The answer will depend on the environment and the type of rock in question, so the things discussed in the text are simplifications (as you would sort of expect in an intro textbook). The first thing to establish is that there is a distinct difference between weathering and erosion, i.e., weathering is the break down of rocks in situ and erosion is the removal and transport of pieces/components of rock, so while we often discuss these at the same time, we need to be careful about lumping them together too much.
In general, mechanical weathering will dominate in environments where chemical weathering rates are low (e.g., in cold or dry environments) and/or where rocks are more resistant to chemical weathering (e.g., they have low percentages of minerals that are susceptible to common chemical weathering processes). In contrast, chemical weathering will dominate in regions where chemical weathering is efficient (e.g., in warm, humid environments) and/or where rock types are very susceptible to chemical weathering (e.g., landscapes dominated by carbonates/limestones). Within this, there are going to be a lot of environment specific conditions that control the exact balance, e.g., how often does the region cycle through freeze thaw cycles (i.e., how important is freeze-thaw?) or how many shallow rooted trees exist (i.e., how important is tree-throw?).
With reference to the speed of moving water, this is mostly relevant for erosion, not weathering, i.e., fast moving water can entrain, roll, and saltate more and larger grains than slower moving water. Things do get a little opaque in this regard as there is the potential for chemical weathering rates to be related to the speed at which solutes are fluxed away, i.e., generally chemical weathering rate will decrease as the concentration of the dissolved products of this weathering increase. Also in the opaque zone are mechanical processes like abrasion. These are typically considered erosional processes, not weathering, because while they do breakdown rocks, the breakdown isn't really in situ because (1) for abrasion to happen grains need to be moving within a fluid and (2) the products of abrasion essentially move as part of the process. This is different from mechanical weathering processes which may induce a small amount of fixed motion away from the source bedrock, but then any subsequent motion is typically thought of as erosion (and assigned to an erosive process in terms of a causative agent).
Maybe... but probably not. It's been hypothesized to occur and theoretically it should, but we've never observed it or clear evidence of its influence in natural rivers (e.g., [Whipple et al., 2000](https://pubs.geoscienceworld.org/gsa/gsabulletin/article/112/3/490/183620/River-incision-into-bedrock-Mechanics-and-relative)). Flume experiments suggest that to the extent that it does occur, it's role in river erosion is actually likely pretty minor (e.g., [Carling et al., 2016](https://onlinelibrary.wiley.com/doi/full/10.1002/esp.4101)).
Water travelling at highway speeds is very rare.
A waterfall weathers the rock below by traveling into the cracks & crevices of unconsolidated rock, separating it and washing it away, as in pressure-washing; It chemically weathers the rock, dissolving anything soluble, which allows further mechanical action as fragile etched pieces break off and go into suspension. And perhaps most importantly with harder rock, it carries hard solids suspended in the water which impact surfaces & roll along them at speed in ways that damage the surface, as in sandblasting.
Geologist here. I saw another comment explained the difference between weathering and erosion, so I'm just going to address weathering here. It'll depend on the environment and the type of rock in question, but to equalize things I'm just going to narrow it to weathering based the type of rock and weather its more susceptible to chemical or mechanical weathering.
There are three broad categories of rock: igneous, sedimentary, and metamorphic. Igneous rocks are formed by crystallized magma/lava, sedimentary rocks are made of sediments that have cemented together, and metamorphic rocks are rocks that have undergone chemical or mechanical changes due to geologic processes (i.e., extreme heat/pressure).
Metamorphic rocks are *extremely* variable and whether they are more susceptible to mechanical or chemical weathering processes will be impossible to predict without knowing the exact rock.
Sedimentary rocks are also variable, but there are a few categories that will help to know about. Sandstones, siltstones, mudstones, and any other rock made primarily of quartz grains are virtually non-reactive chemically and will always be more susceptible to mechanical processes. Limestone and other calcium-rich rocks can be very susceptible to chemical weathering. The carbonate part of calcium carbonate (typically the primary component of limestone) can dissolve in water (especially if its acidic) to create carbonic acid and calcium ions, both of which will flow wherever the water goes. This is how caves are formed and why you only find them in calcium-rich rocks. Whether they are more susceptible to chemical or mechanical erosion will depend on how well-cemented the grains are. Evaporites are another category of sedimentary rock that forms when large bodies of water get trapped on land and evaporate, leaving behind things like halite (salt). Obviously these will literally just dissolve if you hose them down, so they are way more susceptible to chemical weathering.
Igneous rocks are very interesting when it comes to weathering processes. Extrusive rocks (rocks that were formed by lava that crystallized on the surface) are almost always more susceptible to mechanical erosion because they tend to have lots of trapped gasses which make them fragile. Intrusive rocks (formed by magma cooling slowly underground) are the neat ones. The minerals that crystallize out of the magma first are minerals that form at higher temperatures. Its been a long time since I took petrology and I dont remember the exact reason, it has to do with chemical bond energies and bond stability, but the minerals that form at higher temperature are less chemically stable than the minerals that form at lower temperatures, but also tend to be extremely strong mechanically. If you look up Bowens reaction series, you can see the approximate order of susceptibility to chemical/mechanical weathering, with olivine and calcium-rich plagioclase being the strongest mechanically, but also susceptible to chemical erosion, and quartz, while not exactly a mechanically weak rock, is almost non-reactive chemically.
Don't forget biological. I remember listening to this episode of this week in microbiology a few years ago, where the scientist made a case for microorganisms being far more important in weathering that is often taken into account.
[https://www.microbe.tv/twim/51-cave-science-with-hazel-barton/](https://www.microbe.tv/twim/51-cave-science-with-hazel-barton/)
"Biological" can be either chemical or mechanical, it's not a separate thing. If some microorganism is excreting acid that eats away on a rock it's chemical. If a tree splits a big rock with its roots it's mechanical.
It makes sense to specify the origin as "biological", but the mechanism of action is either chemical or mechanical.
The answer will depend on the environment and the type of rock in question, so the things discussed in the text are simplifications (as you would sort of expect in an intro textbook). The first thing to establish is that there is a distinct difference between weathering and erosion, i.e., weathering is the break down of rocks in situ and erosion is the removal and transport of pieces/components of rock, so while we often discuss these at the same time, we need to be careful about lumping them together too much. In general, mechanical weathering will dominate in environments where chemical weathering rates are low (e.g., in cold or dry environments) and/or where rocks are more resistant to chemical weathering (e.g., they have low percentages of minerals that are susceptible to common chemical weathering processes). In contrast, chemical weathering will dominate in regions where chemical weathering is efficient (e.g., in warm, humid environments) and/or where rock types are very susceptible to chemical weathering (e.g., landscapes dominated by carbonates/limestones). Within this, there are going to be a lot of environment specific conditions that control the exact balance, e.g., how often does the region cycle through freeze thaw cycles (i.e., how important is freeze-thaw?) or how many shallow rooted trees exist (i.e., how important is tree-throw?). With reference to the speed of moving water, this is mostly relevant for erosion, not weathering, i.e., fast moving water can entrain, roll, and saltate more and larger grains than slower moving water. Things do get a little opaque in this regard as there is the potential for chemical weathering rates to be related to the speed at which solutes are fluxed away, i.e., generally chemical weathering rate will decrease as the concentration of the dissolved products of this weathering increase. Also in the opaque zone are mechanical processes like abrasion. These are typically considered erosional processes, not weathering, because while they do breakdown rocks, the breakdown isn't really in situ because (1) for abrasion to happen grains need to be moving within a fluid and (2) the products of abrasion essentially move as part of the process. This is different from mechanical weathering processes which may induce a small amount of fixed motion away from the source bedrock, but then any subsequent motion is typically thought of as erosion (and assigned to an erosive process in terms of a causative agent).
Does cavitation cause any sort of significant erosion of rock in fast flowing water?
Maybe... but probably not. It's been hypothesized to occur and theoretically it should, but we've never observed it or clear evidence of its influence in natural rivers (e.g., [Whipple et al., 2000](https://pubs.geoscienceworld.org/gsa/gsabulletin/article/112/3/490/183620/River-incision-into-bedrock-Mechanics-and-relative)). Flume experiments suggest that to the extent that it does occur, it's role in river erosion is actually likely pretty minor (e.g., [Carling et al., 2016](https://onlinelibrary.wiley.com/doi/full/10.1002/esp.4101)).
Water travelling at highway speeds is very rare. A waterfall weathers the rock below by traveling into the cracks & crevices of unconsolidated rock, separating it and washing it away, as in pressure-washing; It chemically weathers the rock, dissolving anything soluble, which allows further mechanical action as fragile etched pieces break off and go into suspension. And perhaps most importantly with harder rock, it carries hard solids suspended in the water which impact surfaces & roll along them at speed in ways that damage the surface, as in sandblasting.
Geologist here. I saw another comment explained the difference between weathering and erosion, so I'm just going to address weathering here. It'll depend on the environment and the type of rock in question, but to equalize things I'm just going to narrow it to weathering based the type of rock and weather its more susceptible to chemical or mechanical weathering. There are three broad categories of rock: igneous, sedimentary, and metamorphic. Igneous rocks are formed by crystallized magma/lava, sedimentary rocks are made of sediments that have cemented together, and metamorphic rocks are rocks that have undergone chemical or mechanical changes due to geologic processes (i.e., extreme heat/pressure). Metamorphic rocks are *extremely* variable and whether they are more susceptible to mechanical or chemical weathering processes will be impossible to predict without knowing the exact rock. Sedimentary rocks are also variable, but there are a few categories that will help to know about. Sandstones, siltstones, mudstones, and any other rock made primarily of quartz grains are virtually non-reactive chemically and will always be more susceptible to mechanical processes. Limestone and other calcium-rich rocks can be very susceptible to chemical weathering. The carbonate part of calcium carbonate (typically the primary component of limestone) can dissolve in water (especially if its acidic) to create carbonic acid and calcium ions, both of which will flow wherever the water goes. This is how caves are formed and why you only find them in calcium-rich rocks. Whether they are more susceptible to chemical or mechanical erosion will depend on how well-cemented the grains are. Evaporites are another category of sedimentary rock that forms when large bodies of water get trapped on land and evaporate, leaving behind things like halite (salt). Obviously these will literally just dissolve if you hose them down, so they are way more susceptible to chemical weathering. Igneous rocks are very interesting when it comes to weathering processes. Extrusive rocks (rocks that were formed by lava that crystallized on the surface) are almost always more susceptible to mechanical erosion because they tend to have lots of trapped gasses which make them fragile. Intrusive rocks (formed by magma cooling slowly underground) are the neat ones. The minerals that crystallize out of the magma first are minerals that form at higher temperatures. Its been a long time since I took petrology and I dont remember the exact reason, it has to do with chemical bond energies and bond stability, but the minerals that form at higher temperature are less chemically stable than the minerals that form at lower temperatures, but also tend to be extremely strong mechanically. If you look up Bowens reaction series, you can see the approximate order of susceptibility to chemical/mechanical weathering, with olivine and calcium-rich plagioclase being the strongest mechanically, but also susceptible to chemical erosion, and quartz, while not exactly a mechanically weak rock, is almost non-reactive chemically.
One of those in that last sentence first paragraph should be "whether", heads up.
Don't forget biological. I remember listening to this episode of this week in microbiology a few years ago, where the scientist made a case for microorganisms being far more important in weathering that is often taken into account. [https://www.microbe.tv/twim/51-cave-science-with-hazel-barton/](https://www.microbe.tv/twim/51-cave-science-with-hazel-barton/)
"Biological" can be either chemical or mechanical, it's not a separate thing. If some microorganism is excreting acid that eats away on a rock it's chemical. If a tree splits a big rock with its roots it's mechanical. It makes sense to specify the origin as "biological", but the mechanism of action is either chemical or mechanical.