The Blake River Megacaldera Complex
, also called the Blake River Group
, is a giant subaqueous caldera
cluster or a nested caldera system that spans across the Ontario
border in Canada
The caldera complex is around 2700 million years old, consisting of a series of overlapping calderas of various ages and sizes. It lies within the southern zone of the Abitibi greenstone belt of the Superior craton and has an area of 3,000 km2.
It is a world-class metallotect with respect to both hydrothermal Cu-Zn massive sulfides and gold-rich massive sulfides.
The Blake River Megacaldera Complex has been a center of major interest since 2006
with numerous excursions at the international, national and local level.
Structure and geographical extent
The Blake River Megacaldera Complex consists of mainly mafic
to intermediate volcanic flows and less abundant felsic
volcanic flows and intercalated pyroclastic rocks
, which underwent three stages of major volcanic activity. The first phase resulted in the creation of the 40-80 km in diameter, eastwest striking Misema Caldera
which has been dated to 2704-2707 Ma. It is a coalescence of at least two large mafic shield volcanoes
that formed more than 2703 million years ago. The second phase resulted in the creation of the 15-30 km in diameter, northwest-southeast New Senator Caldera
which formed 2701-2704 Ma. Its formation consists of thick massive mafic
sequences which has inferred to be a subaqueous lava lake
during the early stages of the caldera's development. The third phase of activity constructed the classic east-northeast striking 2696 Ma Noranda Caldera
which contains a 7-to-9-km-thick succession of mafic and felsic
rocks erupted during five major series of activity.
The Blake River Megacaldera Complex is considered a supervolcano due to its great size and its multiple dikes and vents. The Misema Caldera is in the order of 3500 to 4000 km2, making the complex similar to the Yellowstone Caldera in Wyoming, Lake Toba in Indonesia and strikingly similar in structure to the Olympus Mons caldera on Mars. As a result, the Blake River Group is best categorized as a meganested caldera complex.
Caldera structure and composition
are collapse structures between 2-100 km in diameter derived from ether as a single massive eruption, or it may occur in stages as a result of a series of eruptions. Fractures will form around the edge of the chamber, usually in a roughly circular shape. These ring fractures may in fact be serve as volcanic vents.
If the magma is rich in silica, the caldera is often filled in with ignimbrite, tuff, rhyolite, and other igneous rocks. Silica-rich magma is slow flowing or has high viscosity. As a result, gases tend to become trapped at high pressure within the magma.
When the magma gets near the surface of the Earth, the gas expands quickly, causing explosions and spreading volcanic ash over wide areas.
A silicic or rhyolitic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event. The pre-caldera structure is either a stratovolcano or a shield volcano.