A Guide to the Geology of the Flagstaff Area

Flagstaff section

The Arizona Geological Survey (AZGS) has just released a 53-page illustrated booklet about the Flagstaff area. You can download the booklet here:


According to Michael Conway, Chief of the AZGS, Geologic Extension Service, “This 53-page, Down-to-Earth booklet includes pictures, illustrations and jargon-free text to open the geology of northern Arizona to those who otherwise lack a geology background.”

Sunset crater



General geology as described in the booklet:

The Flagstaff area is on the southern margin of the Colorado Plateau, a 130,000-square-rnile geologic province of vast plains, high mesas and buttes, deep canyons, volcanic fields and isolated mountain clusters. The landscape of this southern Plateau margin is dominated by the young San Francisco volcanic field and the underlying limestone-capped plateau.

The oldest known rocks underlying this part of the Plateau are 1.7-1.8 billion-year-old (Precambrian) granite and schist. These rocks, which make up the original crust of North America, were beveled by erosion and offset by faults that moved again during younger geologic periods.

Horizontal layers of sandstones, limestones, shales, and siltstones of the Paleozoic Era (544 million to 248 million years ago) were deposited on the ancient Precambrian rocks. These younger units, named in ascending order, the Tapeats Sandstone, Bright Angel Shale and Muav Limestone, Martin Formation, Redwall Limestone, Supai Group, Coconino Sandstone, and the Toroweap and Kaibab Formations, were deposited when this part of the continent was a shallow sea floor, a muddy tidal zone, a coastal plain crossed by silt-laden rivers, or a vast desert covered by sand dunes. The Coconino Sandstone and the Toroweap and Kaibab Formations are the only Paleozoic rocks exposed in the area covered by this guidebook.

More rock layers were laid down during the Mesozoic Era (248 to 65 million years ago). The Moenkopi Formation is the only Mesozoic rock that covers large parts of the Flagstaff area. Younger layers of sediment accumulated, but were later eroded away. The total thickness of sedimentary rock deposited during the Paleozoic and Mesozoic Eras may have reached 10,000 ft (3050 m), but much of this was stripped off by erosion.

Beginning about 65 to 75 million years ago, western North America was subjected to intense horizontal compression during an episode of mountain building called the Laramide Orogeny. The Rocky Mountains, for example, were formed during this period. This stress reactivated old faults and created new faults and folds. Vertical movement along these faults elevated the Precambrian basement rocks and the thick sequence of younger sedimentary layers thousands of feet, eventually forming the Colorado Plateau. The exact timing and causes of the uplift are still debated by geologists.

In the Flagstaff area movement along faults deformed once-horizontal layers into long folds, such as the Black Point monocline north of Wupatki National Monument. The uplift also caused formerly sluggish rivers to cut deep canyons into the younger sedimentary layers.

Beginning about 25 million years ago, the crustal rocks of western North America were stretched, thinned, and broken along steep faults. Movement occurred again along the old faults of the Flagstaff area. About 6 million years ago, molten rock (called magma inside the earth and lava when it erupts) migrated upward along some of these fractures and flowed onto the land surface as lava flows. As eruptions continued during the period 3 million to 1000 years ago lava of the San Francisco volcanic field poured onto, exploded through, or was injected into Paleozoic and Mesozoic sedimentary layers of the plateau.

Finally, San Francisco Mountain, the high stratovolcano that towers over the volcanic field, was scoured by glacial ice several times during the last 1.8 million years. Today, running water is cutting into and wearing down this southern flank of the Colorado Plateau.


The geologic features described and illustrated in the booklet include:

San Francisco Volcanic Field

Lava Dome: Mount Elden

Stratovolcano: San Francisco Mountain

Glacial features: Cirques, Moraines, and U-shaped Valley

Young Cinder Cones and Lava Flows: Sunset and SP Craters

Squeeze-up: Bonito Flow

Cinder Dunes and Ventifacts

Moenkopi Formation: Wupatki National Monument

Blowhole: Wupatki National Monument

Fault-aligned Cinder Cones: Wupatki National Monument

Sinkhole: Wupatki National Monument

Graben: Wupatki National Monument

Folding: Black Point Monocline

Entrenched Meanders: Walnut Canyon National Monument

Kaibab Formation: Walnut Canyon National Monument

Coconino Sandstone: Walnut Canyon National Monument

Stream Displaced by a Lava Flow: Grand Falls

Meteor Impact Crater: Barringer Meteor Crater

Laccolith: White Horse Hills (Marble Mountain)

Anatomy of a Cinder Cone: Red Mountain

Tafoni: Red Mountain

Hoodoos (Demoiselles): Red Mountain

Lava Tube: Lava River Cave

Interactive Recreation & Cultural map of Arizona from AZGS

The Arizona Geological Survey has produced a series of interactive maps about Arizona. The newest in this series is the Recreation & Cultural Sites map which showcases the vibrant cultural diversity and outdoor attractions found throughout Arizona. “Map locations include national parks and monuments, Forest Service and Bureau of Land Management lands, federal recreational areas, state parks, Tribal Lands, and cultural and historic sites.” “The online map is hosted at the Arizona Experience website, which commemorates 100 years of statehood and offers an online portal to the people, places, and events that defined Arizona’s past and are shaping its future.”

Arizona recreation sitesThe graphic on the left shows the map, which is an interesting one to explore. If you mouse over one of the icons, it gives you the name of the feature. Clicking on the icon brings up a box with a website link and sometimes a phone number. Click on the website link for a description of the feature and directions to get there.

On the upper right corner of the Recreation & Cultural Sites map is a box “View other maps” which features a drop-down menu. This leads to many more interesting maps.

For instance, if you select “Volcano Map” it brings up a map showing the volcanic fields in Arizona. Clicking on any one of those shows photos of the area and a short explanation. Another interesting map is “Iconic Landscapes.” Clicking on those will bring up photos or videos of the area. In all, there are 17 different maps to explore. (Note: some of the maps initially appear blank, but there is a time-line slider at the bottom. Use it.)

Other interesting products from AZGS upon which I reported include:

Earth Fissures in Arizona

Origin of the Grand Canyon

Sedona’s Sinkholes

A guide to the geology of the Sedona & Oak Creek Canyon area of Arizona

Arizona earthquakes, 1852-2011, a video time line

Precariously Balanced Rocks and earthquakes

Earthquake hazard near Flagstaff assessed, Video

Stephen Hawking, the big bang, invasion of aliens from outer space, the end of the world, and what’s beneath America, review of a Discovery Channel DVD

The Discovery Channel’s Curiosity series of DVD programs can be controversial, entertaining, and enlightening.

The DVD I watched had five programs (total length 198 minutes):

Did God Create the Universe?

This program presents Stephen Hawking’s view of the universe. It follows the history of our understanding of natural phenomena, such as eclipses, from religious superstition to a physical explanation. Hawking believes our universe started with a big bang, it sprang from nothing. Before the big bang there was nothing, including no time and no cause and effect. For Hawking, there is no God, no heaven, no afterlife.

In a separate program on the DVD, a panel of scientists and theologians discuss Hawking’s view. They discuss the concept of multiple universes, and conclude that science can neither prove nor disprove the existence of a creator.

Alien Invasion: Are We Ready?

This program, narrated by actress Michelle Rodriguez, features scientists and military strategists, first discussing the probabilities that some intelligent alien species exist, then speculating on how the invaders would go about getting rid of humans. If these people were the aliens, they would first disable our communications and other electronics with an electro-magnetic pulse (EMP), then kill many people in coastal areas by causing very large tsunamis. They would finish us off with biological warfare. It’s the stuff of good science fiction stories.

What’s Beneath America?

This program, narrated by Martin Sheen, is about the geology and natural resources of North America. It discusses plate tectonics, mountain building and how some of our large deposits of gold, oil, coal,and iron formed. And they got the geology right.

How the World Will End

The title of this program, narrated by Samuel L. Jackson, is a bit of hype, they destroy just North America, not the world. The program speculates on five types of natural disasters.

1) California is destroyed by an ARkStorm (spelled correctly, and yes, I never heard that term before.) Basically a mega-storm with very strong hurricane-force winds, lasts about a month and dumps 10 feet of water on California, causing flooding and land slides. This kind of storm is purely speculative. See an explanation from the U.S. Geologic Survey here.

2) Asteroid impact. This scenario features Meteor Crater (aka Barringer Crater) in Arizona as an example of a “small” impact. The program explains the consequences of a really big strike. The program claims that children today have a 1 in 20 chance of witnessing a really big strike. They fail to mention how they came up with that number.

3) Mega-earthquake in mid-west along the New Madrid fault on the Mississippi River. This program recounts the earthquakes that occurred between December 1811 and February, 1812, all with strengths estimated to be between 7.8 to 8.1. These earthquakes are the strongest to hit the eastern U.S. The program goes on to speculate what could happen if an even bigger one hits. The program puts the odds of this happening within 50 years at 1 in 10. Again they don’t explain how the number was derived.

4) The eastern seaboard of the U.S. is destroyed by a giant tsunami precipitated by a major volcanic eruption in La Palma, Canary Islands. Odds of happening 1 in 1,000, same caveat.

5) Yellowstone super volcano explodes. I wrote about this one last year (see The Yellowstone Super Volcano). This volcano tends to have a major eruption every 600,000 years on average. It has been 640,000 years since the last major eruption. That one covered about half of the U.S. with volcanic ash.

These programs are interesting and well-made. They show very good animations, but tend to overuse stock footage of disaster scenes. The programs are well-worth watching.

The DVD is available from the Discovery Channel here.

For more on the nature of the universe and religion versus science, see my reviews of two other works:

The Unobservable Universe by Scott Tyson

Religion versus Science by Ron Frost

See all my book and DVD reviews here.

The Cinder Cone Caper

In my role as an exploration geologist, I examined many properties submitted to my employer in the hope that they would be of interest for development.  Some of those properties turned out to be scams, especially gold and platinum prospects.

I remember one in particular.  A small company claimed it had discovered gold in one of the basaltic cinder cones near Flagstaff, Arizona.  I was assigned to go take a look.

spCinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity.  Cinder cones are probably one of the last places you would expect to find gold.

When I arrived on the property, I spoke to the owner and asked him where he had found gold.  I dutifully collected samples there and at other spots on the property.  There was no visible sign of gold, but that’s not unusual.

The first red flag appeared when the owner offered to have his on-site assayer analyze the samples for me.  So, just to humor him, I split some of the samples and offered a portion for assay by the owner’s chemist.  Not surprisingly, the chemist reported gold in all the samples.

When I got back to Tucson, I again split the samples and gave a portion to our in-house assayer and sent the rest to an independent assayer.  Again not surprisingly neither our chemist nor the independent assayer found any gold.

Conclusion: the submitter’s chemist was either incompetent, or a crook.  He could have just faked the numbers, but there are many ways to “salt” the sample.  The most common are dropping a gold chloride solution on the sample before assay or adding a little gold dust to the sample.

One of the most blatant ways of salting mine workings is the shotgun method.  I once examined an adit that had blebs of gold on its walls, real gold.  The pattern suggested that gold dust was put into a shotgun shell and fired at the walls.  And that owner could not explain why the original miners did not notice and recover the gold along the adit walls themselves.

Caveat emptor.

See also:

Yellowstone Super Volcano

Young Volcanic Fields of Arizona

The dual nature of Hawaiian volcanos

Old mines of the Tucson Mountains

Surprising Structure of the Copper Deposits near Green Valley, Arizona

British Balloon to spew sulfur, another wacky geoengineering scheme

In order to forestall dread global warming, many, usually wacky, schemes have been proposed. The latest is from a group of British academics who want to simulate the cooling emissions of volcanoes by tethering a balloon about 1 kilometer into the sky and pumping up and dispersing a chemical mixture of sulfates to reflect sunlight. Of course, the EPA has for years been trying to reduce sulfur emissions from coal plants and automobile exhausts.


The academics have garnered a £1.6m ($2.5 million) British government grant to build a scale model of their device.

“The whole weight of this thing is going to be a few hundred tonnes. That’s the weight of several double-decker buses. So imagine how big a helium balloon do you need to hold several double-decker buses – a big balloon. We’re looking at a balloon which is possibly 100-200m in diameter. It’s about the same size as Wembley stadium,” said the Oxford engineering lecturer Hugh Hunt in an interview earlier this year. This hose would be just like a garden hose, 20km long and we pump stuff up the pipe.”

Read the whole story from the British newspaper, the Guardian.

For more wacky geoengineering schemes, see my post:

Geoengineering Wacky Schemes to Control Climate

Volcanoes may have greater influence on climate than previously thought

A newly published French study of last year’s eruption of the Eyjafjallajökull Volcano in Iceland suggests that models have underestimated the aerosol formation and hence cooling effect of volcanic eruptions “by 7 to 8 orders of magnitude.”

The Abstract reads:

Volcanic eruptions caused major weather and climatic changes on timescales ranging from hours to centuries in the past. Volcanic particles are injected in the atmosphere both as primary particles rapidly deposited due to their large sizes on time scales of minutes to a few weeks in the troposphere, and secondary particles mainly derived from the oxidation of sulfur dioxide. These particles are responsible for the atmospheric cooling observed at both regional and global scales following large volcanic eruptions. However, large condensational sinks due to preexisting particles within the plume, and unknown nucleation mechanisms under these circumstances make the assumption of new secondary particle formation still uncertain because the phenomenon has never been observed in a volcanic plume. In this work, we report the first observation of nucleation and new secondary particle formation events in a volcanic plume. These measurements were performed at the puy de Dôme atmospheric research station in central France during the Eyjafjallajokull volcano eruption in Spring 2010. We show that the nucleation is indeed linked to exceptionally high concentrations of sulfuric acid and present an unusual high particle formation rate. In addition we demonstrate that the binary H2SO4 – H2O nucleation scheme, as it is usually considered in modeling studies, underestimates by 7 to 8 orders of magnitude the observed particle formation rate and, therefore, should not be applied in tropospheric conditions. These results may help to revisit all past simulations of the impact of volcanic eruptions on climate.

Besides primary ash, the researchers say that sulfur dioxide, which oxidizes to sulfuric acid, can act as cloud-forming nuclei that can change the precipitation over a region. The clouds would also partially reflect solar irradiance and therefore contribute to cooling.

UPDATE: New NASA paper says volcanoes primarily responsible for increased SO2:

Recently, the trend, based on ground-based lidar measurements, has been tentatively attributed to an increase of SO(2) entering the stratosphere associated with coal burning in Southeast Asia. However, we demonstrate with these satellite measurements that the observed trend is mainly driven by a series of moderate but increasingly intense volcanic eruptions primarily at tropical latitudes.

See also:

Iceland volcano and its effect on life Photos

Icelandic Volcanoes Geologic Setting

Katla volcano in Iceland may be priming to erupt

Yellowstone Super Volcano

Carbon Dioxide and the Greenhouse Effect

Young Volcanic Fields of Arizona

The volcanic history of Arizona spans more than one billion years. There are seven volcanic fields which have erupted within the last four million years. One of them entombs pottery of local inhabitants of the time. One dammed the Colorado River in the Grand Canyon several times. Another produces world-class gemstones, and another is associated with the 1887 earthquake which shook Southern Arizona and Northern Sonora.

The map below, from the Arizona Geological Survey shows the location of these recent and older volcanic fields. This article discusses those shown in red. For more information on rock names, see the Igneous rock naming page to the left.


Uinkaret Volcanic Field (UI on the map)

The Uinkaret volcanic field lies on the north rim of the Grand Canyon in northwestern Arizona. Four major eruptions of basalt from this field flowed into the Grand Canyon and dammed the Colorado River between 725,000 and 475,000 years ago, between 400,000 and 275,000 years ago, between 225,000 and 150,000 years ago, and between150,000 and 75,000 years ago. (Source).

Dating was done by radiometric methods using the ratio Argon-40 to Argon-39.

Some of the eruptions flowed down the canyon as much as 75 miles. Some of the dams reached more than 700 feet high. When the river eventually over-topped and broke the dams there were great floods. There is no consensus on how long the dams lasted. Some think they may have lasted up to 20,000 years and formed large reservoirs.

According to the Smithsonian Institution, “One lava flow, from Little Springs, south of Pliocene Mount Trumbull, has a cosmogenic helium age of 1300 +/- 500 years BP. Pottery sherds dated at between 1050 and 1200 AD were found within the Little Springs lava flow, which occurred about the same time as the Sunset Crater eruption in the San Francisco volcanic field to the SE.”

San Francisco Volcanic Field (FL near Flagstaff)

SFpeakThe San Francisco volcanic field near Flagstaff has been active for about 6 million years. The oldest eruptions occurred near the town of Williams. Sunset Crater, a cinder cone east of Flagstaff, is less than 1,000 years old.

spThe US Geological Survey says, “It is likely that eruptions will occur again in the San Francisco Volcanic Field. With an average interval of several thousand years between past periods of volcanic activity, it is impossible to forecast when the next eruption will occur. U.S. Geological Survey (USGS) scientists believe that the most probable sites of future eruptions are in the eastern part of the field and that the eruptions are likely to be small. These future eruptions may provide spectacular volcanic displays but should pose little hazard because of their small size and the relative remoteness of the area.”

The San Francisco mountains, which include Humphreys Peak, Arizona’s highest mountain at 12,633 feet, is a stratovolcano which erupted between 1 million and 400,000 years ago. This volcanic mountain consists of interspersed layers of andesitic lava, cinders, ash, and volcanic mudflows.

The younger volcanic cones and their flows are basaltic, such as SP crater (71,000 years old) and Sunset Crater. According to the USGS,

“Most of the more than 600 volcanoes in the San Francisco Volcanic Field are basalt cinder cones. Basalt has the lowest viscosity of all common magmas. Cinder cones are relatively small, usually less than 1,000 feet tall, and form within months to years. They are built when gas-charged frothy blobs of basalt magma are erupted as an upward spray, or lava fountain. During flight, these lava blobs cool and fall back to the ground as dark volcanic rock containing cavities created by trapped gas bubbles. If small, these fragments of rock are called “cinders” and, if larger, “bombs.” As the fragments accumulate, they build a cone-shaped hill. Once sufficient gas pressure has been released from the supply of magma, lava oozes quietly out to form a lava flow. This lava typically squeezes out from the base of the cone and tends to flow away for a substantial distance because of its low viscosity. SP Crater, 25 miles north of Flagstaff, is an excellent example of a cinder cone and its associated lava flow.”

SunsetCraterThe volcanic field also contains lava domes. These are dome-shaped, steep-sided piles of viscous dacite and rhyolite lava. These can expand like balloons when lava wells up inside the dome, or the dome can break and develop by buildup of successive layers.

A side story, the cinder cone gold scam: Among my duties as an exploration geologist was examining properties submitted to our company by third parties. One submitted property was a gold prospect in one of the basaltic cinder cones. It is extremely unlikely that gold occurs in cinder cones, but a friend of one of my company’s directors was interested, so I looked.

Upon arriving at the property and meeting the owner, I asked where he thought the gold was. I collected samples at those places, and others. The property owner just happened to have an assay lab on the property and offered to analyze the samples I collected. Just to see what would happen, I gave him a few handfuls of some of the samples. Miraculously, his lab found gold in the samples. When I got back to Tucson, I had the remaining material assayed by a reputable laboratory which failed to find any gold. Now, that result is a truly amazing “nugget effect.” Conclusion: the assayer was either incompetent or crooked.

Springerville Volcanic Field (SP)

Most of the basaltic flows in the Springerville volcanic field are between 2.1 million and 300,000 years old. Some older flows, 6- to 8 million year old flows occur to the south. There are no stratovolcanoes here, only about 400 cinder cones and their associated flows.

San Carlos Volcanic Field (SC)

The San Carlos volcanic field is on the San Carlos Indian Reservation south of Springerville. The basalt and peridotite lavas erupt between 7 million and 500,000 years ago. Peridotite, from whence the term peridot comes, is a coarse igneous rock consisting mainly of the minerals pyroxene and olivine. Peridot is gem-quality olivine, a magnesium-iron silicate. It is claimed that 90% of the world’s peridot comes from Peridot Mesa in this volcanic field. Peridot Mesa is a diatreme (a breccia-filled vent formed by gaseous explosion) and was followed by lava flows.

San Bernardino Volcanic Field (SB)

The San Bernardino volcanic field in the southeast corner of Arizona contains about 130 vents and cinder cones, maars, and flows of olivine basalt. These were erupted between 1 million and 27,000 years ago. An earthquake in 1887 was centered just south of the border in the San Bernardino Valley.

Maars are craters created by a steam blast. Paramore Crater is the largest of these, about 1 km by 1.5km and about 60 m deep.

In the northern extension of the San Bernardino Valley, the San Simon Valley, there are still many hot springs.

Sentinel Volcanic Field (SE)

The Sentinel volcanic field, west of Gila Bend, AZ, contains basalt flows 2- to 6 million years old. This field may be related to the larger Pinacate field to the south.

Pinacate Volcanic Field (PI)

Most of the Pinacate volcanic field is in Sonora, just south of the Arizona border. Pinacate contains eleven giant-maar craters and hundreds of cinder cones. (See photos here and here.) The field has been active for 2 or 3 million years and eruptions have occurred as recently as 13,000 years ago. (Source.)


A map showing young volcanic fields in Arizona and New Mexico may be found here. This is a PDF file. It shows a remarkable straight line of volcanic fields from San Carlos in Arizona through Taos, NM. It also gives ages of the volcanic units. (You have to enlarge the view.) The map was prepared as part of an assessment for geothermal energy.

Some Fallout from Icelandic Volcanoes

A story on the Scientific American website proclaims “Ice cap thaw may awaken Icelandic volcanoes.” “A thaw of Iceland’s ice caps in coming decades caused by climate change may trigger more volcanic eruptions by removing a vast weight and freeing magma from deep below ground, scientists said on Friday.”

This contention is based on the fact that rocks melt at a lower temperature under lower pressure. The question is, will removal of some or all of the ice cap in Iceland result in a pressure load decrease that will make a significant difference in melting temperature and therefore produce more magma?

The relationship between pressure and melting temperature of basaltic lava, the type in Iceland, was estimated by the Carnegie Geophysical Laboratory to be:

Tm = 1391.5 + 0.01297 * P

Tm is the melting temperature in degrees Centigrade, and P is the pressure in bars (or atmospheres, 1 atmosphere = 1.01 bars or about 14.7 pounds per square inch at sea level). We need be concerned only with the last term of that formula which says that a one atmosphere change in pressure results in a 0.013º C change in melting point (rounding the number to two significant figures).

The pressure of one atmosphere is about equivalent to the pressure or weight of 10 meters of ice, so one meter of ice would result in a temperature change of about 0.0013º C. The thickest ice in Iceland is about 500 meters. Complete removal of that ice would lower the melting point of rock about 0.65 ºC, not very significant considering the base melting point is nearly 1400 ºC. The earth’s normal geothermal gradient (the change in temperature with depth), is about 20- to 30º C around most of the planet, and about 40º C at tectonically active spots like Iceland. The removal of 500 meters of ice giving a temperature difference of 0.65 ºC is the equivalent of a depth difference of about 16 meters. Big deal. It is very unlikely that this small pressure difference would stimulate additional volcanic activity.

Ash is formed when magma is rapidly cooled and fractured by steam. Removal of the surficial water source could result in less ash formation and make the Icelandic volcanoes behave more like the Hawaiian volcanoes.


Change of Melting Point of Diopside with Pressure, by Hatten S. Yoder Jr., Geophysical Laboratory, Carnegie Institution of Washington, © 1952 The University of Chicago Press.

Another type of fallout:

The banning of air travel in Europe, which is causing economic chaos, was based on predicted damage to aircraft from ash clouds. The ban was based on an advisory from the British Meteorological Office’s London Volcanic Ash Advisory Centre. That organization based their advisory on a computer program originally developed to monitor nuclear fallout. The computer model predicted dangerous concentrations of ash at various places around Europe. The Met office failed to send up any weather balloons to check actual conditions. Because there is a valid possibility that flying through an ash cloud can damage aircraft, the major airlines made test flights in the last few days to check conditions and found no danger. This is another example showing that relying on computer modeling, rather than real data, can cause unnecessary economic loss and concern.

Geologic Setting of Icelandic Volcanoes

The ash spewing from the Eyjafjallajokull volcano in Iceland is disrupting air traffic throughout Europe. Volcanic ash is essentially sharp-edged glass with particles ranging from sand-sized to microscopic. These particles can wreak havoc with jet engines and your lungs. The last time this volcano erupted, the eruption lasted more than a year, from December 1821 until January 1823.

Iceland sits astride the Mid-Atlantic Ridge, a boundary between two tectonic plates, where new crust is being formed by volcanic eruptions as the plates diverge, i.e., they are moving away from each other. Movement on this structure over the last 180 million years or so has separated Europe from North America, and Africa from South America. The island of Iceland is engaged in a geologic race between the spreading motion which is ripping the island apart, and the volcanoes which are building the island up.

Other islands of the Atlantic Ocean created by the volcanism of the Mid-Atlantic Ridge are The Azores, Bermuda, Madeira, The Canary Islands, Ascension, St. Helena, and Tristan da Cunha. But, you probably haven’t heard about any volcanoes erupting in Bermuda because that island group lacks one other geologic phenomenon: the “hot spot.” Iceland also sits above a mantle plume or “hot spot” where magma from deep in the mantle forces its way to the surface. The Hawaiian Islands were formed, and are being formed, by such a hot spot. In Hawaii, the westward movement of the Pacific plate passes over this hot spot and eruptions produce new islands. Iceland, however, is not moving in such a manner.


The general geology of Iceland is shown on the map below from the Nordic Volcanology Institute.


The pink area on the map represents the rifts along the Mid-Atlantic Ridge where the crust is separating and the volcanoes are most active. Of Iceland’s 100 most active volcanoes, 25 have erupted in recent history, and 35 volcanoes have erupted in the last 10,000 years. Eleven volcanoes have erupted between 1900 and 1998. Most of the eruptions were from fissures or shield volcanoes and involve the effusion of basaltic lava.

The 1783 to 1784 eruption at Laki fissure and the adjoining Grímsvötn volcano was the largest single historic eruption of basaltic lava (12 cubic km). Benjamin Franklin, who at the time was serving as ambassador to France remarked on this eruption. The ash cloud caused the “year without summer.” That eruption of basalt lava and clouds of poisonous hydrofluoric acid/sulfur-dioxide compounds killed over 50% of Iceland’s livestock population, leading to famine which killed approximately 25% of the population. It remains to be seen if the current eruption will be as long lasting. So far, the ash cloud from the current eruption has risen to 30,000 feet which affects airline travel, but it has stayed below the stratosphere, so the climate effects are not likely to be as drastic as those in 1783.

The Boston Globe has a series of photographs of the volcano and flooding it caused. Boston Globe photos: http://tinyurl.com/y6vslk4

Arizona Geological History Chapter 5: Jurassic Time

Jurassic Time, the age of dinosaurs, was from 241- to 145 million years ago. See geologic time chart. The super-continent of Pangea was breaking up and the Atlantic Ocean was born along a spreading axis.

Paleomap 152

During the Jurassic there were no Rocky Mountains. The ancestral Rockies of the Paleozoic had eroded away and the current Rocky Mountains were yet to be born. Northern Arizona, and all of what is now the Colorado Plateau was a featureless desert of blowing sand, much like the Sahara Desert today. These sands became the Wingate Sandstone, Kayenta formation, Navajo Sandstone, and Entrada Sandstone that form the arches and cliffs of parks in southern Utah such as Arches National Monument, and Zion National Park. The cartoon below shows the paleogeography.

The real action was in southern Arizona. Magmatism begun in the Triassic Periodcontinued and moved inland, so that southern Arizona and California contained a magmatic arch and subduction zone with development of many volcanoes on the western edge of the continent. (See the hatched line in the global map, first figure above.) This subduction zone still exists along the west coast of North and South America. The figure below shows a cross-section of a subduction zone, magmatic arc, and spreading axis. To be in proper orientation for our purposes, consider that you are looking toward the south, with the Pacific Ocean on the right and the incipient Atlantic Ocean labeled “back-arc basin” in the figure.

Subduction zone 1

In Jurassic time, southern Arizona was a volcanic field, and some of the volcanoes collapsed into calderas. Remnants of these calderas are recognized in the dragoon mountains near Courtland-Gleeson, in Tombstone, at the southern end of the Huachuca mountains, in the Canelo Hills, and in the Santa Rita mountains. Gold, silver, and copper is associated with the subvolcanic intrusions of these calderas. Many of the historic mining camps of southern Arizona were founded on these deposits. The Juniper Flat granite just north of Bisbee has been dated at about 180 million years and the copper deposit at Bisbee is presumed to be about the same age.**

The Jurassic was also a time of other structural complications. According to Tosdal et al. “In southeastern Arizona, movement along northwest-striking fault systems broke the area into elongate structural blocks, forming topographic highs and basins in which terrigenous clastic* and volcanic rocks accumulated.” The Canelo Hills volcanics are some of the rocks deposited at this time. Tosdal continues: ” In northwestern Sonora, southern Arizona, and southeastern California, a system of sinistral strike-slip faults, The Mojave-Sonora megashear, cut obliquely across the magmatic arc, as much as 800 km of aggregate displacement along these faults may have occurred in Jurassic time.”

At the end of Jurassic time, and extending into the following Cretaceous period, the style of tectonism changed from strike-slip shearing to normal faulting (one side down relative to the other side). This formed basins which received sediments and volcanic deposits, and eventually formed the basin which held the Cretaceous-age Bisbee Sea.

Glance Conglomerate, up to 2,000 meters thick, is the youngest Jurassic deposit in southern Arizona and forms the base of the Cretaceous Bisbee group of rocks. The Glance represents high-energy deposition of alluvial fans by debris flows and rivers along a mountain front.

For most of Jurassic time, global temperatures are estimated to have been 15 -to 20 F warmer than today, the same as in the preceding Triassic Period. Most of the land area was hot and steamy, but in southwestern North America, it was arid. Plant life consisted mainly of conifers and palm-like cycadeoids. Flowering plants had not yet evolved. On land, this was the age of dinosaurs, including flying reptiles. There were some primitive mammals, and abundant insects.

Mid-Jurassic volcanism caused atmospheric carbon dioxide to rise from about 1,500 ppm to about 2,500 ppm (vs. 390 currently) by late Jurassic time. But while carbon dioxide remained high, Jurassic time ended with an ice age. There is evidence of glaciation on some continents, but apparently temperatures did not get as cold as in the previous ice age in late Paleozoic time nor as cold as the glacial epochs of the current ice age.

Next time, the Cretaceous Period: bad news for dinosaurs.

* Geologic Terms

Clastic: Of or belonging to or being a rock composed of fragments of older rocks (e.g., conglomerates or sandstone)

Sinistral strike-slip: If standing on one side of a fault, the other side would appear to move left. The San Andreas fault is a dextral (right) strike-slip fault.

Subduction: A geological process in which one edge of a crustal plate is forced sideways and downward into the mantle below another plate

Terrigenous: deposited on the earth’s crust.

**Age dating of the Juniper Flat granite yielded an age of 171 mya by potassium-argon method and an age of 182-184 mya by rubidium-strontium method.


Lipman, P.W., and Hagstrum, J.T., 1992, Jurassic ash-flow sheets, calderas, and related instrusions of the Cordilleran volcanic arc in southeastern Arizona, GSA Bulletin, v.104.

Tosdal, R.M., Haxel, G.B., and Wright, J.E., 1989, Jurassic Geology of the Sonoran Desert Region, Southern Arizona, Southeastern California, and Northernmost Sonora, in Arizona Geological Society Digest 17.