Observe rock textures

Whole rock or grains?

The size of a whole rock is different from the size of the grains within it. Texture describes grain size, shape and arrangement. A visible grain is not an individual atom. Not every rock shows grains at the scale being viewed

1. 1Size How large are the individual grains? A large specimen can contain small grains
2. 2Shape Are grain outlines rounded, angular or elongated? Do not use the outline of the whole specimen
3. 3Arrangement How do the grains meet, and do many share a preferred direction?

Describe what is shown

A record saying grains are about 2 mm across, rounded and arranged in varied directions describes texture. Saying the whole specimen weighs 90 g describes a different property. The models ahead show selected patches, not whole rocks or photographs

1. 1Rounded fragments Model A has rounded fragments surrounded by finer material. That background is material, not empty space
2. 2Interlocking grains In B and C, irregular grain outlines fit together along shared boundaries
3. 3Preferred direction Many elongated grains in D run mainly left to right. A preferred direction does not require every edge to be exactly parallel

Describe the outlines of the larger fragments within Model A, rather than the rectangular edge of the drawing

A view has a scale

The field-width label tells you how wide the represented patch is. A bar labelled 3 mm represents that distance within the model, not a promise that the bar measures 3 mm on your screen. Shading helps separate grains; it does not identify minerals

1. 1Compare like views At the same represented scale, smaller grain outlines mean smaller modelled grains. B and C whole-patch views each show 12 mm across
2. 2Enlarge the centre The centre view shows a 6 mm-wide part of the same 12 mm patch in the same display space. Its grains look larger, but the rock has not changed
3. 3Use the locator The dashed rectangle on the small whole-patch picture shows the region displayed in the main view

Predict what happens to the grains themselves when the centre of this model is displayed at a larger scale

Keep Whole patch selected and compare B with C: both show 12 mm across. Hold C fixed and switch between Whole patch and Centre enlarged; follow the central grains and the 3 mm bar. Return to Whole patch, then compare the grain directions in B and D

What does changing only the View control do?

Observation is not the whole explanation

These models supply texture observations. They do not supply a rock's exact mineral identities, formation history or age. Interlocking or aligned grains alone must not be treated as a complete rock-identification rule

1. 1Drawing shades The colours separate regions in the model. They are not mineral tests or measurements of natural colour
2. 2Water flow A two-dimensional texture drawing does not measure connected pores or fractures. Model A's fine background proves neither free water flow nor complete blockage
3. 3Next evidence A wider property or origin claim needs relevant observations or tests and later scientific knowledge, not just a stronger-sounding description

A limited claim can be supported

For Model D, many elongated grains run mainly left to right is a supported record. It describes the visible arrangement and allows individual edges to differ. You do not need to know the rock's age before recording that observation

1. 1Keep the observation Grain direction is visible in the supplied model
2. 2Do not add a history The same picture does not establish an exact age or unique formation process. Supported observation and unsupported inference can occur together

From grains to a careful record

Describe grain size, shape and arrangement. Check the represented scale before comparing sizes. State what the observation supports without inventing mineral identities, formation, age or water-flow results