Soon after the picture on the left was taken, "pioneer" plants, such as fireweed and Canada thistle, started to populate the area. Eventually, a recognizable forest will fill the view. Click on the pictures for enlarged views.
Biological succession is a concept that tries to explain the way living things repopulate an area after a disturbance. Examples of disturbances range from floods to fires to logging to lava flows to explosive volcanic eruptions. Each type of disturbance presents unique challenges to the organisms that will eventually fill the area. The eruption of Mt. St. Helens in 1980 produced a fairly extreme example of a disturbance in an ecosystem. The surrounding area has been and is a good case study for observing the stages of biological succession. The pictures below illustrate the concept. Immediately after the eruption, all of the living things near the mountain were obliterated by the heat and force (left picture). Five years later (right picture), the forest had not yet grown back, but there were certain types of plants already thriving. These early plants, with hearty root systems, do well in the open environment left by the eruption, but they don't do as well once shade plants start to develop. Gradually, these early plants will be pushed out of the way, and the forests will likely return to the fir and hemlock type that existed before the eruption
Washington DNR Image
The Juan de Fuca plate is subducting, or diving, under the North American plate from northern California to British Columbia. Just inland from the plate boundary is a line of volcanoes, including Mt. St. Helens. Click on the picture for an enlarged view.
From a geological standpoint, Mt. St. Helens is a good representative of all of the volcanoes in the Pacific Northwest. The reason the volcanoes exist in that area is due to the subduction (diving underneath) of one crustal plate under another. As the rock forming the plate sinks lower into the underlying magma it heats up. This heated rock rises up and eventually bursts through the overlying plate (in this case the North American plate). Mt. St. Helens is simply the most recent location where the heating caused a burst.
Image: View from Boundary Trail at Norway Pass - Click for a live view from Volcano Cam
J. Quiring, USGS