Most geological processes are slow. Mountains rise a few millimetres a year. A continent drifts about as fast as your fingernails grow. A river spreads a layer of mud a fraction of a millimetre thick over its floodplain each year. Over a human lifetime these changes look like nothing. Over millions of years they build the Himalaya, open oceans and lay down kilometres of rock.

Geologists call these vast spans deep time. This module is about getting a feel for it, because almost every idea later in the course depends on it.

Numbers like "4.54 billion years" are too big to picture, so here is a trick: squeeze all of Earth's history into a single 24-hour day. The Earth forms at midnight and "now" is the next midnight. On this clock one hour is about 190 million years, one minute about 3 million years and one second about 52,000 years.

Geologists split that history into four eons, the largest named units of geological time: the Hadean, Archaean, Proterozoic and Phanerozoic. The coloured bands on the clock are the eons. Notice how much of the day passes before animals appear, and how crowded the last hour is: the lower bar enlarges it.

Three things stand out. First, for nearly the whole day life is microbial: animals with shells and skeletons arrive only after 9 pm. Second, the events that shaped India's modern landscape all crowd into the last hour. The Rajmahal Traps and then the Deccan Traps, huge outpourings of basalt lava, erupt at about 23:23 and 23:39, and India collides with Asia at about 23:44. Third, our own species appears about six seconds before midnight, and all of recorded history fits into the last tenth of a second.

The other half of deep time is what slow processes can do with it. The arithmetic is simple, distance = rate × time, but our intuition is not. A plate moving 5 cm a year covers 50 km in a million years and 500 km in ten million, which is how India crossed an ocean. Pick a process and a span of time below and compare the answer with things you know.

This is why geologists are relaxed about very slow rates. A layer of mud 0.2 mm thick a year sounds like nothing, yet a basin that keeps subsiding for 10 million years can fill with 2 km of it. Many of the rocks in this course are the sums of tiny yearly increments.

The same slowness applies to weathering, the chemical breakdown of rock by water and air. Turning bedrock into soil happens at roughly a few centimetres per thousand years. That pace matters for the climate, as the box at the end of the module explains.

Clock times like "23:39" are only as good as the ages behind them. Most of those ages come from radiometric dating. Some atoms are unstable: over time they turn into a different element, a process called radioactive decay. The original atom is the parent and what it becomes is the daughter. Each kind of parent decays at its own fixed rate, described by its half-life: the time it takes for half of the parent atoms to decay. Heat, pressure and chemistry don't change it.

Uranium-238, for example, decays (through a chain of steps) to lead-206 with a half-life of about 4.5 billion years, roughly the age of the Earth. That makes it ideal for old rocks. A favourite target is zircon, a tough little crystal that takes in uranium but almost no lead when it forms, so nearly all the lead inside it was made by decay. The oldest known zircons, from Western Australia, are about 4.4 billion years old. The 4.54-billion-year age of the Earth itself comes from meteorites, leftovers from when the planets formed, dated in the same way. Radiocarbon, with a half-life of about 5,700 years, is only useful for the last ~50,000 years, which is why it plays almost no part in dating rocks.

Radiometric ages give numbers, but the order of events comes from reading the rocks themselves. That is the subject of Stratigraphy.

How could anyone have guessed the Earth was old before radiometric dating existed? In the 1780s the Scottish farmer and naturalist James Hutton watched rivers carry sand to the sea, and reasoned that the sandstones in the hills must have formed the same slow way. In 1788 he sailed to Siccar Point, on the coast east of Edinburgh. There, nearly vertical layers of old sedimentary rock are cut off by an eroded surface, with gently tilted red sandstone lying on top. To make that, the lower layers had to be laid down flat, buried, turned into rock, tilted upright, lifted and worn flat, before the upper layers were laid on top. Each of those steps was slow, so the whole sequence needed an immense span of time. His friend John Playfair wrote that "the mind seemed to grow giddy by looking so far into the abyss of time". The buried erosion surface is an unconformity, which the Intrusions & unconformities module explains in full.

This idea is called uniformitarianism, and it is the foundation of everything in this course. When we look at a rock, we're asking which ordinary process, repeated for long enough, could have made it.

Modern geologists apply it with two caveats. First, the laws of physics and chemistry stay the same, but the rates and conditions don't. Most of the time change is slow, but now and then something sudden happens: an asteroid impact, or the eruption of a flood basalt like the Deccan Traps, can reshape the planet in a geological instant. Second, the early Earth was a different place. It was hotter inside, its air had almost no oxygen for its first two billion years, and there were no land plants until about 470 million years ago, so rocks weathered and soils formed differently. The present is a key to the past, but not a photograph of it.