Stop
4
Garnet
Amphibolite
This stop is west of Stop 3. (Google Earth)
Temperature and
depth of burial for the amphibolite facies. Amphibolite is on
the continental/typical geothermal gradient.
Stop 4
The index minerals for this facies are hornblende, garnet, and actinolite and are generally associated with a mafic protolith. They can also contain serpentine, talc and tremolite if ultramafic rocks like peridotite are in the protolith. On the island it formed at depths up to 35 km+/- and temperatures about 600°C and is approximately 122 +/-3 MYO MYO (Grove M., Bebout G.E. 2008). The garnet crystals are the small red pieces in the very dark amphibolite matrix. Amphibolite forms on the continental/typical geothermal gradient.
This is behind the airport to the northwest. There are small
hills called knockers in this area, they stand up where the rocks are more
resistant to weathering compared with the surrounding rock. This is where you
find the garnet amphibolite unit.
Garnet amphibolite
outcrops
Garnet amphibolite, the small red grains are the garnet,
the dark matrix is amphibole.
Heavily weathered serpentinite unit with
actinolite/tremolite seams.
Enroute
to Stop 5 is an alternate trip to visit the soapstone quarry, near the airport.
Soapstone is a soft material that has good thermal properties, its formed when you have ultramafic rocks in the protolith. Native Americans would craft bowls out of it on the island.
The 1st picture is of the soapstone before its carved,
some pieces are already round, others have to be crafted into the exterior
round shape. In the 2nd and 3rd pictures they are in the
process of cutting the interior part of the bowl.