Construction of the current edifice, or main part, of the volcano began during the last half million years on the same site where ancient Mount Rainier stood 1 to 2 million years ago. Before these volcanoes were exposed to the surface, large amounts of magma rose, erupted, pooled, and slowly cooled below the surface. A portion of this cooled magma, identified as Tatoosh granodiorite, may be dated to between 18 and 14 million years old. About 10 million years ago, western North America was uplifted and erosion processes washed away the overlying rock, exposing the underlying granodiorite. Rocks observed at the volcano indicate a history of minor pyroclastic flows, or rapidly moving rocks and ash, and hydrothermally altered rocks, or rocks that show changes due to high water temperatures. There are two smoking craters on Mount Rainier that appear to be preventing glacial erosion and symbolize a young, active volcano with potential for future volcanic eruptions. The volcanism that formed Mount Rainier that we see today is related to its location within the Cascade Range.
The Cascade Range, in which Mount Rainier stands, is a perfect example of a fundamental concept in geology. It is a convergent joint interaction between tectonic plates, where two plates collide, and the resulting volcanic chain that forms parallel to and inward from the plate boundary. This particular area features a subduction zone, or collision zone, where a slowly sliding, dense oceanic plate, known as the Juan de Fuca Plate, plunges beneath the less dense North American tectonic plate. As the Juan de Fuca plate sinks deeper into the earth, temperatures and pressures increase, causing the plate to partially melt and release less dense rock material (water, gases, etc.). The less dense material rises, melting and absorbing the surrounding rock as it rises to form magma. This magma will eventually accumulate and form magma chambers just below the earth's crust. These cameras act like a can of soda, sitting idle most of the time unless there is a sudden failure. Can you guess what could upset the balance in the chamber and trigger a volcanic eruption? Earthquake! Just as a sudden and violent jolt from a soda can causes the liquid to explode when opened, volcanoes respond to this rapid change in motion and pressure by erupting at the Earth's surface.
We see volcanic history through layers deposited after an explosion. Similar volcanoes like Crater Lake and Mount St. Helens follow the same principles. Stratigraphy, layers of layers or bedrock, acts as a time machine for geoscientists to measure, record, and interpret each layer at a corresponding point in history. Additionally, each bed can be measured by its elevation, or general direction, and its slope, the angle at which the bed "sinks" from the horizontal. These measurements allow geologists to get a better idea of the general orientation of the bed and its relationship to surrounding formations and the landscape (see more about course and diving). By examining rocks in the field, scientists can paint a clearer picture of Earth's history and anticipate future geological events. The eruption history of Mount Rainier is less frequent and less voluminous than that of surrounding volcanoes because the magma is more fluid and less sticky during eruption. Its volcanic past dates back 500,000 years, but it has a quieter history compared to the explosiveness of other Cascade volcanoes. Mount Rainier consists primarily of a collection of andesite and dacite lava flows and pumice outbursts. The types of rocks found in this volcano show differences in chemical composition compared to other volcanoes in the region, such as Mount St. Helens.
ca 500 to 420– The first 80,000 years of rock records from Mount Rainier show a very active volcanic period. Thick, highly visible layers of pyroclastic flows covered by layers of lava flow can be examined by diving radially away from the current peak. These steeply sloping strata indicate that the volcano grew rapidly to a height similar to what it is today. The remains of icy columns from the lava flows can be seen today near the Sunrise visitor facility.
ca 420 to 280 - Wand:This period left minimal geological evidence and minor lava accumulation due to infrequent and relatively small eruptions. The Rampart Ridge lava flow is the only record geologists have of the 380,000-year-old eruption, which has since been dramatically eroded by glacial activity.
280 to 160 ca – Mowich:Between 280,000 and 180,000 years ago, the lava accumulated extensively and rapidly. The preservation of 200,000-year-old rocks found on the upper west flank indicate that the volcano reached its maximum height at this time. The increased inflow of magma allowed dikes, or narrow transport routes, to be cut through the soft underside of the overlying rock to facilitate the large movement of magma through volcanic vents at the surface.
160 to 40–Little Tahoma:Eruptions gradually subsided 160,000 years ago, and erosion processes dominated thereafter. Two large lava flows occurred 130,000 and 105,000 years ago respectively. Both showed extremely hot flow characteristics and fluids from deep sources and not lateral to the Mount Rainier magmatic pipe system.
40 to 15 years-hit point: The eruption rate increased during this period, which led to the reconstruction of the upper mountain. This period coincides with the last great Ice Age, which occurred 20,000 years ago and caused a clash of volcanic deposition and glacial erosion.
11 ka to date – Holocene eruption period:Holocene eruptive history is well preserved in the rock record, identified by layers of volcanic ash deposited after glacial retreat. Tephra deposition events and lahar flows influenced the topography between 11,000 and 6,700 years ago. A large mudflow event occurred about 5,600 years ago, triggered by the collapse of weakened rock with sufficient water content into the volcano's edifice. The Osceola mudflow was 4 km away.3in volume and reached the western end of Puget Sound. This event resulted in an open horseshoe-shaped crater to the northeast, similar to the Mount St. Helens crater in 1980. The volcano was relatively inactive until about 2,700 years ago, when eruptions resumed. Episodes of tephra falls, pyroclastic flows, lahars, and lava flows contributed to the growth of the modern edifice.
Icing was also an integral part of the creation of the Mount Rainier building. This region was dominated by glaciers both before and after the modern development of the cone. At different periods in its volcanic history, glacial erosion struggled with building processes. During glacial maximums, the valleys surrounding the edifice were filled with ice, which extended 100 km (60 mi) up the side of Mount Rainier. There are two main ice ages in the history of Mount Rainier:
Hayden Creek -170 bis 130 ka:This period included long valleys that filled the Alpine glaciation. The remnants of Hayden Creek are 600 m (nearly 2,000 ft) above the adjacent valley floor, indicating that the glaciers were very thick.
Evans Creek -22 bis 15 ka:The Evans Creek glaciation was significantly smaller than the glaciers that dominated the topography during the Hayden Creek period. The slopes of Mount Rainier were covered by ice caps, and all other glaciers furthest from the volcano were completely enclosed in valleys.
Two smaller glaciation periods occurred during the Late Pleistocene (12,000 years ago) and at the end of the Little Ice Age (mid-19th century). Glaciation in the Pacific Northwest had a major impact on the topography of Mount Rainier. The slopes of Mount Rainier have been covered in ice and snow for most of its 500,000-year history. In many cases, the ice formation was so thick that even lava could not melt through its mass. Instead, the ice cooled the lava below. This caused lava flows to harden and settle atop ridges radiating from the edifice of Mount Rainier as the glaciers that filled the valley began to retreat. Under current conditions, lava would melt the confined ice sheet and eruptions would form destructive lahars. Most of this evidence was gathered by examining the 40,000-year-old Ricksecker Point lava flow, which has helped scientists understand the importance of glaciers channeling lava flows on Mount Rainier. Evidence for horizontal lava movement through glacial terrain is documented in the horizontally oriented columnar fractures found at the edge or boundary of the Ricksecker Point lava flow.
Mount Rainier is still an active volcano, its behavior having remained relatively the same for half a million years. It will continue to grow, erupt and collapse over time, and volcanic activity could cause problems for surrounding residents and air travel. An eruption would also affect the local landscape and recreational areas, as well as natural resources. When an outbreak occurs, there are warning signs that precede the big event. Scientists would detect changes on their monitoring equipment, such as B. small earthquakes below the volcano, subtle deformation or expansion of the earth, and an increase in volcanic gas emissions and ground temperatures. Geologists monitor this data to better prepare for future events and how to deal with disasters.