In essays, it is very tempting to put too much into one sentence. Phrasing becomes muddled when a sentence is too long. A very good essay sentence can often contain only one thought: Mid-ocean ridges are a series of mountain ranges. Often, however, you want to say something more. For example, we may want to add something about how mid-ocean ridges are formed, how big they are and where to find them. We do this through inserting what are known as subordinate (or dependent) clauses. Subordinate clauses are those parts of the sentence, often banked by commas, that add additional information to things mentioned in the sentence.

For example:

Mid-ocean ridges, which stretch for a total distance of over 40,000 kilometres, are a series of mountain ranges.

The sentence Mid-ocean ridges are a series of mountain ranges makes perfect sense without the subordinate clause which stretch for a total distance of over 40,000 kilometres. The subordinate clause just makes the sentence more interesting.

When more than one subordinate clause is introduced, the sentence becomes more complex, and care must be taken to make sure the reader can follow it.

Mid-ocean ridges, which are the longest series of mountain ranges on earth, stretching for a total distance of over 40,000 kilometres, are found under Indian, Antarctic and Pacific Oceans, the Norwegian Sea and the Arctic Basin.

We can follow every section of this sentence because of the commas. By separating out the subordinate clauses, the commas guide the reader through the sentence, making it very easy to read.

Often, however, even with commas, a sentence containing too many subordinate clauses becomes long and unwieldy. Try the following:

Mid-ocean ridges, formed by the separation of two oceanic plates, by the process of the sea floor spreading at divergent or spreading boundaries, something discovered by extensive exploration of the sea floor after World War II, and important because, as the plates move apart, new oceanic lithosphere is formed along those divergent boundaries, are the longest series of mountain ranges on earth, stretching for a total distance of over 40,000 kilometres, and are found under the Atlantic, Indian, Antarctic and Pacific Oceans, the Norwegian Sea and the Arctic Basin, with the crack-like valley, or rift, running down the centre of the Mid-Atlantic Ridge, the place in which nearly all earthquakes in the Atlantic occur, with similar activity taking place in mid-ocean ranges with similar rifts, for example in the Pacific and Indian Oceans.

This sentence is horrible to read. There are quite a few ways to divide it up so that it is easier to follow. The following is a seven sentence solution. Obviously, in an assignment, you would turn this into a paragraph and not a series of bullet points. I've put the bullet points in for clarity.

Mid-ocean ridges are formed by the separation of two oceanic plates.
The process involves the sea floor spreading at divergent or spreading boundaries, something discovered by extensive exploration of the sea floor after World War II.
The process is important because, as the plates move apart, new oceanic lithosphere is formed along those divergent boundaries, resulting in the formation of the longest series of mountain ranges on earth.
These stretch for a total distance of over 40,000 kilometres, and are found under the Atlantic, Indian, Antarctic and Pacific Oceans, the Norwegian Sea and the Arctic Basin.
Running down the centre of the Mid-Atlantic Ridge is a deep, crack-like valley or rift.
It is here that nearly all earthquakes in the Atlantic occur.
In other mid-ocean ridges with similar rifts, for example the Pacific and Indian Oceans, earthquake activity is also recorded.

Long sentences become even more muddling if the subject becomes separated from its subordinate clause. Try this:

Mid-ocean ridges, important because, as the plates move apart, new oceanic lithosphere is formed along those divergent boundaries, stretching for a total distance of over 40,000 kilometres, are found under the Atlantic, Indian, Antarctic and Pacific Oceans, the Norwegian Sea and the Arctic Basin, discovered by extensive exploration of the sea floor after World War II, formed by the separation of two oceanic plates, by the process of the sea floor spreading at divergent or spreading boundaries.

What stretches for 40,000 kilmeteres? The ridges, the lithosphere or the boundaries?
Were the oceans, the Norwegian Sea and Arctic Basin discovered by extensive exploration of the sea floor after World War II?
Was World War II formed by the separation of two oceanic plates?

Ambiguity

Sometimes we write sentences that are very easily misunderstood.

These tools are dangerous for children - they are too small for them.
If the atmosphere is too cold to support life, it will eventually disappear.
After Professor Noakes told his student about the experiment, he wished he hadn't.

What is too small? The tools or the children?

What will disappear? The atmosphere or life?

Who wished he hadn't? Professor Noakes or his student?

Ambiguity occurs when you have two items - the tools, the children, the atmosphere, life, Professor Noakes, his student - followed by a pronoun (pronouns are him, her, they, I, it - words standing in place of the person or item's name) that could be standing in for either item/person.

Tips:

When you read Earth Science text books or papers, note how they are written as well as their content. Most are written very clearly. Try and make your text similarly clear.
Always ask yourself: “what exactly am I trying to say?’
When you've decided, write it down in the simplest possible way.
If you find your sentence getting long and convoluted, stop writing. Say what you are trying to say out loud, as if to a friend. See if that helps clarify your thought. Then write the thought down in short sentences.