Volcanic Rocks

Intro

Volcanic (or extrusive) rocks have cooled from molten material that either flowed out onto the surface of the earth or were blasted up into the air and settled back onto the surface of the earth. Volcanic igneous rocks are widespread throughout Colorado. At one time, about sixty percent of Colorado was covered by volcanic rocks but much of that coverage has since eroded.

Volcanic rocks come in a variety of forms, depending on the type of eruption from which it originated. For example, in a violent eruption ash fall, ash flow, and lava flow may occur, all of which produce different types of volcanic igneous rock.

Many of these rocks originated in the San Juan volcanic field, which is in the southwestern region of the state. There, many large caldera eruptions generated hundreds of cubic miles of pyroclastic debris.

Ash Falls

When volcanic debris is ejected into the atmosphere it eventually settles back down to the surface as ash fall. Volcanic igneous rocks that result from this process are usually of uniform in grain size, which are tiny flakes of glass. Some ash ejected into the atmosphere may float for years before settling back down to the surface. Ash falls provide very good age datums that may be correlated for thousands of miles in some cases. A very important ash fall deposit in Colorado is the Lava Creek B ash that was created by Yellowstone Caldera around 640,000 years ago. This ash has been instrumental in providing age constraints on numerous formations around Colorado, such as the Browns Park Formation.

Ash Flows

Ash flows follow explosive volcanic eruptions and occur when dense ash sinks and flows down the flanks of an erupting volcano. These scorching hot flows can travel hundreds of miles per hour, knocking down trees and obliterating anything else in their path. Ash flows can form volcanic igneous rocks such as welded tuffs, where the incredible temperature of the flow causes ash to fuse together. Ash flows may also contain various sizes of other pyroclastic material.

Following an epic period in the region’s geologic history, Colorado now has ash-flow tuffs that cover thousands of square miles in the southwestern part of the state. Between 25 and 37 million years ago when ash flows were erupting with gusto, it is estimated that two-thirds of the state was covered with ash flow deposits. The Fish Canyon Tuff surrounding the present-day site of Creede, Colorado is one of the largest ash flows in the world. It contains approximately 1,200 cubic miles of material that was deposited red hot and has a welded zone more than a half-mile thick.

Breccias

Breccias a rock formed from angular gravel and boulder-sized clasts that are cemented together in a matrix. The angular nature of the clasts indicates that they have not been transported very far from their source and have not been water worn smooth. Volcanic breccias (agglomerates) comprise blocks of lava in an ash matrix, and are the product of an explosive eruption.

Breccia can be further divided according to:

• Class – breccia can be divided into two broad classes: clast supported – where the clasts touch each other and the matrix fills the voids; and matrix supported – where the clasts are not in contact and the matrix surrounds each clast;
• Clast size – fine (2 – 6mm), medium (6 – 20mm), coarse (20 – 60mm), very coarse (> 60mm);
• Sorting – a breccia comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;
• Lithology – a breccia where the clasts represent more than one rock type is termed polymictic (or petromictic), while one where the clasts are of a single rock type are monomictic (or oligomictic).

Calderas

Calderas form during massive volcanic eruptions where large portions of the surface collapse into the emptying magma chamber below. This type of eruption generates hundreds of cubic miles of ash, which in turn creates tremendous amounts of volcanic rock.

Colorado has experienced at least 20 caldera eruptions. The San Juan volcanic field is one of the most notable areas in Colorado where this type of eruption occurred. Hundreds of square miles of Colorado’s southwest are covered by ash flow tuff generated by these eruptions. The world’s largest caldera, La Garita, is also located in the San Juan volcanic field. This caldera extruded over 1,200 cubic miles of ash flow, which is also the world’s largest ash flow deposit.

Columnar Jointing

Columnar joints forms as volcanic rock cools and contracts and is thus a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing can occur in cooling lava flows and ash flow tuffs (ignimbrites), as well as in some shallow intrusions. Joint growth is perpendicular to the surface of the volcanic flow.

Column diameters vary from a few centimeters to three meters (10 feet), and can be as much as 30 meters (about 100 feet) in length. They are typically parallel and straight (colonnade), but can also be curved (entablature). The number of sides of the individual columns can vary from three to eight, with six sides (hexagonal in cross section) being the most common.

We are used to seeing columnar jointing have vertical joints, particularly in basalt flows. They form at right angles to the isotherms (lines of equal temperature). In a horizontal lava flow the isotherms are normally horizontal because the flow loses heat to the cooler ground below and the cooler air above: the columns thus are vertical.

At first glance here one might think there is a violation of that relationship. However, the dike is intruded into rock and is losing heat to each of its sides as it cools. Therefore, the isotherms are vertical and since columns form perpendicular to isotherms, they are horizontal. See more about intrusive rocks (like dikes) on our plutonic rocks page.

Lava Flows

Lava flows come in a variety of styles depending upon the composition of erupted material. Two major categories of lava flow in Colorado areas basalt (magnesium and iron rich) flows and rhyolite (silica rich) flows.

Cinder Cone Flows

A cinder cone or scoria cone is a steep, straight-sided conical hill of pyroclastic material called tephra (volcanic debris) that accumulates around and downwind from a volcanic vent. Cinder cones range in size from tens of meters to 300 meters tall. The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles “frozen” into place as magma exploded into the air and then cooled quickly. Many cinder cones have a bowl-shaped crater at the summit.During the waning stage of a cinder-cone eruption, the magma has lost most of its gas content. This gas-depleted magma does not fountain, but oozes quietly into the crater or beneath the base of the cone as lava. Lava rarely issues from the top (except as a fountain) because the loose, un-cemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent. Because it contains so few gas bubbles, the molten lava is denser than the bubble-rich cinders. Thus, it often burrows out along the bottom of the cinder cone, lifting the less-dense cinders like a cork on water, and advances outward, creating a lava flow around the cone’s base. When the eruption ends, a symmetrical cone of cinders sits at the center of a surrounding pad of lava. However, if the crater is fully breached, the remaining walls form an amphitheater or horseshoe shape around the vent.

Cinder cones are commonly found on the flanks of shield volcanoes, stratovolcanoes, and calderas. Colorado’s youngest cinder cone volcano is located near Dotsero, Colorado and has been dated to be about 4,150 +/- 300 years old. More information is available in the CGS OF-08-14 Geologic Map of the Dotsero Quadrangle, Eagle and Garfield Counties, Colorado report.