Soil Composition and infiltration rates

When a drop of water falls from the sky, it can either fall down to the ground or get intercepted by vegetation and other non-natural materials. If it gets intercepted by, lets say, a leaf, the drop of water can either get taken up by animals / plants, get evaporated back into the air or can flow downwards by drip flow or stem flow.

If it doesn't get intercepted, it will hit the ground. If it hits an impermeable surface, the same things can happen to it as it would if it lands on a leaf. 

But if it falls to the soil, the water drop will most likely be infiltrated into the ground. As it goes below the water table, it percolates the rock layer below the soil. But what are the situations where the droplet DOES NOT infiltrate the ground ? 

1. Saturated ground - this means the soil's air spaces between particles, which is what makes it porous and "permeable", is filled up with water from the rain. Every space is taken up, so no more water can enter and fill spaces. 
2. Infiltration rate is slower than the rate of rainfall - Infiltration rate is the speed at which water permeates the ground and enters the air spaces in the material. This rate is largely dependent on the type of soil in that area.

The diagram shows how soils can be categorized according to what type of composition they have. Soils are made from clay, sand and silt, which can be found in different degrees in each type of landscapes.

• Clay (slowest infiltration rate) - this type of particle is the smallest of the 3, usually 1/2000th mm thick. They have small air spaces and less spaced out air gaps; particles are very close together so water will infiltrate slower. There's also strong electrical bonds between each particle which adds to it's cohesion. 
• Silt (Medium infiltration rate) - 1/2000 - 1/200th mm thick, medium density, medium porosity. 
• Sand (High infiltration rate) - Sand has the fastest infiltration rate out of all 3 materials. The particles are often larger, 1/20 - 1mm thick and are large, round and spaced out. 

Hence, if the soil is mostly clay, and there is great rainfall plus a steep gradient, then most of the water will become surface overland flow, instead of being infiltrated into the ground. Gradient matters, as if the gradient is high, gravity pulls the rain downwards faster than if it was a gentle, shallow gradient.

How to Date Rocks

How do Geographers date rocks ? Give them flowers!

Jokes aside, the way Geologists date rocks is not too complicated. It all stemmed from Albert Einstein's theory that radioactivity is the energy for which chemical energy is emitted from any substance, rock or soil over time (energy change).

1. Collect a sample - if you have a large rock segment, cut a piece out or gather some smaller rocks from the environment. This will mean one can easily transport the rock to the rock laboratory. 
2. Using specialised equipment in the rock laboratory, the rock sample can be crushed and made into a power form, which has the consistency of sugar. 
3. The main thing that determines how old the rock is, is it's mineral contents. How much minerals is embedded in the rock tells us how old it is because of Einstein's theory of radioactivity. 
4. Scientists have to determine the amount of Lead and Uranium in the sample. Uranium radioactively converts into Lead over time as it decays and reaches "half life". To do that, a mass spectrometer must be used to separate out the large Uranium particles and the small, tiny Lead particles that the former disintegrates into. 
5. We know the rate of decay for Uranium (the total time). Therefore, the amount of uranium to lead can be proportioned (in a ratio) and this can tell us how old the rock is. 

Dating rocks is all about proportion of Uranium to Lead particles in the sample. Rocks form as sand and soils deposit layer by layer, over many millions of years. If any pebbles are deposited over it, they get buried on top of more layers of sand, silt and soil until gradually all we see is the top layer of young grass growing on a hill.

If asked to determine how old a rock is just by looking - this is what you could do:

1. Spot any fossils embedded in the rock. These tell you which time era the rock was formed in, and if you're good at paleontology, then the rest is quite simple. 
2. Categorize the rock type - is it sedimentary, igneous rock or metamorphic rock ? This can tell you generally the age of the rock and how it is formed.