Wildfires can start naturally due to lightning strikes, volcanic eruptions and sparks created by a rock fall. Of these natural causes, lightning accounts for a little more than 10% of the fires started in the United States. However, the remaining 90% of the wildfires are caused by human activities. These include: unattended campfires, burning debris, discarded cigarettes and arson.
Source: Lightning-caused Fires | NIFC
Source: Human-caused Fires | NIFC