Rocks and the Rock Cycle

Rocks and minerals

People often use the words rock and mineral as if they mean the same thing. To a geologist, they do not.

• MineralA naturally occurring, solid, inorganic substance with a definite chemical makeup and an orderly crystal structure. Quartz, feldspar, mica, and calcite are minerals.
• RockA solid mixture of one or more minerals (and sometimes other natural material, such as bits of shells or plants). Granite, for example, is made mostly of quartz, feldspar, and mica.

Geologists sort rocks into three big families by how they formed, not by color or size. That is useful, because how a rock formed tells you where it has been.

It is tempting to read the diagram like a clock: igneous, then sedimentary, then metamorphic, then around again. Real rocks do not follow a fixed order. Any rock can take any path that its surroundings allow:

• A granite can be lifted to the surface and weathered into sand without ever becoming metamorphic.
• A sandstone can be buried and squeezed into quartzite, then lifted up and weathered into sand again, never melting at all.
• A metamorphic rock can be squeezed even harder and turn into a different metamorphic rock, like slate becoming schist.
• A rock can sit in one place, unchanged, for hundreds of millions of years.

What decides the path is where the rock is. Rock at the surface meets air, water, ice, and living things, so it gets weathered and eroded. Rock buried deep gets hot and squeezed, so it changes or melts. Uplift and burial move rock between those two worlds.

James Hutton at Siccar Point

In June 1788, the Scottish geologist James Hutton went by boat along the coast east of Edinburgh with his friends John Playfair and Sir James Hall. Hutton had already found similar places on the Isle of Arran and near Jedburgh, and he was hunting for a better one. At a rocky headland called Siccar Point, he found it.

At the bottom were layers of gray rock called greywacke, standing almost straight up. Lying across their worn-down tops were gently tilted layers of red sandstone. A surface like this, where one set of rock layers sits on the eroded edges of much older ones, is called an unconformity. In Hutton's own words: "at Siccar Point, we found a beautiful picture of this junction washed bare by the sea."

Hutton reasoned it out step by step. The gray layers must have formed as flat layers of sediment. Then they were hardened, tipped up on end, lifted, and worn down. Only after all that was the red sandstone laid down on top. Today geologists estimate that the gap between the two sets of rock is tens of millions of years. Playfair later wrote that "the mind seemed to grow giddy by looking so far into the abyss of time."

Crystal size: a cooling clock

How can a geologist tell whether an igneous rock cooled underground or at the surface, millions of years after it happened? Look at the crystals. Magma that cools slowly, deep underground, gives its minerals time to grow, so granite has big crystals you can see. Lava that cools quickly at the surface makes basalt with tiny crystals. Lava that cools almost instantly makes obsidian, a glass with no crystals at all. So crystal size works like a clock that recorded how fast the rock cooled, and that tells you where it formed.

Fossils in sedimentary rock

Fossils are found almost only in sedimentary rocks. That makes sense once you know how they form: an animal or plant is buried by sediment, and the sediment hardens into rock around it. Magma would destroy remains, and the heat and pressure that make metamorphic rock usually destroy fossils too. So when you find a seashell fossil in a rock on top of a mountain, it tells you that the rock formed under the sea and was later lifted up.

In a stack of sedimentary layers that has not been flipped over, the layers at the bottom were laid down first. That lets geologists put rocks, and the fossils inside them, in order from oldest to youngest.

Mary Anning at Lyme Regis

Mary Anning was born in 1799 in Lyme Regis, a seaside town on the south coast of England. The cliffs there are made of layers of limestone and shale, about 200 million years old, from the Early Jurassic period. Her father, a cabinetmaker, collected fossils on the beach and sold them from his shop, and he taught Mary to find and clean them. After he died in 1810, Mary sold her finds to help pay the family's debts.

In 1811, when Mary was 12, her brother Joseph found a strange-looking fossil skull. About a year later, in 1812, Mary carefully dug out the rest of its skeleton, about 5 meters long. Scientists first thought it was a crocodile. It turned out to be an ichthyosaur, an extinct sea reptile. Ichthyosaur bones had turned up before: in 1699 Edward Lhuyd published drawings of some, though he thought they came from a fish. The Annings' find was the first ichthyosaur skeleton to come to the attention of scientists in London, and Everard Home described it in 1814. In 1823 she found the first complete skeleton of a plesiosaur, a reptile with a long neck and flippers. It looked so odd that the famous French scientist Georges Cuvier at first thought it might be a fake. It was real.

Mary also found a pterosaur, a flying reptile, and studied coprolites: fossil poo. Her finds helped convince scientists that Earth is very old and that whole groups of animals can go extinct. Even so, the scientists who bought her fossils often left her name out of their papers, and the Geological Society of London would not let her join. It did not elect its first women Fellows until 1919, more than 70 years after she died.

Credits

Written by Mary, Jugami's Earth science teacher. The specimen drawings, the rock cycle diagram, and the field-notebook illustrations are original, hand-coded SVG made for Jugami. The texture drawings are simplified sketches, not photographs, so real specimens will look different.