Question

(a)
Part A
Based on the information in the stimulus and Map 1, which statements best explain why opal is found in places such as Australia?
​
Select the two correct answers.​
A
Australia has large amounts of sandstone and silica.
B
Opal can only be formed where an ocean comes in contact with land.
C
An inland sea once covered central to eastern Australia.
D
Australia has a very hot, desertlike climate in most areas.
E
Australia has very few active volcanoes across the continent.
(b)

Part B
Which statement best explains how the conditions identified in Part A affect the availability of opal?​
A
Weathering processes no longer occur in Australia, so no new opal is formed.
B
Gems do not need an exact combination of elements or minerals to form, so opal forms more easily.
C
Opal formation occurs over large periods of time, so the number of opal deposits is limited.
D
Volcanoes in Australia no longer provide high temperatures and pressures, so no new opal is formed.
Use the information about opal and your knowledge of science to answer the questions.
Opal
Earth processes form many different types of minerals and gems. One example is a mineral-like substance known as opal. Opal can be found in different areas across the Earth, such as Louisiana, Nevada, and Mexico. Australia is estimated to produce as much as 90 percent of the world's opal supply. Map 1 shows some of the major opal deposits in Australia.





The process of mineral and gem formation, shown in Figure 1, can help explain how opal is distributed. When sandstone is weathered, it releases grains of sand. Water can then move the sand and other minerals into the cracks formed in other types of rock along the way. When the water evaporates, sand and other materials are left behind. This mixture of sand and other minerals hardens over time, eventually forming opal.





There are several environmental concerns to consider when mining opal. Some opal mines use equipment that releases harmful substances into the soil, air, and water. These machines can also heavily compact the soil in these mining areas, which can affect how the soil absorbs water. In some places, large amounts of material are excavated from an open pit in order to recover minerals available in small concentrations. The small concentration increases the amount of material that must be excavated, damaging the landscape for many years, even after a mine has shut down.

Answer 1

### Part A
The two statements that best explain why opal is found in places such as Australia are:

**A**: Australia has large amounts of sandstone and silica.
**C**: An inland sea once covered central to eastern Australia.

### Part B
The statement that best explains how the conditions identified in Part A affect the availability of opal is:

**C**: Opal formation occurs over large periods of time, so the number of opal deposits is limited.