Leukonychia (or leuconychia), also known as white nails or milk spots, is a medical term for white discoloration appearing on nails. It is derived from the Greek words leuko (white) and onux (nail). The most common cause is injury to the base of the nail (the matrix) where the nail is formed.
Extinction Events. 1  There were many small-scale, sporadic extinction events during the Cambrian Period. 2  A large-scale mass extinction event occurred approximately 500 million years ago. 3  Scientists believe was caused by a significant decrease in global temperature. A large-scale mass extinction event occurred approximately 500 million years ago. 2  Scientists believe was caused by a significant decrease in global temperature. 3  Decrease in global temperature created an ice age-further eliminated many of the oceanic species.
2. Chicxulub. The Chicxulub crater is 112 miles wide, about the distance from Los Angeles to San Diego, and is now underneath the Yucatan peninsula in Mexico. According to scientists, the asteroid hit Earth 65 million years ago, thought to be a hint on what caused the mass extinction of the dinosaurs. (NASA). 3. Sudbury. The Sudbury Crater, in Ontario, Canada, is believed to been formed by a meteor blast over a billion years ago.