https://bereket-haileab.geology.sites.carleton.edu/files/original/21c8f7af8b1b09103bc2980661d42f6e.jpg 3b2ad2b8995680e12c0c8a6a187cd94c Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Igneous Rocks Subject The topic of the resource This collection covers both volcanic and intrusive rocks, including rhyolites, granites, diorites, andesites, basalts, gabbros, diabases, pumices, obsidians, syenites, phonolites, peridotites, carbonatites, dacites, trachyte, dunites, xenoliths, ignimbrites, and pegmatites. Creator An entity primarily responsible for making the resource Bereket Haileab Source A related resource from which the described resource is derived Housed in Anderson Hall, Carleton College. Format The file format, physical medium, or dimensions of the resource Thin sections, thin section scans, hand samples, hand sample images, photomicrographs, chemical analyses (major elements, trace elements and REE analyses for selected samples). Thin section and hand sample Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource BH250-248 Subject The topic of the resource Hornblende Gabbro Description An account of the resource Major Minerals: hornblende, plagioclase BH250-248 is an excellent example of hornblende gabbro, a coarse-grained mafic intrusive rock that reminds students that magmas do not always follow the simplified pathways we draw on Bowen’s Reaction Series. As I often tell my students, “a magma will do what it wants to do.” While many introductory diagrams suggest that gabbros are dominated by pyroxene and calcium-rich plagioclase, natural systems are often far more complex. In this sample, the magma crystallized a mineral assemblage dominated by hornblende and plagioclase, demonstrating that water content, pressure conditions, oxygen fugacity, and evolving magma chemistry can significantly alter crystallization pathways. Instead of producing a classic pyroxene-rich gabbro, this hydrous magma stabilized amphibole, resulting in a hornblende-rich variety that records a different magmatic story. In hand sample, hornblende gabbro typically appears dark and coarse grained, with black to dark green hornblende crystals intergrown with lighter plagioclase feldspar. The slow cooling of magma at depth allowed large, interlocking crystals to develop, producing the phaneritic texture typical of intrusive rocks. The plagioclase crystals in this thin section are particularly well developed and commonly display textbook examples of polysynthetic albite twinning, visible in crossed polarized light as alternating light and dark bands. These feldspars form the framework of the rock and are intergrown with large amphibole grains that occupy interstitial spaces and in some areas partially envelop earlier-formed plagioclase crystals. The thin sections provide beautiful petrographic evidence of amphibole crystallization. In plane-polarized light, hornblende displays its characteristic green to olive-brown pleochroism, with color changes reflecting shifts in crystal orientation. Several grains show the distinctive amphibole cleavage directions intersecting at approximately 56° and 124°, making them excellent teaching examples for mineral identification. Under crossed polars, hornblende exhibits moderate to high interference colors, while some grains show signs of alteration along fractures and cleavage traces. The large amphibole grains appear to have crystallized relatively late in the magma’s history, filling spaces between plagioclase crystals and recording the increasingly hydrous nature of the residual melt. One of the most instructive features of BH250-248 is that it demonstrates how magmatic systems evolve beyond idealized textbook examples. If the original magma contained elevated water content—common in subduction-related settings—amphibole may become stable earlier and more abundantly than pyroxene. This is why hornblende gabbros are commonly associated with convergent margin plutonic complexes, where water released from subducting oceanic lithosphere modifies mantle-derived magmas. The sample’s provenance from San Diego County, California, places it within the broader context of the Peninsular Ranges Batholith, a major Mesozoic arc-related intrusive complex formed during subduction along the western margin of North America. BH250-248 is therefore an excellent teaching specimen because it reinforces an important geological truth: natural rocks rarely read the textbook before they form. As I often tell my students, a magma is going to do what it wants to do because it is already armed with what it needs—its chemistry, volatile content, temperature, pressure conditions, and crystallization history all help determine the minerals that will ultimately form. Magmas evolve dynamically, respond to changing physical and chemical conditions, and often produce mineral assemblages that surprise us. That unpredictability is precisely what makes igneous petrology both challenging and fascinating. This sample is a perfect reminder that Earth’s magmatic systems are governed by thermodynamics and chemistry, yet they still seem to have personalities of their own. Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Location: San Diego County, California, USA Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample 248. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Thin section and hand sample Creator An entity primarily responsible for making the resource Bereket Haileab Relation A related resource <iframe width="100%" height="480" src="https://drive.google.com/file/d/1unYz9NwWYwk5Y1xp7VRVMQRL5MtmZ8rm/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/135pP4Y3DUmI_bXckQAxxxDSLcNRjZdko/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1twjiDEo0ezJQJu2sKme2KYynMwjY6W0p/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1RJAzbQwKgklaZwoLVZy0kMLmo4WRUDhl/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1xzleQFVjpOyKmSVsth7Xs4mGu8ieIUPO/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1M2mT7tFmrisyNllIwG5QwC74OidtV2mg/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1lX-yHOSoBOadXCvaf18Lz8V1YQRLzRSe/preview" allow="autoplay"></iframe> <br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1i5FRH3hnlEPLDUQUn6tTrPiz1ggbkNEl/preview" allow="autoplay"></iframe> Albite twinning amphibole amphibole cleavage Amphibole-rich Gabbro arc magmatism Bowen’s Reaction Series calcic plagioclase Coarse-grained Convergent Margin crystallization gabbro Hornblende Hornblende gabbro hydrous magma igneous Interference Colors interlocking texture mafic Mafic Intrusive Rock magma evolution Peninsular Ranges Batholith Phaneritic texture Plagioclase Pleochroism plutonic rock polysynthetic twinning San Diego County geology Subduction zone magmatism