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Tom's Thumb zoom.
Bottom is McDowell Mountains and Tom's Thumb.
Geologically this is young rock, but the climate and consequent weathering has removed much of the softer and more fragmented volcanic material and deposited it in the short alluvial fans and nearby flat valleys. It appears Tom Thumb is a resistant remainder of a much higher mountain.
How was Tom's thumb formed? About 1.4 million years ago, rising magma intruded overlying rhyolite and quartzite rock layers and lithified into granite. Crystals formed and produced a course-grain rock in pockets where magma cooled slowly; faster cooling magma holds fewer crystals and a finer grain.
Exposed granite has been dramatically rounded by erosion, with horizontal and vertical joint patterns in larger rock masses. This rounded, light colored rock contrasts sharply with the dark, jagged basalt and rhyolite found throughout the McDowell range. SOUNDS FAMILIAR - I wonder why it's not called monzogranite like in Joshua Tree National Park? Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma.
I found this https://rocks.comparenature.com/en/gran ... -1-119-999: Granite is a very hard, granular, crystalline igneous rock which consists mainly of quartz, mica, and feldspar and is often used as building stone Monzogranite is a type of igneous rock and belongs to biotite granite rocks that are considered to be the final fractionation product of magma.
Texture of Granite is Granular, Phaneritic whereas that of Monzogranite is Porphyritic whatever that means???
the difference between granite and monzograniteis that granite is a group of igneous and plutonic rocks composed primarily of feldspar and quartz. Usually contains one or more dark minerals, which may be mica, pyroxene, or amphibole. Common colors are gray, white, pink, and yellow-brown while monzogranite is a kind of biotite granite rock, considered to be the final fractionation product of magma.
Bottom is McDowell Mountains and Tom's Thumb.
Geologically this is young rock, but the climate and consequent weathering has removed much of the softer and more fragmented volcanic material and deposited it in the short alluvial fans and nearby flat valleys. It appears Tom Thumb is a resistant remainder of a much higher mountain.
How was Tom's thumb formed? About 1.4 million years ago, rising magma intruded overlying rhyolite and quartzite rock layers and lithified into granite. Crystals formed and produced a course-grain rock in pockets where magma cooled slowly; faster cooling magma holds fewer crystals and a finer grain.
Exposed granite has been dramatically rounded by erosion, with horizontal and vertical joint patterns in larger rock masses. This rounded, light colored rock contrasts sharply with the dark, jagged basalt and rhyolite found throughout the McDowell range. SOUNDS FAMILIAR - I wonder why it's not called monzogranite like in Joshua Tree National Park? Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma.
I found this https://rocks.comparenature.com/en/gran ... -1-119-999: Granite is a very hard, granular, crystalline igneous rock which consists mainly of quartz, mica, and feldspar and is often used as building stone Monzogranite is a type of igneous rock and belongs to biotite granite rocks that are considered to be the final fractionation product of magma.
Texture of Granite is Granular, Phaneritic whereas that of Monzogranite is Porphyritic whatever that means???
the difference between granite and monzograniteis that granite is a group of igneous and plutonic rocks composed primarily of feldspar and quartz. Usually contains one or more dark minerals, which may be mica, pyroxene, or amphibole. Common colors are gray, white, pink, and yellow-brown while monzogranite is a kind of biotite granite rock, considered to be the final fractionation product of magma.
Mar 10 2023