Newtonian: fluids, such as water and most gases which have a constant viscosity.

Thus there exist a number of forms of viscosity: Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity. It is not a fundamental law of nature but an approximation that holds in some materials and fails in others. Newton's law of viscosity, given above, is a constitutive equation (like Hooke's law, Fick's law, Ohm's law). Viscosity, the slope of each line, varies among materials James Clerk Maxwell called viscosity fugitive elasticity because of the analogy that elastic deformation opposes shear stress in solids, while in viscous fluids, shear stress is opposed by rate of deformation. Note that the rate of shear deformation is which can be also written as a shear velocity. Hence, through this method, the relation between the shear stress and the velocity gradient can be obtained. This equation can be expressed in terms of shear stress Thus as expressed in differential form by Isaac Newton for straight, parallel and uniform flow, the shear stress between layers is proportional to the velocity gradient in the direction perpendicular to the layers: Where μ is the proportionality factor called the viscosity. Combining these three relations results in the equation The applied force is proportional to the area and velocity of the plate and inversely proportional to the distance between the plates. If this force causes the substance between the plates to undergo shear flow at velocity u (as opposed to just shearing elastically until the shear stress in the substance balances the applied force), the substance is called a fluid. Assuming that the plates are very large, with a large area A, such that edge effects may be ignored, and that the lower plate is fixed, let a force F be applied to the upper plate. The relationship between the shear stress and the velocity gradient can be obtained by considering two plates closely spaced at a distance y, and separated by a homogeneous substance. In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force. Laminar shear, the non-constant gradient, is a result of the geometry the fluid is flowing through (e.g. The study of flowing matter is known as rheology, which includes viscosity and related concepts. All real fluids (except superfluids) have some resistance to stress and therefore are viscous, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.

For example, high-viscosity felsic magma will create a tall, steep stratovolcano, because it cannot flow far before it cools, while low-viscosity mafic lava will create a wide, shallow-sloped shield volcano. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).( 1) Thus, water is 'thin', having a lower viscosity, while honey is 'thick', having a higher viscosity. In everyday terms (and for fluids only), viscosity is 'thickness'. Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. But most importantly, highly viscous lava is associated with explosive eruptions and dangerous pyroclastic flows.Clear liquid above has lower viscosity than the substance below. Viscous lava will trap pockets of gas within the rock, and not let them pop as bubbles on the surface. It can also clog up the volcanic vent and form blocks that resist the flow of lava. Instead of rivers of lava, you can get crumbling piles of rock flowing down hill. When lava has a high viscosity, it’s very thick and doesn’t flow very well at all. Classic examples of shield volcanoes are Mauna Kea and Mauna Loa in Hawaii, as well as Olympus Mons on Mars. And over time, volcanoes made from low lava viscosity are wide and have a shallow slope these are known as shield volcanoes. You can also get bubbles of lava filled with volcanic gasses that burble and pop on the surface of the lava.

This creates the classic rivers of lava, with channels, puddles and fountains. When lava has low viscosity, it can flow very easily over long distances. Even though lava is 100,000 times more viscous than water, it can still flow great distances. And the lava viscosity defines the size and shape of a volcano. You can measure lava in terms of viscosity as well. Water has low viscosity, while corn syrup, for example, is highly viscous. When it comes to liquids, viscosity is a measurement of how thick or syrupy it is.

Volcano viscosity definition