Grade 6 Science Practice Test: Plate Tectonics, the Rock Cycle and the Water Cycle

Matching fossils of a land or freshwater animal on continents now separated by an ocean suggest the land was once connected and has since drifted apart. Even a shallower ocean would still be salt water that a freshwater reptile could not cross.

Heat from deep inside Earth makes mantle rock flow very slowly in convection currents, and this movement, along with the sinking of cold, dense plates, moves the plates. Tides and winds act only on water and air at the surface.

Plates that move apart form a divergent boundary, where new crust is made. At convergent boundaries (including subduction zones) plates move toward each other, and at transform boundaries they slide past each other.

The rock is youngest at the ridge and gets steadily older with distance on both sides, which is the pattern of seafloor spreading: crust forms at the ridge and is carried outward. Sinking crust happens at trenches, not at ridges.

The rock moved 50 km every 2 million years, so in 1 million years it moved 50 ÷ 2 = 25 km. 50 km is the distance for 2 million years, not 1.

Where an ocean plate sinks (subduction), a deep trench forms at the bend, and melting rock rises to build volcanoes on the continent, as in the Andes. A ridge and rift valley form where plates move apart.

Continental crust is too thick and light to sink far into the mantle, so when two continents collide the rock is squeezed, folded and pushed up into high mountains. Plates sliding past each other form a transform boundary, which does not build tall ranges.

The San Andreas Fault is a transform boundary. The plates stick, build up stress and then slip suddenly, causing earthquakes. Crust is neither made nor destroyed there, so trenches and new ocean floor do not form.

Weathering, erosion, deposition, compaction and cementation form sedimentary rock, such as sandstone. Igneous rock forms from cooled magma or lava, and metamorphic rock forms when existing rock is changed by heat and pressure.

Crystal size depends on cooling time: magma that cools slowly deep underground has time to grow large crystals, while lava that cools fast at the surface forms tiny crystals. Rock made from cemented grains is sedimentary, not igneous.

When heat and pressure change a rock without melting it, the result is metamorphic rock; marble is metamorphosed limestone. If the rock had melted and then cooled, it would form igneous rock.

Any rock can become igneous rock, but only by melting into magma and then cooling into solid rock. Heating and squeezing without melting would make a metamorphic rock instead.

Water vapor in the warm air touches the cold glass, cools and condenses into droplets, the same process that forms clouds and dew. The water does not pass through the glass; it comes from the air.

Plants take up water through their roots and release water vapor through tiny openings in their leaves. This is transpiration, and the vapor condensed inside the bag. The bag was added in the morning on a sunny day, so frost or rain cannot explain the drops.

Soil lets rainwater infiltrate and become groundwater, but pavement blocks it, so the water runs off the surface into drains. The same amount of rain falls on both areas; the difference is what happens after it lands.