Why do the continents look like they fit together?

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Across
  1. 1. TO BREAK INTO PIECES.
  2. 6. VOLCANIC MATERIAL FROM INSIDE EARTH.
  3. 7. A SCIENTIST WHO STUDIES THE STRUCTURE & HISTORY OF EARTH.
  4. 10. ABLE TO FLOAT.
  5. 12. A MAJOR FAULT OR CRACK IN EARTH'S SURFACE.
  6. 14. TO PROPOSE AN EXPLANATION OR THEORY.
  7. 15. A PERSON WHO MAKES MAPS.
Down
  1. 2. TO COME TOGETHER.
  2. 3. REMNANTS OF THE CORES OF PLANETS.
  3. 4. THE TYPE OF MATERIAL THAT AN OBJECT IS MADE OF.
  4. 5. THE OUTER LAYER OF EARTH.
  5. 8. EARTHQUAKES PRODUCE 2 TYPES OF ____________ WAVES.
  6. 9. THE SECTION OF EARTH BETWEEN THE CRUST AND THE CORE.
  7. 11. A LONG, STEEP VALLEY.
  8. 13. THE AMOUNT OF MATTER WITHIN A GIVEN SPACE.