BH250-118c
Title
BH250-118c
Subject
Amphibolite
Description
Major Minerals: amphibole, biotite, orthoclase
BH250-118, BH250-118b, and BH250-118c are samples collected from the Morton Gneiss in Morton, Minnesota. The Morton Gneiss is one of the oldest known rocks in North America and is a widely studied example of high-grade metamorphic rock. It is prominently exposed in southwestern Minnesota, particularly around the town of Morton, from which it takes its name. Here, active quarries reveal striking outcrops of the gneiss, displaying its distinctive banding. Historically, the rock was quarried for use in tabletops, building walls, and tombstones, and quarrying continues today under the ownership of a company based near St. Cloud. The site has also been of academic interest, with recent Carleton College graduate Emma Watson (Class of 2023) completing her comprehensive thesis on the Morton Gneiss.
The Morton Gneiss is Archean in origin, with its protolith (original rock) forming around 3.6 billion years ago. Since its formation, it has undergone multiple episodes of metamorphism and deformation, most notably during the Penokean orogeny (~1.85 billion years ago) and later thermal/metamorphic overprinting during the Proterozoic. It is a banded, migmatitic gneiss, showing evidence of both solid-state metamorphism and partial melting (migmatization). The rock features interlayered felsic bands (BH250-118), mafic bands (BH250-118b), and amphibolite (BH250-118c), each representing different compositions, origins, and metamorphic histories.
Amphibolite (BH250-118c) is present as lenses or layers within the Morton Gneiss and represents metamorphosed mafic rocks that may have originated as basaltic or gabbroic intrusions, or as volcanic sediments. Amphibolite in this setting is dominated by hornblende and plagioclase, typically lacking quartz. It can occur conformably with the gneissic layering or as discordant bodies, suggesting emplacement as dikes during or before metamorphism.
Geologically, the Morton Gneiss is significant because it preserves a record of early Earth crustal evolution, including crust formation, tectonism, and high-grade metamorphism. It forms part of the Minnesota River Valley subprovince, which is itself a key component of the Superior Province of the Canadian Shield. Interestingly, in the past, outcrops of the Morton Gneiss were considered as potential storage sites for radioactive waste, and remnants of that period—including test wells and excavated pits—can still be seen in certain locations.
BH250-118, BH250-118b, and BH250-118c are samples collected from the Morton Gneiss in Morton, Minnesota. The Morton Gneiss is one of the oldest known rocks in North America and is a widely studied example of high-grade metamorphic rock. It is prominently exposed in southwestern Minnesota, particularly around the town of Morton, from which it takes its name. Here, active quarries reveal striking outcrops of the gneiss, displaying its distinctive banding. Historically, the rock was quarried for use in tabletops, building walls, and tombstones, and quarrying continues today under the ownership of a company based near St. Cloud. The site has also been of academic interest, with recent Carleton College graduate Emma Watson (Class of 2023) completing her comprehensive thesis on the Morton Gneiss.
The Morton Gneiss is Archean in origin, with its protolith (original rock) forming around 3.6 billion years ago. Since its formation, it has undergone multiple episodes of metamorphism and deformation, most notably during the Penokean orogeny (~1.85 billion years ago) and later thermal/metamorphic overprinting during the Proterozoic. It is a banded, migmatitic gneiss, showing evidence of both solid-state metamorphism and partial melting (migmatization). The rock features interlayered felsic bands (BH250-118), mafic bands (BH250-118b), and amphibolite (BH250-118c), each representing different compositions, origins, and metamorphic histories.
Amphibolite (BH250-118c) is present as lenses or layers within the Morton Gneiss and represents metamorphosed mafic rocks that may have originated as basaltic or gabbroic intrusions, or as volcanic sediments. Amphibolite in this setting is dominated by hornblende and plagioclase, typically lacking quartz. It can occur conformably with the gneissic layering or as discordant bodies, suggesting emplacement as dikes during or before metamorphism.
Geologically, the Morton Gneiss is significant because it preserves a record of early Earth crustal evolution, including crust formation, tectonism, and high-grade metamorphism. It forms part of the Minnesota River Valley subprovince, which is itself a key component of the Superior Province of the Canadian Shield. Interestingly, in the past, outcrops of the Morton Gneiss were considered as potential storage sites for radioactive waste, and remnants of that period—including test wells and excavated pits—can still be seen in certain locations.
Coverage
Location: Granite Falls, Minnesota, USA
44°49'7.94"N, 95°33'52.44"W
44°49'7.94"N, 95°33'52.44"W
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 246. Housed at Carleton College in Minnesota.
Type
Thin section
Relation
Collection
Citation
Bereket Haileab, “BH250-118c,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/352.