Hornblendite
“Nearly pure hornblende from magmatic roots”
Hornblendite is a dark, dense ultramafic rock composed almost entirely of hornblende amphibole. It forms as a cumulate rock in the roots of magmatic arc volcanoes when hornblende crystallizes and accumulates at the base of magma chambers deep in the crust.
hornblende (90%+), plagioclase (minor), magnetite, apatite, titanite, olivine (rare)
Duke Island (Alaska, USA), Fiordland (New Zealand), Kohistan Arc (Pakistan), Sierra Nevada arc roots (California), Adamello (Italy)
5-6
Hornblendite cumulates found in the exposed roots of ancient volcanic arcs show that hornblende crystallization plays a much bigger role in arc magma evolution than scientists previously recognized.
Test Your Knowledge
Where in a volcanic arc system does hornblendite typically form?
Related Specimens
Want to identify rocks yourself?