A mountain is a landform that rises prominently above its surroundings, generally exhibiting steep slopes, a relatively confined summit area, and considerable local relief.
Mountains generally are understood to be larger than hills, but the term has no standardized geological meaning. Very rarely do mountains occur individually. In most cases, they are found in elongated ranges or chains.
- Encyclopaedia Britannica
Yellow = continental crust (lithosphere)
thin region under the ocean (visible as the GIF progresses) = oceanic crust, also part of the lithopshere
Gray = upper mantle/lithosphere
Orange = lower mantle/asthenosphere
Himalayan mountains formation
Here we see the 6,000 kilometre-plus journey of the India landmass before its collision with Asia (Eurasian Plate) about 40 to 50 million years ago.
This GIF shows a sideways view of what happened as these landmasses collided. Observe the rock folding and forming mountains.
Isostasy (Greek ísos “equal”, stásis “standstill”) is the state of gravitational equilibrium between Earth’s crust (or lithosphere) and mantle such that the crust “floats” at an elevation that depends on its thickness and density.
How can the mantle be both solid and plastic?
You might be wondering how it is possible that Earth’s mantle is rigid enough to break during an earthquake, and yet it convects and flows like a very viscous liquid.
It does so because it isn’t a solid or a liquid – it is really a a non-Newtonian fluid. That means it responds differently to stresses depending on how quickly the stress is applied.
A good example of this behaviour is seen in Silly Putty. It can bounce, yet can break if you pull on it sharply. Also, it can deform like a liquid if stress is applied slowly. In this photo, Silly Putty was placed over a hole. In response to gravity, it slowly flowed into the hole. Similarly, the mantle will flow when placed under the slow but steady stress of a growing (or melting) ice sheet.
text here adapted from BC Open Textbooks, Geology, 9.4 Isostasy
A concept map of today’s lesson
8.MS-ESS2-1. Use a model to illustrate that energy from Earth’s interior drives convection that cycles Earth’s crust, leading to melting, crystallization, weathering, and deformation of large rock formations, including generation of ocean sea floor at ridges, submergence of ocean sea floor at trenches, mountain building, and active volcanic chains.
ESM-PE.1.3.1 Explain and justify the topographic features typically found at each type of tectonic boundary (convergent, divergent, transform).
ESSENTIAL KNOWLEDGE Students apply, as well as engage and reason with, the following concepts in the performance expectations: Tectonic processes create distinct landforms, such as uplifted, folded, and thrusted mountains and downthrown rift valleys.
Enduring Understanding 2C: Earth’s landscapes emerge from the interactions among the atmosphere, hydrosphere, lithosphere, biosphere, cryosphere and human activity.
Benchmarks for Science Literacy, AAAS
The interior of the earth is hot. Heat flow and movement of material within the earth cause earthquakes and volcanic eruptions and create mountains and ocean basins. Gas and dust from large volcanoes can change the atmosphere. 4C/M1
There are a variety of different land forms on the earth’s surface (such as coastlines, rivers, mountains, deltas, and canyons). 4C/M8** (BSL)
The earth’s plates sit on a dense, hot, somewhat melted layer of the earth. The plates move very slowly, pressing against one another in some places and pulling apart in other places, sometimes scraping alongside each other as they do. Mountains form as two continental plates, or an ocean plate and a continental plate, press together. 4C/M12** (BSL)
There are worldwide patterns to major geological events (such as earthquakes, volcanic eruptions, and mountain building) that coincide with plate boundaries. 4C/M13** (BSL)
Earthquakes often occur along the boundaries between colliding plates, and molten rock from below creates pressure that is released by volcanic eruptions, helping to build up mountains. Under the ocean basins, molten rock may well up between separating plates to create new ocean floor. Volcanic activity along the ocean floor may form undersea mountains, which can thrust above the ocean’s surface to become islands. 4C/H5