Mount Hood formed as a stratovolcano from layers of lava flows, ash from the air and volcanic mudflows. Its primary construction is from a family of rocks known as andesite. The initial volcano formed more than 500,000 years ago, and it has seen long periods of inactivity followed by short periods of activity that formed the mountain.Know More
The National Science Foundation reports that two types of magma mixed a few weeks before each of Mount Hood's historic eruptions. This mixing is unlike what occurs in many other stratovolcanos, even those within the same Cascade Range as Mount Hood, like Mount St. Helens, only 60 miles away. Mixing the two lava types causes the pressure to build, causing an eruption, but it is usually less violent than the eruptions of other volcanoes. This same mixing provides the ingredients required for formation of andesite, leading scientists to believe Mount Hood formed in the same way as other andesite volcanoes.
While eruptions built the base of Mount Hood, weathering and erosion have formed the visible mountain. The movement of glaciers from the top of the mountain helps to break down the rock that forms its shape. These movements break through lava domes to allow new flow to escape.
While scientists consider Mount Hood an active volcano, they expect its next eruptions to include small lava flows. Models indicate an eruption is not likely to cause major damage in Portland, Ore., only 45 miles away.Learn more about Geology
A plateau is formed by a process of geological uplift, either due to the collision of continental plates, pressure from magma below or the burial of land by volcanic lava and ash flows, resulting in a high, flat surface. These are very similar to processes that give rise to mountains and mountain ranges. The only major difference is that a plateau is flat while mountains are peaked.Full Answer >
A laccolith is a geological structure that forms when magma pushes through layers of rock above it and pools in a dome shape. Laccoliths are characterized by their shape, as they are typically flat on the bottom and rounded or dome shaped on top. These structures are also called plutonic formations or igneous intrusions, which are related to sills.Full Answer >
Shale forms from the pressure of layers of sediment compressing bits of silt that settle into the clay on the bottom of bodies of water. The compressed clay and silt become shale over time. Shale is a sedimentary rock.Full Answer >
Breccia forms when shards of rock, over at least 0.08 inches in length, are cemented together a relatively short distance from where they were originally broken off. Often this occurs at the base of an outcrop of rock where mechanical weathering occurs.Full Answer >