by rapid subduction of thin oceanic crust and large back-arc basins. In contrast, the younger 1,890‑million-year-old volcanics indicate evidence of crustal thickening. This was due to long-term growth of the volcanic arcs by continuous volcanic activity and tectonic thickening associated with arc collisions and successive arc deformation. This in turn followed a massive mountain building event called the Trans-Hudson orogeny. The Cretaceous period 145-66 million years ago was a period for active kimberlite volcanism in the Western Canadian Sedimentary Basin of Alberta and Saskatchewan. The Fort à la Corne kimberlite field in central Saskatchewan formed 104 to 95 million years ago during the Early Cretaceous. Unlike most kimberlite fields on Earth, the Fort à la Corne kimberlite field formed during more than one eruptive event. Its kimberlites are among the most complete examples on Earth, preserving kimberlite pipes and maar volcanoes. The Northern Alberta kimberlite province consists of three kimberlite fields known as the Birch Mountains, Buffalo Head Hills and the Mountain Lake cluster. The Birch Mountains kimberlite field consists of eight kimberlite pipes known as Phoenix, Dragon, Xena, Legend and Valkyrie, dating approximately 75 million years old. The Buffalo Head Hills kimberlite field was dominated by explosive kimberlite volcanism from 88 million years ago to 81 million years ago, forming maars. Kimberlites of the Buffalo Head Hills field are similar to those associated with the Fort à la Corne kimberlite field in central Saskatchewan. The kimberlite pipes of the Mountain Lake cluster were formed during a similar timespan with the Birch Mountains field 77 million years ago. Formation of the Pacific Northwest The Canadian portion of the Pacific Northwest began forming during the early Jurassic period when a group of active volcanic islands collided against a pre-existing continental margin and coastline of Western Canada. These volcanic islands, known as the Intermontane Islands by geoscientists, were formed on a pre-existing tectonic plate called the Intermontane Plate about 245 million years ago by subduction of the former Insular Plate to its west during the Triassic period. This subduction zone records another subduction zone called the Intermontane Trench under an ancient ocean between the Intermontane Islands and the former continental margin of Western Canada called the Slide Mountain Ocean. This arrangement of two parallel subduction zones is unusual in that very few twin subduction zones exist on Earth; the Philippine Mobile Belt off the eastern coast of Asia is an example of