The Three Rock Families

A rock is a mixture of minerals grown or glued together. Geologists sort every rock on Earth into just three families by asking one question. How did it form? Igneous rocks crystallized from molten material. Sedimentary rocks were assembled at the surface from pieces of older rocks, dissolved minerals, or the remains of living things. Metamorphic rocks are older rocks changed by heat and pressure without ever melting.

You do not have to memorize the family for every specimen. The family shows up in the rock's texture, which means the size, shape, and arrangement of what the rock is made of. Interlocking crystals point to one family, cemented grains to another, and aligned bands to a third. Learn the three textures and you can pick up a rock you have never seen and work out how it formed. That skill, not memorization, is what this chapter and the exam are after.

Igneous rocks form when molten rock cools and crystallizes. The most useful fact in this chapter is that crystal size tells you the cooling speed. Magma that cools slowly deep underground, insulated by the rock around it, gives its atoms time to organize into large crystals you can see. That produces an intrusive igneous rock like granite, which is a coarse mix of pink feldspar, gray quartz, and dark mica. Lava that erupts and cools quickly at the surface hardens before large crystals can grow. That produces an extrusive rock like basalt, whose crystals are too small to see without a microscope. Cool lava fast enough and you get no crystals at all. Obsidian is natural glass. If escaping gas froths the lava first, you get pumice, a rock so full of holes that it floats. Scoria is the dark, iron-rich version, full of the same gas holes but heavier, so it sinks.

Composition is the second thing to check, and you already know it from the volcanoes chapter. On one side are felsic rocks, which are light-colored and high in silica, such as granite and rhyolite. On the other are mafic rocks, which are dark and dense, such as basalt and gabbro. Cooling speed sets the crystal size. Composition sets the color and density. Those two properties are exactly how the Mineral Composition of Igneous Rocks chart on page 14 is laid out.

Sedimentary rocks are how the surface recycles older rock. Weathering breaks older rocks into fragments, and water and wind carry those fragments away. They settle in layers wherever the energy runs out, in places like river deltas, lake bottoms, and sea floors. Bury the layers and squeeze the water out, which is called compaction. Then let dissolved minerals glue the grains together, which is called cementation. Now the sediment is rock. These clastic rocks are named by particle size. Microscopic clay makes shale, sand makes sandstone, and rounded pebbles make conglomerate. The 2026 Reference Tables no longer print a sedimentary identification scheme, so know these names by heart. The rock cycle infographic on page 15 does show where each one forms.

Two other paths lead to the same family. Chemical sedimentary rocks form straight out of water, the way rock salt and gypsum crystallized as ancient seas evaporated. Organic sedimentary rocks are built from living things. Most limestone is compacted shells and coral, and coal is compressed plant material. Because all of this happens gently at the surface, sedimentary rocks are the only family that preserves fossils. They also form flat layers called strata, which makes them easy to recognize from across a valley. Nearly all of Earth's history is read from these layers.

Take any rock, bury it deep in a mountain-building collision, and heat it under enormous pressure without quite melting it. The minerals recrystallize, grow, and rotate until they line up at right angles to the squeezing force. The result is a metamorphic rock, and that alignment is called foliation. You can see it in the parallel mica flakes that make schist glitter, and in the bold light and dark banding of gneiss. Metamorphism that happens across whole regions during collisions is called regional metamorphism. The narrow baked zone around a magma intrusion is called contact metamorphism.

Every metamorphic rock has a parent rock, and the exam asks about those pairs often. Shale hardens into slate. Sandstone fuses into quartzite, so tightly that it breaks through its own grains rather than around them. Limestone recrystallizes into marble, which still fizzes in acid but has lost its fossils. Heat any of them further and they head toward schist and gneiss. Metamorphic rocks record the pressures that build mountains. That is why New York is full of them, as the next section shows.

A specimen is on the table with three field observations. Name its family. The rock name and an explanation appear after you answer, and a streak counter keeps track of how you are doing.

The 2026 Reference Tables handle rocks differently from the older edition. Page 14 carries the Mineral Composition of Igneous Rocks chart and Bowen’s Reaction Series. Page 15 is a Rock Cycle Infographic that names dozens of rocks and shows where each one forms. The igneous chart on page 6 runs cooling environment vertically, with extrusive on top and intrusive below, so crystal size grows as you go down. Composition runs horizontally, from felsic on the left through mafic on the right, with the mineral percentage bands underneath. The sedimentary scheme on page 7 splits by origin. Clastic rocks are sorted by particle size, followed by the chemically and organically formed rocks. The metamorphic scheme sorts by foliated versus nonfoliated, then by grain size, and its right-hand column names each rock's parent.

Practice the standard exam moves. Given a crystal size and composition, name the igneous rock. Given a sediment size, name the clastic rock it becomes. Given a parent rock, name its metamorphic product. Every one of those answers is sitting on the open chart, waiting for someone who knows its layout.

The state is a three-family sampler. The Palisades are the great cliff face across the Hudson from Westchester. They are the edge of a thick sheet of dark igneous rock that squeezed between sedimentary layers as Pangaea ripped apart. The Catskills and most of western New York are stacked sedimentary strata. This enormous pile of ancient sea floors and river deltas is full of fossils and quarried as bluestone. And the basement under it all is metamorphic. It includes the gneiss of the Hudson Highlands and the schist that Manhattan's skyscrapers anchor into. The Adirondacks are a dome of deeply cooked metamorphic rock more than a billion years old. Every family, one state, and the bedrock map on page 8 lets you trace all of it.

• Mineral Composition of Igneous Rocks and Bowen’s Reaction Series (page 14). Crystal size vertically, composition horizontally, mineral bands below. Find granite and basalt first; they anchor the whole chart.
• Schemes for Sedimentary and Metamorphic Rock Identification (page 7). Clastic rocks by particle size; metamorphic rocks by foliation, with the parent rock column on the right.
• Generalized Bedrock Geology of New York State (pages 8 and 9). Trace the three families across the state, from the Palisades sill to the Adirondack dome.
• Mineral Identification Flowchart (pages 16 and 17). The ingredients list from the last chapter, still open on your desk.

Download the current tables from NYSED (2026 Revised Edition, used from the 2026-27 school year): Reference Tables for Earth and Space Sciences.

• Family follows formation: igneous rocks crystallize from melt, sedimentary rocks assemble at the surface, metamorphic rocks are transformed by heat and pressure without melting.
• In igneous rocks, crystal size records cooling speed: intrusive means slow and coarse (granite), extrusive means fast and fine (basalt).
• Clastic sedimentary rocks are named by particle size: shale, sandstone, conglomerate. Chemical and organic paths add rock salt, limestone, and coal.
• Only sedimentary rocks preserve fossils and strata.
• Foliation, the banding and alignment of minerals, is the metamorphic signature, and every metamorphic rock has a parent: shale to slate, sandstone to quartzite, limestone to marble.
• Texture, not memorization, identifies the family, and the charts on pages 14 and 15 do the rest.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.