Question 1

Why is the ocean salty?

Accepted Answer

Because rivers have been carrying dissolved minerals into the sea for billions of years, and the salt has nowhere to go.

**The main source is rock weathering.** Rain is slightly acidic — atmospheric carbon dioxide dissolves in it to form weak carbonic acid. As it falls on land and flows over rock, it slowly dissolves minerals, carrying ions including sodium, chloride, calcium and magnesium into rivers and eventually the sea.

**Why the ocean accumulates it and rivers do not.** This is the crucial part. Water leaves the ocean almost exclusively by **evaporation**, and evaporation takes only the water — the dissolved salts stay behind. Rivers are constantly flushed; the ocean is a terminal destination with no outflow. Over geological time, salt concentrates.

**A second source** is **hydrothermal vents** on the ocean floor, where seawater percolates through hot volcanic rock, dissolving minerals and returning them. Underwater volcanism contributes too.

**Why it is not getting steadily saltier.** Ocean salinity has been roughly stable for a very long time, because salts are also **removed** — deposited as sediment, incorporated into marine organisms' shells and skeletons, precipitated in evaporite deposits, and absorbed into new seafloor rock. Input and output are approximately balanced.

**Why sodium chloride dominates.** Different ions have different **residence times** in seawater. Calcium is rapidly taken up by organisms building shells, so it does not accumulate. Sodium and chloride are less readily removed by biological processes, so they build up and account for the overwhelming majority of dissolved salt — around 85%.

**Salinity averages about 3.5%**, or 35 grams per litre, but varies: higher where evaporation is strong and rainfall low, lower near river mouths and melting ice.

**Some inland lakes are far saltier.** The Dead Sea and Great Salt Lake have no outflow at all, so they concentrate salt much further.

Question 2

How does fossilisation actually happen?

Accepted Answer

Rarely, and through several different processes — the important point being that fossilisation requires an unusual sequence of events, which is why the fossil record is a biased sample of past life

Question 3

How do mountains form, and why do they stop growing?

Accepted Answer

Mostly through tectonic collision thickening the crust, and they stop growing because erosion and gravity eventually balance uplift — so a mountain range's height reflects an equilibrium rather than

Question 4

What is plate tectonics?

Accepted Answer

The theory that the Earth's outer shell is divided into rigid plates that move relative to one another over a weaker layer beneath — explaining earthquakes, volcanoes, mountain ranges and the

Question 5

Why does the sea look blue when water is colourless?

Accepted Answer

Two effects combine, and one of them surprises people: water is not actually colourless. Water genuinely absorbs red light. Water molecules absorb light at the red end of the visible spectrum through

Question 6

What causes tides, and why are there two a day?

Accepted Answer

Tides are caused by the difference in gravitational pull across the Earth, primarily from the Moon and secondarily from the Sun. The two-a-day pattern follows from that difference rather than from

Question 7

What is groundwater, and what is an aquifer?

Accepted Answer

Groundwater is water held beneath the surface in the spaces within rock and sediment; an aquifer is a body of rock permeable enough to yield usable quantities of it. Where it sits. Rainfall that

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