First Advisor

Michael L. Cummings

Term of Graduation

Spring 1987

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Geology






Volcanism -- Oregon -- Troy Region, Basalt -- Oregon -- Troy Region, Geology -- Oregon -- Troy Region



Physical Description

1 online resource (3, xxi, 220 pages)


Explosion structures occur in flows of Grande Ronde Basalt in the study area near Troy, Oregon. Data from nineteen stratigraphic sites indicate that the maximum number of flows that contain explosion structures at any one site is six. In the informally named Troy flow, explosion structures are widespread.

Each flow that contains explosion structures can be divided into two cooling units. The first cooling units occupy troughs in the pre-eruption topography and are up to 10 meters thick. The second cooling units contain the explosion structures and are up to 100 meters thick. The thickness of flows that contain explosion structures range from 10 meters to 150 meters. A plot of the thickness of an explosion structure against the total thickness of the flow is linear with slope of approximately 0.5. The breccias within explosion structures average 42% of the total thickness of a flow.

The overall shape of an explosion structures is similar to a three-dimensional nested arch with a central spine of breccia that cuts through the uppermost arches. Jointing patterns follow the shape of the arches. The linear trends of the central spines within explosion structures of the Troy flow parallel either the northeast-trending Grande Ronde (N 3° E) fault system or the northwest-trending dike system in the area (N 15° W).

Two processes operate during the formation of explosion structures 1) mixing and 2) fragmentation. These two processes produce unique intraflow zones within the second cooling unit. Petrographic textures of these intraflow zones range from vitrophyric to intersertal to intergranular. All three textures can be observed in thin bands or layers in samples from the upper intraflow zones of the second cooling units. Individual bands or layers are twisted, pinched, and swirled due to mixing. Fragmentation and mixing produce a vertically stratified central spine composed of three main types of clasts: vesicular to nonvesicular, scoriaceous, and pahoehoe types. Clast sizes range from lapilli in the outer matrix-supported margin to block in the inner clast-supported core.

Broad overall trends occur in geochemical data for the Troy flow and a flow stratigraphically above the Troy flow. Concentrations of particular elements increase or decrease in samples towards the base of the flow relative to the uppermost sample. K, La, Eu, and Ta are enriched and Fe and Co depleted greater than 10% towards the base of a flow in areas away from explosion structures. Particular elements are enriched (Ce, Hf) or depleted (Th) less than 10% towards the base. Where explosions structures are present within the flow, these broad overall trends are less pronounced and few elements display these trends of enrichment or depletion.


In Copyright. URI: This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).


If you are the rightful copyright holder of this dissertation or thesis and wish to have it removed from the Open Access Collection, please submit a request to and include clear identification of the work, preferably with URL.

Persistent Identifier