On a field trip, a student finds
three different rocks. They note the appearance and
structure of each rock and record their observations when a small amount of
hydrochloric acid is added. Which of the rocks is likely to be
a sedimentary rock?
Sedimentary rocks are formed from
the deposition, accumulation, and solidification of small mineral or organic
particles. When looking at a sedimentary rock,
we can often see the individual layers that have been deposited and solidified
together. Sedimentary rocks tend to be softer
than igneous or metamorphic rocks. So, it seems that rock A might be a
sedimentary rock. But let’s take a closer look at the
descriptions and observations the student recorded about each rock.
One of the key observations about
rock C is that it contains small holes. Rocks that contain small holes are
typically igneous volcanic rocks. Igneous volcanic rocks are formed
when lava above the Earth’s surface quickly cools. Gases that were trapped in the
magma below the Earth’s surface are released as the lava cools, leaving small holes
in the rock. So, rock C, which is very hard and
contain small holes, does not appear to be a sedimentary rock.
Both rock A and rock B react with
hydrochloric acid, producing an effervescence or bubbles. The mineral responsible for this
reaction is calcite. Calcite is the crystalline form of
calcium carbonate. Calcium carbonate reacts with
hydrochloric acid to produce calcium chloride, carbon dioxide gas, and water. We see the carbon dioxide being
produced as effervescence or bubbles.
There are two common rocks that are
a white color and contain the mineral calcite. Limestone is a soft, smooth, white
rock that contains calcite. When limestone is exposed to high
temperature and high pressure, it can be transformed into marble, a hard, white rock
with a rough texture. Limestone is a sedimentary
rock. Marble is a rock formed from the
conversion of existing rock. So, marble is a metamorphic
From our discussion, we now know
that the rock, which is likely to be a sedimentary rock, is rock A.