Mexico.-Setting:The range runs north to south, from just south of the Sonora–Arizona border southeast through eastern Sonora, western Chihuahua, Sinaloa, Durango, Zacatecas, Nayarit, Jalisco, Aguascalientes to Guanajuato, where it joins...
in the center of the southernmost Basin and Range Province. In the adjacent figure the province includes the Great Basin
Great Basin
The Great Basin is the largest area of contiguous endorheic watersheds in North America and is noted for its arid conditions and Basin and Range topography that varies from the North American low point at Badwater Basin to the highest point of the contiguous United States, less than away at the...
section (22a, including part of the Mojave Desert), the Sonoran Desert
Sonoran Desert
The Sonoran Desert is a North American desert which straddles part of the United States-Mexico border and covers large parts of the U.S. states of Arizona and California and the northwest Mexican states of Sonora, Baja California, and Baja California Sur. It is one of the largest and hottest...
(22b) & Salton Trough (22c) sections, the Mexican Altiplano (22d), and Sacramento section (22e).
Geology
It is generally accepted that basin and range topography is the result of extension and thinning of the lithosphereLithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
, which is composed of crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
and upper mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. Extensional environments like the Basin and Range are characterized by listric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing a horst and graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
geometry, where horst refers to the upthrown fault block and graben to the down dropped fault block.
The average crustal thickness of the Basin and Range Province is approximately 30 – 35 km and is comparable to extended continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
around the world. The crust in conjunction with the upper mantle comprises the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...
. The base of the lithosphere beneath the Basin and Range is estimated to be about 60 – 70 km. Opinions vary regarding the total extension of the region, however the median estimate is about 100% total lateral extension. Total lateral displacement in the Basin and Range varies from 60 – 300 km since the onset of extension in the Early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
with the southern portion of the province representing a greater degree of displacement than the north. Evidence exists to suggest that extension initially began in the southern Basin and Range and propagated north over time.
Tectonics
The tectonic mechanisms responsible for lithospheric extension in the Basin and Range province are controversial, and several competing hypotheses attempt to explain it. Key events preceding Basin and Range extension in the western United States include a long period of compression due to the subductionSubduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
of the Farallon Plate
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...
under the west coast of the North American continental plate which stimulated the thickening of the crust. Most of the pertinent tectonic plate movement associated with the Basin and Range Province occurred in Neogene
Neogene
The Neogene is a geologic period and system in the International Commission on Stratigraphy Geologic Timescale starting 23.03 ± 0.05 million years ago and ending 2.588 million years ago...
time and continues to the present. By Early Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
time, much of the Farallon Plate had been consumed, and the seafloor spreading ridge that separated the Farallon Plate from the Pacific Plate
Pacific Plate
The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....
(East Pacific Rise
East Pacific Rise
The East Pacific Rise is a mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, and the Antarctic Plate...
) approached North America. In the Middle Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
, the East Pacific Rise was subducted beneath North America ending subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
along this part of the Pacific margin; however, the Farallon Plate continued to subduct into the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
. The movement at this boundary divided the East Pacific Rise and spawned the San Andreas
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
transform fault
Transform fault
A transform fault or transform boundary, also known as conservative plate boundary since these faults neither create nor destroy lithosphere, is a type of fault whose relative motion is predominantly horizontal in either sinistral or dextral direction. Furthermore, transform faults end abruptly...
, generating an oblique strike-slip component. Today, the Pacific Plate moves north-westward relative to North America, a configuration which has given rise to increased shearing along the continental margin
Continental margin
The continental margin is the zone of the ocean floor that separates the thin oceanic crust from thick continental crust. Continental margins constitute about 28% of the oceanic area....
.
The tectonic activity responsible for the extension in the Basin and Range is a complex and controversial issue among the geoscience community. The most accepted hypothesis suggests that crustal shearing associated with the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a continental strike-slip fault that runs a length of roughly through California in the United States. The fault's motion is right-lateral strike-slip...
caused spontaneous extensional faulting similar to that seen in the Great Basin. However, plate movement alone does not account for the high elevation of the Basin and Range region. The western United States is a region of high heat flow
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
which lowers the density of the lithosphere and stimulates isostatic
Isostasy
Isostasy is a term used in geology to refer to the state of gravitational equilibrium between the earth's lithosphere and asthenosphere such that the tectonic plates "float" at an elevation which depends on their thickness and density. This concept is invoked to explain how different topographic...
uplift as a consequence. Lithospheric regions characterized by elevated heat flow are weak and extensional deformation
Deformation
In materials science, deformation is a change in the shape or size of an object due to an applied force or a change in temperature...
can occur over a broad region. Basin and Range extension is therefore thought to be unrelated to the kind of extension produced by mantle upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
which may cause narrow rift zones, such as the Afar Triple Junction
Afar Triple Junction
The Afar Triple Junction is a junction of three tectonic rifts centered in the Afar Depression, informally known as the Afar Triangle, of northeastern Africa. Here, the Red Sea Rift meets the Aden Ridge and the East African Rift...
. Geologic processes that elevate heat flow are varied, however some researchers suggest that heat generated at a subduction zone is transferred to the overriding plate as subduction proceeds. Fluids along fault zones then transfer heat vertically through the crust. This model has led to increasing interest in geothermal systems
Geothermal gradient
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is 25–30°C per km of depth in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied...
in the Basin and Range, and requires consideration of the continued influence of the fully subducted Farallon plate in the extension responsible for the Basin and Range Province.
Metamorphic core complexes
In some localities in the Basin and Range, metamorphic basement is visible at the surface. Some of these are metamorphic core complexMetamorphic core complex
Metamorphic core complexes are exposures of deep crust exhumed in association with largely amagmatic extension. They form, and are exhumed, through relatively fast transport of middle and lower continental crust to the Earth's surface...
(MCC), an idea that was first developed based on studies in the Basin and Range Province. A metamorphic core complex occurs when lower crust is brought to the surface as a result of extension. MCCs in the Basin and Range were not interpreted as being related to crustal extension until after the 1960s. Since then, similar deformational patterns have been identified in MCCs in the Basin and Range and has led geologists to examine them as a group of related geologic features formed by Cenozoic crustal extension. The study of metamorphic core complexes has provided valuable insight into the extensional processes driving Basin and Range formation.
Volcanism
Prior to the Eocene Epoch (55.8 ±0.2 to 33.9 ±0.1 Ma) the convergence rate of the Farallon and North American Plates was fast, the angle of subduction was shallow, and the slab width was huge. During the EoceneEocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
the Farallon Plate
Farallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...
subduction
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
-associated compressive forces of the Laramide
Laramide orogeny
The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...
, Sevier
Sevier orogeny
The Sevier orogeny was a mountain-building event that affected western North America from Canada to the north to Mexico to the south. This orogeny was the result of convergent boundary tectonic activity between approximately 140 million years ago and 50 Ma. The Sevier River area of central Utah...
and Nevada orogenies ended, plate interactions changed from orthogonal compression to oblique strike-slip, and volcanism in the Basin and Range Province flared up (Mid-Tertiary ignimbrite flare-up
Mid-Tertiary ignimbrite flare-up
The Mid-Tertiary ignimbrite flare-up, as the name implies, is a dramatic period of volcanic eruptions in mid-Tertiary time, approximately 25-40 million years ago, centered in the western United States...
). It is suggested that this plate continued to be underthrust until about 19 Ma, at which time it was completely consumed and volcanic activity ceased, in part. Olivine basalt from the oceanic ridge erupted around 17 Ma and extension began.
Mineral resources
In addition to small amounts of NevadaNevada
Nevada is a state in the western, mountain west, and southwestern regions of the United States. With an area of and a population of about 2.7 million, it is the 7th-largest and 35th-most populous state. Over two-thirds of Nevada's people live in the Las Vegas metropolitan area, which contains its...
petroleum
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
, the Basin and Range province supplies nearly all the copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and most of the gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, and barite
Barite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
mined in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
.