8. Human-made rocks

As minerals have evolved, so have the rocks that they make up. ‘Human-made rocks’ explains that we are entering a remarkable new phase of history that is of planetary—and perhaps wider—importance. Humans have begun to modify the chemistry and mineralogy of the Earth’s surface, and this has included the manufacture of many new types of mineral, mineral compounds, and mineraloids such as extracting metals from ores and creating plastics and glass. Humans have created new rock types including bricks, ceramics, cement, and concrete. These, along with our subterranean activities and the potential ‘technofossils’ of all that makes up our towns and cities, will change the Earth’s rocks forever.

3. Sedimentary rocks

The Earth has the greatest abundance and variety of sedimentary strata in our star system, dating back some 3.8 billion years. ‘Sedimentary rocks’ describes the processes that create different types of sedimentary rocks, which form a key guideline in reconstructing Earth’s history. Particles are created by both physical and chemical weathering. These particles are then moved by different modes of transport and sorted by size, which affect the resulting sedimentary rock. Sedimentary layers can remain as soft muds or loose sands for millions of years, but most buried sedimentary layers harden and turn into rock, under the combined effects of increasing heat and pressure, and of changes in chemical environment.

2. First rocks on a dead Earth

‘First rocks on a dead Earth’ describes the formation of the planet Earth from the collision of the precursor planets Tellus and Theia. The surface of the newly born Earth had a surface magma ocean. As this magma cooled, the first minerals formed. The earliest rocks on Earth date back to the Archaeon Eon. During that time, plate tectonics started up, which determined the nature of all subsequent rocks on Earth. The processes of fractional melting and impact of cooling rate on crystal sizes is explained along with the different types of igneous rocks—basalts, andesites, diorites, rhyolites, and granites—formed at mid-ocean ridges, subduction zones, and plate collision zones.

7. Rocks on other planets

Rocks are not just an Earthly phenomenon. They make up the surfaces of some of the planets of the solar system and, in one form or another, those all of the many moons, innumerable comets, and other small objects that orbit out to great distances from the Sun. They likely also lie deep beneath the immensely thick fluid envelopes of the gas giants. ‘Rocks on other planets’ describes what scientists have discovered about the rocks of Mercury, Venus, Mars, our moon, the giant gas planets, and distant moons and planets. The early exploration of exoplanets suggests that an even wider variety of rock formations likely exists on star systems other than ours.

5. Rocks in the deep

It is over 6,000 km to the centre of the Earth, but our direct experience of its rocks goes to little more than 3 km below the surface in the deepest mines on Earth. ‘Rocks in the deep’ shows that we can find out more by assessing rock fragments brought from deeper levels by tectonic or volcanic processes; by analysing patterns of change in the gravitational and magnetic fields; by detecting seismic waves that have travelled through the Earth; or by recreating conditions of the deep Earth in the laboratory. It describes what is known about the lower crust, the upper mantle, the deep mantle, and the core.

4. Rocks transformed

‘Rocks transformed’ outlines the processes of metamorphism and describes the different types of metamorphism: regional, contact, and impact. Regional metamorphism is the most common form and occurs in mountain belt zones where the crust is much thicker. High pressures and temperatures result in recrystallization in the rocks. As temperatures and pressures increase, the new crystals that form are bigger. The original chemical composition of the rocks affects the resulting metamorphic rocks. Muds become slates and mica-schists, while limestones become marbles. Contact metamorphism takes place at the boundaries of magma bodies and impact metamorphism is seen when meteorites crash into the Earth’s surface.

6. Living rocks, evolving rocks

Life on Earth has been dated back to 3.8 billion years ago with organic carbon found in metamorphosed mudstones, but new evidence suggests it may go back even further. ‘Living rocks, evolving rocks’ describes the earliest rocks that show evidence of life, initially in anoxic conditions, such as stromatolites and banded iron formations. The Precambrian also saw widespread microbial mats growing across the seafloor. Around 550 million years ago there was a ‘Cambrian Explosion’ of metazoan marine organisms, including reef-builders and plankton rain, the consequences of which can be seen clearly in the rocks. Land-living organisms evolved in the Silurian and Devonian periods resulting in peat bogs and bone beds.

1. Primordial rocks

A rock is a piece of solid matter made up of minerals or with a composition that is fixed within certain limits. ‘Primordial rocks’ considers the first minerals that appeared some millions of years after the Big Bang, during the dramatic finale of supernovae. These comprised: carbon in the form of diamond; a few carbides and nitrides; a few oxides including corundum; and a form of olivine, a member of the silicate minerals that dominate the surface of Earth. The first rocks—asteroids—were a result of a second generation of stars. Remnants from the primordial planetary building processes are seen as meteorites, which are divided into ‘stony’ achondrites and ‘iron-rich’ chondrites.