https://bereket-haileab.geology.sites.carleton.edu/files/original/853740d71d4ff2dcfaf6ea6de04f611c.jpg c08eb6fdee1da0fe492ff893e29a02f1 https://bereket-haileab.geology.sites.carleton.edu/files/original/4f4bd4e0c085abde308a4b7221f7011a.jpg 471234128e199484374b3765233164c5 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). 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 BH250-98 Subject The topic of the resource Granite with Basaltic Dike Description An account of the resource Major Minerals: plagioclase, biotite Minor Minerals: chlorite, clinopyroxene BH250-97 and BH250-98 are excellent teaching specimens that preserve the intrusion of a basaltic dike into a pre-existing granitic host rock. These samples provide a clear example of intrusive relationships, chilled-margin development, magma flow textures, and the influence of cooling rate on crystal size within igneous rocks. The dark mafic dike sharply cuts across the lighter coarse-grained granite, making the intrusive contact easily visible both in hand sample and thin section. The granitic host rock consists primarily of quartz and feldspar with minor biotite, whereas the basaltic dike is dominated by plagioclase microlites and mafic minerals including clinopyroxene and altered ferromagnesian phases. In thin section, the dike displays a pronounced fine-grained chilled margin adjacent to the granite contact. This chilled zone formed when the hot basaltic magma was rapidly cooled against the comparatively cold granitic country rock. Rapid heat loss at the margin inhibited crystal growth, producing a dark, very fine-grained to nearly glassy texture. Toward the center of the dike, crystal size increases progressively as cooling became slower and more uniform. The interior contains larger plagioclase laths and more developed intergranular textures, reflecting a lower cooling rate away from the contact. This gradual transition from cryptocrystalline margin to coarser-grained interior provides an excellent natural example of conductive cooling in mafic intrusions. One of the most striking petrographic features preserved in these samples is the alignment of plagioclase laths along the dike margins. The preferred orientation of the elongate feldspar crystals records magma flow during emplacement. Near the contacts, the plagioclase grains are strongly aligned parallel to the dike walls, indicating laminar flow of the basaltic melt as it moved through the fracture system. Toward the center of the dike, this flow fabric becomes progressively weaker and more random as crystallization proceeded and magma motion diminished. The large-format thin section spanning the full width of the dike is particularly valuable because it preserves the complete textural transition from granite contact, through the chilled margin, into the coarser central portion of the intrusion, and back to the opposite contact. Such sections allow students to directly observe how cooling gradients influence crystal nucleation, grain size, and texture development across a single intrusive body. These samples were used extensively in an introductory geology class project at Carleton College in which students investigated cooling-rate variations within basaltic dikes. Students measured grain-size distributions across the dike width and analyzed the relationship between crystal size and distance from the dike margin. Their work demonstrated that crystal size generally increases toward the center of the dike, consistent with slower cooling rates away from the contact zone. The project also explored the role of dike thickness in cooling history and crystal development. The accompanying student paper, The Cooling Rates of Two Dikes from Granite Falls, MN, developed quantitative approaches for analyzing crystal-size distributions using petrographic images and digital image-processing techniques. The study concluded that both dikes cooled more rapidly at the margins than in the center and that wider dikes produced significantly larger crystals due to prolonged cooling times. The photomicrographs further illustrate these relationships. The chilled margins contain abundant tiny plagioclase microlites set within a dark fine-grained groundmass, whereas the interior portions contain larger feldspar crystals and more developed intergranular textures. In crossed polarized light, plagioclase laths exhibit bright interference colors and locally show preferred orientation parallel to the dike contact, reinforcing evidence for magma flow during emplacement. Overall, BH250-97 and BH250-98 are outstanding educational specimens because they connect field relationships, hand-sample observations, petrography, crystallization kinetics, and quantitative cooling-rate analysis into a single integrated teaching example. They demonstrate how intrusive igneous bodies record both thermal history and magma dynamics during emplacement within the crust. Key Petrographic Features • Sharp intrusive contact between basaltic dike and granite host • Chilled margins along dike contacts • Progressive grain-size increase toward dike center • Flow alignment of plagioclase laths • Intergranular to pilotaxitic basaltic texture • Large-format thin section preserving full dike profile • Evidence of conductive cooling and magma flow dynamics Educational Significance • Demonstrates intrusive cross-cutting relationships • Excellent example of chilled-margin development • Useful for teaching cooling-rate effects on crystal growth • Connects petrography with quantitative image analysis • Used in student research projects on igneous textures and crystallization dynamics 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: Granite Falls, Minnesota, USA Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample 98. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Hand sample and thin section Large thin sections 2-3 inches Relation A related resource <iframe width="100%" height="480" src="https://drive.google.com/file/d/1L8bp5Q1ZVMPRQ6du2joqzpVB1My3GAcd/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1_SE0KXfDJjim_UZzD8SgQe3_mOxduNs4/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1-NriA04ZqWPhw3owZSQUniWJxquLqe1F/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1fpqiNpKBewmGU2bsHhh08QGs_8bIZjUD/preview" allow="autoplay"></iframe>&nbsp;<br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1BP8s9VECvME7V6opyvySMRt-EMDuqXaB/preview" allow="autoplay"></iframe><iframe width="100%" height="480" src="https://drive.google.com/file/d/1xk6mFYkM8He5ZtpC7JfNE9WKPXEafJ9e/preview" allow="autoplay"></iframe> Creator An entity primarily responsible for making the resource Bereket Haileab basalt Basaltic Basaltic dike basaltic intrusion Biotite alteration Chilled margin conductive cooling cooling rate Crystal Growth crystal nucleation dike dike crystallization dike emplacement felsic Grain size variation granite granite host rock igneous Igneous petrology igneous textures Intrusion–host contact intrusive contact mafic Mafic intrusion magma dynamics magma flow texture Magmatic flow fabric Mineral alignment Petrography plagioclase alignment plagioclase microlites plutonic volcanic